split dev_queue

[cor.git] / block / blk-settings.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Functions related to setting various queue properties from drivers
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/memblock.h>     /* for max_pfn/max_low_pfn */
#include <linux/gcd.h>
#include <linux/lcm.h>
#include <linux/jiffies.h>
#include <linux/gfp.h>
#include <linux/dma-mapping.h>

#include "blk.h"
#include "blk-wbt.h"

unsigned long blk_max_low_pfn;
EXPORT_SYMBOL(blk_max_low_pfn);

unsigned long blk_max_pfn;

void blk_queue_rq_timeout(struct request_queue *q, unsigned int timeout)
{
        q->rq_timeout = timeout;
}
EXPORT_SYMBOL_GPL(blk_queue_rq_timeout);

/**
 * blk_set_default_limits - reset limits to default values
 * @lim:  the queue_limits structure to reset
 *
 * Description:
 *   Returns a queue_limit struct to its default state.
 */
void blk_set_default_limits(struct queue_limits *lim)
{
        lim->max_segments = BLK_MAX_SEGMENTS;
        lim->max_discard_segments = 1;
        lim->max_integrity_segments = 0;
        lim->seg_boundary_mask = BLK_SEG_BOUNDARY_MASK;
        lim->virt_boundary_mask = 0;
        lim->max_segment_size = BLK_MAX_SEGMENT_SIZE;
        lim->max_sectors = lim->max_hw_sectors = BLK_SAFE_MAX_SECTORS;
        lim->max_dev_sectors = 0;
        lim->chunk_sectors = 0;
        lim->max_write_same_sectors = 0;
        lim->max_write_zeroes_sectors = 0;
        lim->max_discard_sectors = 0;
        lim->max_hw_discard_sectors = 0;
        lim->discard_granularity = 0;
        lim->discard_alignment = 0;
        lim->discard_misaligned = 0;
        lim->logical_block_size = lim->physical_block_size = lim->io_min = 512;
        lim->bounce_pfn = (unsigned long)(BLK_BOUNCE_ANY >> PAGE_SHIFT);
        lim->alignment_offset = 0;
        lim->io_opt = 0;
        lim->misaligned = 0;
        lim->zoned = BLK_ZONED_NONE;
}
EXPORT_SYMBOL(blk_set_default_limits);

/**
 * blk_set_stacking_limits - set default limits for stacking devices
 * @lim:  the queue_limits structure to reset
 *
 * Description:
 *   Returns a queue_limit struct to its default state. Should be used
 *   by stacking drivers like DM that have no internal limits.
 */
void blk_set_stacking_limits(struct queue_limits *lim)
{
        blk_set_default_limits(lim);

        /* Inherit limits from component devices */
        lim->max_segments = USHRT_MAX;
        lim->max_discard_segments = USHRT_MAX;
        lim->max_hw_sectors = UINT_MAX;
        lim->max_segment_size = UINT_MAX;
        lim->max_sectors = UINT_MAX;
        lim->max_dev_sectors = UINT_MAX;
        lim->max_write_same_sectors = UINT_MAX;
        lim->max_write_zeroes_sectors = UINT_MAX;
}
EXPORT_SYMBOL(blk_set_stacking_limits);

/**
 * blk_queue_make_request - define an alternate make_request function for a device
 * @q:  the request queue for the device to be affected
 * @mfn: the alternate make_request function
 *
 * Description:
 *    The normal way for &struct bios to be passed to a device
 *    driver is for them to be collected into requests on a request
 *    queue, and then to allow the device driver to select requests
 *    off that queue when it is ready.  This works well for many block
 *    devices. However some block devices (typically virtual devices
 *    such as md or lvm) do not benefit from the processing on the
 *    request queue, and are served best by having the requests passed
 *    directly to them.  This can be achieved by providing a function
 *    to blk_queue_make_request().
 *
 * Caveat:
 *    The driver that does this *must* be able to deal appropriately
 *    with buffers in "highmemory". This can be accomplished by either calling
 *    kmap_atomic() to get a temporary kernel mapping, or by calling
 *    blk_queue_bounce() to create a buffer in normal memory.
 **/
void blk_queue_make_request(struct request_queue *q, make_request_fn *mfn)
{
        /*
         * set defaults
         */
        q->nr_requests = BLKDEV_MAX_RQ;

        q->make_request_fn = mfn;
        blk_queue_dma_alignment(q, 511);

        blk_set_default_limits(&q->limits);
}
EXPORT_SYMBOL(blk_queue_make_request);

/**
 * blk_queue_bounce_limit - set bounce buffer limit for queue
 * @q: the request queue for the device
 * @max_addr: the maximum address the device can handle
 *
 * Description:
 *    Different hardware can have different requirements as to what pages
 *    it can do I/O directly to. A low level driver can call
 *    blk_queue_bounce_limit to have lower memory pages allocated as bounce
 *    buffers for doing I/O to pages residing above @max_addr.
 **/
void blk_queue_bounce_limit(struct request_queue *q, u64 max_addr)
{
        unsigned long b_pfn = max_addr >> PAGE_SHIFT;
        int dma = 0;

        q->bounce_gfp = GFP_NOIO;
#if BITS_PER_LONG == 64
        /*
         * Assume anything <= 4GB can be handled by IOMMU.  Actually
         * some IOMMUs can handle everything, but I don't know of a
         * way to test this here.
         */
        if (b_pfn < (min_t(u64, 0xffffffffUL, BLK_BOUNCE_HIGH) >> PAGE_SHIFT))
                dma = 1;
        q->limits.bounce_pfn = max(max_low_pfn, b_pfn);
#else
        if (b_pfn < blk_max_low_pfn)
                dma = 1;
        q->limits.bounce_pfn = b_pfn;
#endif
        if (dma) {
                init_emergency_isa_pool();
                q->bounce_gfp = GFP_NOIO | GFP_DMA;
                q->limits.bounce_pfn = b_pfn;
        }
}
EXPORT_SYMBOL(blk_queue_bounce_limit);

/**
 * blk_queue_max_hw_sectors - set max sectors for a request for this queue
 * @q:  the request queue for the device
 * @max_hw_sectors:  max hardware sectors in the usual 512b unit
 *
 * Description:
 *    Enables a low level driver to set a hard upper limit,
 *    max_hw_sectors, on the size of requests.  max_hw_sectors is set by
 *    the device driver based upon the capabilities of the I/O
 *    controller.
 *
 *    max_dev_sectors is a hard limit imposed by the storage device for
 *    READ/WRITE requests. It is set by the disk driver.
 *
 *    max_sectors is a soft limit imposed by the block layer for
 *    filesystem type requests.  This value can be overridden on a
 *    per-device basis in /sys/block/<device>/queue/max_sectors_kb.
 *    The soft limit can not exceed max_hw_sectors.
 **/
void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_sectors)
{
        struct queue_limits *limits = &q->limits;
        unsigned int max_sectors;

        if ((max_hw_sectors << 9) < PAGE_SIZE) {
                max_hw_sectors = 1 << (PAGE_SHIFT - 9);
                printk(KERN_INFO "%s: set to minimum %d\n",
                       __func__, max_hw_sectors);
        }

        limits->max_hw_sectors = max_hw_sectors;
        max_sectors = min_not_zero(max_hw_sectors, limits->max_dev_sectors);
        max_sectors = min_t(unsigned int, max_sectors, BLK_DEF_MAX_SECTORS);
        limits->max_sectors = max_sectors;
        q->backing_dev_info->io_pages = max_sectors >> (PAGE_SHIFT - 9);
}
EXPORT_SYMBOL(blk_queue_max_hw_sectors);

/**
 * blk_queue_chunk_sectors - set size of the chunk for this queue
 * @q:  the request queue for the device
 * @chunk_sectors:  chunk sectors in the usual 512b unit
 *
 * Description:
 *    If a driver doesn't want IOs to cross a given chunk size, it can set
 *    this limit and prevent merging across chunks. Note that the chunk size
 *    must currently be a power-of-2 in sectors. Also note that the block
 *    layer must accept a page worth of data at any offset. So if the
 *    crossing of chunks is a hard limitation in the driver, it must still be
 *    prepared to split single page bios.
 **/
void blk_queue_chunk_sectors(struct request_queue *q, unsigned int chunk_sectors)
{
        BUG_ON(!is_power_of_2(chunk_sectors));
        q->limits.chunk_sectors = chunk_sectors;
}
EXPORT_SYMBOL(blk_queue_chunk_sectors);

/**
 * blk_queue_max_discard_sectors - set max sectors for a single discard
 * @q:  the request queue for the device
 * @max_discard_sectors: maximum number of sectors to discard
 **/
void blk_queue_max_discard_sectors(struct request_queue *q,
                unsigned int max_discard_sectors)
{
        q->limits.max_hw_discard_sectors = max_discard_sectors;
        q->limits.max_discard_sectors = max_discard_sectors;
}
EXPORT_SYMBOL(blk_queue_max_discard_sectors);

/**
 * blk_queue_max_write_same_sectors - set max sectors for a single write same
 * @q:  the request queue for the device
 * @max_write_same_sectors: maximum number of sectors to write per command
 **/
void blk_queue_max_write_same_sectors(struct request_queue *q,
                                      unsigned int max_write_same_sectors)
{
        q->limits.max_write_same_sectors = max_write_same_sectors;
}
EXPORT_SYMBOL(blk_queue_max_write_same_sectors);

/**
 * blk_queue_max_write_zeroes_sectors - set max sectors for a single
 *                                      write zeroes
 * @q:  the request queue for the device
 * @max_write_zeroes_sectors: maximum number of sectors to write per command
 **/
void blk_queue_max_write_zeroes_sectors(struct request_queue *q,
                unsigned int max_write_zeroes_sectors)
{
        q->limits.max_write_zeroes_sectors = max_write_zeroes_sectors;
}
EXPORT_SYMBOL(blk_queue_max_write_zeroes_sectors);

/**
 * blk_queue_max_segments - set max hw segments for a request for this queue
 * @q:  the request queue for the device
 * @max_segments:  max number of segments
 *
 * Description:
 *    Enables a low level driver to set an upper limit on the number of
 *    hw data segments in a request.
 **/
void blk_queue_max_segments(struct request_queue *q, unsigned short max_segments)
{
        if (!max_segments) {
                max_segments = 1;
                printk(KERN_INFO "%s: set to minimum %d\n",
                       __func__, max_segments);
        }

        q->limits.max_segments = max_segments;
}
EXPORT_SYMBOL(blk_queue_max_segments);

/**
 * blk_queue_max_discard_segments - set max segments for discard requests
 * @q:  the request queue for the device
 * @max_segments:  max number of segments
 *
 * Description:
 *    Enables a low level driver to set an upper limit on the number of
 *    segments in a discard request.
 **/
void blk_queue_max_discard_segments(struct request_queue *q,
                unsigned short max_segments)
{
        q->limits.max_discard_segments = max_segments;
}
EXPORT_SYMBOL_GPL(blk_queue_max_discard_segments);

/**
 * blk_queue_max_segment_size - set max segment size for blk_rq_map_sg
 * @q:  the request queue for the device
 * @max_size:  max size of segment in bytes
 *
 * Description:
 *    Enables a low level driver to set an upper limit on the size of a
 *    coalesced segment
 **/
void blk_queue_max_segment_size(struct request_queue *q, unsigned int max_size)
{
        if (max_size < PAGE_SIZE) {
                max_size = PAGE_SIZE;
                printk(KERN_INFO "%s: set to minimum %d\n",
                       __func__, max_size);
        }

        /* see blk_queue_virt_boundary() for the explanation */
        WARN_ON_ONCE(q->limits.virt_boundary_mask);

        q->limits.max_segment_size = max_size;
}
EXPORT_SYMBOL(blk_queue_max_segment_size);

/**
 * blk_queue_logical_block_size - set logical block size for the queue
 * @q:  the request queue for the device
 * @size:  the logical block size, in bytes
 *
 * Description:
 *   This should be set to the lowest possible block size that the
 *   storage device can address.  The default of 512 covers most
 *   hardware.
 **/
void blk_queue_logical_block_size(struct request_queue *q, unsigned short size)
{
        q->limits.logical_block_size = size;

        if (q->limits.physical_block_size < size)
                q->limits.physical_block_size = size;

        if (q->limits.io_min < q->limits.physical_block_size)
                q->limits.io_min = q->limits.physical_block_size;
}
EXPORT_SYMBOL(blk_queue_logical_block_size);

/**
 * blk_queue_physical_block_size - set physical block size for the queue
 * @q:  the request queue for the device
 * @size:  the physical block size, in bytes
 *
 * Description:
 *   This should be set to the lowest possible sector size that the
 *   hardware can operate on without reverting to read-modify-write
 *   operations.
 */
void blk_queue_physical_block_size(struct request_queue *q, unsigned int size)
{
        q->limits.physical_block_size = size;

        if (q->limits.physical_block_size < q->limits.logical_block_size)
                q->limits.physical_block_size = q->limits.logical_block_size;

        if (q->limits.io_min < q->limits.physical_block_size)
                q->limits.io_min = q->limits.physical_block_size;
}
EXPORT_SYMBOL(blk_queue_physical_block_size);

/**
 * blk_queue_alignment_offset - set physical block alignment offset
 * @q:  the request queue for the device
 * @offset: alignment offset in bytes
 *
 * Description:
 *   Some devices are naturally misaligned to compensate for things like
 *   the legacy DOS partition table 63-sector offset.  Low-level drivers
 *   should call this function for devices whose first sector is not
 *   naturally aligned.
 */
void blk_queue_alignment_offset(struct request_queue *q, unsigned int offset)
{
        q->limits.alignment_offset =
                offset & (q->limits.physical_block_size - 1);
        q->limits.misaligned = 0;
}
EXPORT_SYMBOL(blk_queue_alignment_offset);

/**
 * blk_limits_io_min - set minimum request size for a device
 * @limits: the queue limits
 * @min:  smallest I/O size in bytes
 *
 * Description:
 *   Some devices have an internal block size bigger than the reported
 *   hardware sector size.  This function can be used to signal the
 *   smallest I/O the device can perform without incurring a performance
 *   penalty.
 */
void blk_limits_io_min(struct queue_limits *limits, unsigned int min)
{
        limits->io_min = min;

        if (limits->io_min < limits->logical_block_size)
                limits->io_min = limits->logical_block_size;

        if (limits->io_min < limits->physical_block_size)
                limits->io_min = limits->physical_block_size;
}
EXPORT_SYMBOL(blk_limits_io_min);

/**
 * blk_queue_io_min - set minimum request size for the queue
 * @q:  the request queue for the device
 * @min:  smallest I/O size in bytes
 *
 * Description:
 *   Storage devices may report a granularity or preferred minimum I/O
 *   size which is the smallest request the device can perform without
 *   incurring a performance penalty.  For disk drives this is often the
 *   physical block size.  For RAID arrays it is often the stripe chunk
 *   size.  A properly aligned multiple of minimum_io_size is the
 *   preferred request size for workloads where a high number of I/O
 *   operations is desired.
 */
void blk_queue_io_min(struct request_queue *q, unsigned int min)
{
        blk_limits_io_min(&q->limits, min);
}
EXPORT_SYMBOL(blk_queue_io_min);

/**
 * blk_limits_io_opt - set optimal request size for a device
 * @limits: the queue limits
 * @opt:  smallest I/O size in bytes
 *
 * Description:
 *   Storage devices may report an optimal I/O size, which is the
 *   device's preferred unit for sustained I/O.  This is rarely reported
 *   for disk drives.  For RAID arrays it is usually the stripe width or
 *   the internal track size.  A properly aligned multiple of
 *   optimal_io_size is the preferred request size for workloads where
 *   sustained throughput is desired.
 */
void blk_limits_io_opt(struct queue_limits *limits, unsigned int opt)
{
        limits->io_opt = opt;
}
EXPORT_SYMBOL(blk_limits_io_opt);

/**
 * blk_queue_io_opt - set optimal request size for the queue
 * @q:  the request queue for the device
 * @opt:  optimal request size in bytes
 *
 * Description:
 *   Storage devices may report an optimal I/O size, which is the
 *   device's preferred unit for sustained I/O.  This is rarely reported
 *   for disk drives.  For RAID arrays it is usually the stripe width or
 *   the internal track size.  A properly aligned multiple of
 *   optimal_io_size is the preferred request size for workloads where
 *   sustained throughput is desired.
 */
void blk_queue_io_opt(struct request_queue *q, unsigned int opt)
{
        blk_limits_io_opt(&q->limits, opt);
}
EXPORT_SYMBOL(blk_queue_io_opt);

/**
 * blk_queue_stack_limits - inherit underlying queue limits for stacked drivers
 * @t:  the stacking driver (top)
 * @b:  the underlying device (bottom)
 **/
void blk_queue_stack_limits(struct request_queue *t, struct request_queue *b)
{
        blk_stack_limits(&t->limits, &b->limits, 0);
}
EXPORT_SYMBOL(blk_queue_stack_limits);

/**
 * blk_stack_limits - adjust queue_limits for stacked devices
 * @t:  the stacking driver limits (top device)
 * @b:  the underlying queue limits (bottom, component device)
 * @start:  first data sector within component device
 *
 * Description:
 *    This function is used by stacking drivers like MD and DM to ensure
 *    that all component devices have compatible block sizes and
 *    alignments.  The stacking driver must provide a queue_limits
 *    struct (top) and then iteratively call the stacking function for
 *    all component (bottom) devices.  The stacking function will
 *    attempt to combine the values and ensure proper alignment.
 *
 *    Returns 0 if the top and bottom queue_limits are compatible.  The
 *    top device's block sizes and alignment offsets may be adjusted to
 *    ensure alignment with the bottom device. If no compatible sizes
 *    and alignments exist, -1 is returned and the resulting top
 *    queue_limits will have the misaligned flag set to indicate that
 *    the alignment_offset is undefined.
 */
int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
                     sector_t start)
{
        unsigned int top, bottom, alignment, ret = 0;

        t->max_sectors = min_not_zero(t->max_sectors, b->max_sectors);
        t->max_hw_sectors = min_not_zero(t->max_hw_sectors, b->max_hw_sectors);
        t->max_dev_sectors = min_not_zero(t->max_dev_sectors, b->max_dev_sectors);
        t->max_write_same_sectors = min(t->max_write_same_sectors,
                                        b->max_write_same_sectors);
        t->max_write_zeroes_sectors = min(t->max_write_zeroes_sectors,
                                        b->max_write_zeroes_sectors);
        t->bounce_pfn = min_not_zero(t->bounce_pfn, b->bounce_pfn);

        t->seg_boundary_mask = min_not_zero(t->seg_boundary_mask,
                                            b->seg_boundary_mask);
        t->virt_boundary_mask = min_not_zero(t->virt_boundary_mask,
                                            b->virt_boundary_mask);

        t->max_segments = min_not_zero(t->max_segments, b->max_segments);
        t->max_discard_segments = min_not_zero(t->max_discard_segments,
                                               b->max_discard_segments);
        t->max_integrity_segments = min_not_zero(t->max_integrity_segments,
                                                 b->max_integrity_segments);

        t->max_segment_size = min_not_zero(t->max_segment_size,
                                           b->max_segment_size);

        t->misaligned |= b->misaligned;

        alignment = queue_limit_alignment_offset(b, start);

        /* Bottom device has different alignment.  Check that it is
         * compatible with the current top alignment.
         */
        if (t->alignment_offset != alignment) {

                top = max(t->physical_block_size, t->io_min)
                        + t->alignment_offset;
                bottom = max(b->physical_block_size, b->io_min) + alignment;

                /* Verify that top and bottom intervals line up */
                if (max(top, bottom) % min(top, bottom)) {
                        t->misaligned = 1;
                        ret = -1;
                }
        }

        t->logical_block_size = max(t->logical_block_size,
                                    b->logical_block_size);

        t->physical_block_size = max(t->physical_block_size,
                                     b->physical_block_size);

        t->io_min = max(t->io_min, b->io_min);
        t->io_opt = lcm_not_zero(t->io_opt, b->io_opt);

        /* Physical block size a multiple of the logical block size? */
        if (t->physical_block_size & (t->logical_block_size - 1)) {
                t->physical_block_size = t->logical_block_size;
                t->misaligned = 1;
                ret = -1;
        }

        /* Minimum I/O a multiple of the physical block size? */
        if (t->io_min & (t->physical_block_size - 1)) {
                t->io_min = t->physical_block_size;
                t->misaligned = 1;
                ret = -1;
        }

        /* Optimal I/O a multiple of the physical block size? */
        if (t->io_opt & (t->physical_block_size - 1)) {
                t->io_opt = 0;
                t->misaligned = 1;
                ret = -1;
        }

        t->raid_partial_stripes_expensive =
                max(t->raid_partial_stripes_expensive,
                    b->raid_partial_stripes_expensive);

        /* Find lowest common alignment_offset */
        t->alignment_offset = lcm_not_zero(t->alignment_offset, alignment)
                % max(t->physical_block_size, t->io_min);

        /* Verify that new alignment_offset is on a logical block boundary */
        if (t->alignment_offset & (t->logical_block_size - 1)) {
                t->misaligned = 1;
                ret = -1;
        }

        /* Discard alignment and granularity */
        if (b->discard_granularity) {
                alignment = queue_limit_discard_alignment(b, start);

                if (t->discard_granularity != 0 &&
                    t->discard_alignment != alignment) {
                        top = t->discard_granularity + t->discard_alignment;
                        bottom = b->discard_granularity + alignment;

                        /* Verify that top and bottom intervals line up */
                        if ((max(top, bottom) % min(top, bottom)) != 0)
                                t->discard_misaligned = 1;
                }

                t->max_discard_sectors = min_not_zero(t->max_discard_sectors,
                                                      b->max_discard_sectors);
                t->max_hw_discard_sectors = min_not_zero(t->max_hw_discard_sectors,
                                                         b->max_hw_discard_sectors);
                t->discard_granularity = max(t->discard_granularity,
                                             b->discard_granularity);
                t->discard_alignment = lcm_not_zero(t->discard_alignment, alignment) %
                        t->discard_granularity;
        }

        if (b->chunk_sectors)
                t->chunk_sectors = min_not_zero(t->chunk_sectors,
                                                b->chunk_sectors);

        return ret;
}
EXPORT_SYMBOL(blk_stack_limits);

/**
 * bdev_stack_limits - adjust queue limits for stacked drivers
 * @t:  the stacking driver limits (top device)
 * @bdev:  the component block_device (bottom)
 * @start:  first data sector within component device
 *
 * Description:
 *    Merges queue limits for a top device and a block_device.  Returns
 *    0 if alignment didn't change.  Returns -1 if adding the bottom
 *    device caused misalignment.
 */
int bdev_stack_limits(struct queue_limits *t, struct block_device *bdev,
                      sector_t start)
{
        struct request_queue *bq = bdev_get_queue(bdev);

        start += get_start_sect(bdev);

        return blk_stack_limits(t, &bq->limits, start);
}
EXPORT_SYMBOL(bdev_stack_limits);

/**
 * disk_stack_limits - adjust queue limits for stacked drivers
 * @disk:  MD/DM gendisk (top)
 * @bdev:  the underlying block device (bottom)
 * @offset:  offset to beginning of data within component device
 *
 * Description:
 *    Merges the limits for a top level gendisk and a bottom level
 *    block_device.
 */
void disk_stack_limits(struct gendisk *disk, struct block_device *bdev,
                       sector_t offset)
{
        struct request_queue *t = disk->queue;

        if (bdev_stack_limits(&t->limits, bdev, offset >> 9) < 0) {
                char top[BDEVNAME_SIZE], bottom[BDEVNAME_SIZE];

                disk_name(disk, 0, top);
                bdevname(bdev, bottom);

                printk(KERN_NOTICE "%s: Warning: Device %s is misaligned\n",
                       top, bottom);
        }
}
EXPORT_SYMBOL(disk_stack_limits);

/**
 * blk_queue_update_dma_pad - update pad mask
 * @q:     the request queue for the device
 * @mask:  pad mask
 *
 * Update dma pad mask.
 *
 * Appending pad buffer to a request modifies the last entry of a
 * scatter list such that it includes the pad buffer.
 **/
void blk_queue_update_dma_pad(struct request_queue *q, unsigned int mask)
{
        if (mask > q->dma_pad_mask)
                q->dma_pad_mask = mask;
}
EXPORT_SYMBOL(blk_queue_update_dma_pad);

/**
 * blk_queue_dma_drain - Set up a drain buffer for excess dma.
 * @q:  the request queue for the device
 * @dma_drain_needed: fn which returns non-zero if drain is necessary
 * @buf:        physically contiguous buffer
 * @size:       size of the buffer in bytes
 *
 * Some devices have excess DMA problems and can't simply discard (or
 * zero fill) the unwanted piece of the transfer.  They have to have a
 * real area of memory to transfer it into.  The use case for this is
 * ATAPI devices in DMA mode.  If the packet command causes a transfer
 * bigger than the transfer size some HBAs will lock up if there
 * aren't DMA elements to contain the excess transfer.  What this API
 * does is adjust the queue so that the buf is always appended
 * silently to the scatterlist.
 *
 * Note: This routine adjusts max_hw_segments to make room for appending
 * the drain buffer.  If you call blk_queue_max_segments() after calling
 * this routine, you must set the limit to one fewer than your device
 * can support otherwise there won't be room for the drain buffer.
 */
int blk_queue_dma_drain(struct request_queue *q,
                               dma_drain_needed_fn *dma_drain_needed,
                               void *buf, unsigned int size)
{
        if (queue_max_segments(q) < 2)
                return -EINVAL;
        /* make room for appending the drain */
        blk_queue_max_segments(q, queue_max_segments(q) - 1);
        q->dma_drain_needed = dma_drain_needed;
        q->dma_drain_buffer = buf;
        q->dma_drain_size = size;

        return 0;
}
EXPORT_SYMBOL_GPL(blk_queue_dma_drain);

/**
 * blk_queue_segment_boundary - set boundary rules for segment merging
 * @q:  the request queue for the device
 * @mask:  the memory boundary mask
 **/
void blk_queue_segment_boundary(struct request_queue *q, unsigned long mask)
{
        if (mask < PAGE_SIZE - 1) {
                mask = PAGE_SIZE - 1;
                printk(KERN_INFO "%s: set to minimum %lx\n",
                       __func__, mask);
        }

        q->limits.seg_boundary_mask = mask;
}
EXPORT_SYMBOL(blk_queue_segment_boundary);

/**
 * blk_queue_virt_boundary - set boundary rules for bio merging
 * @q:  the request queue for the device
 * @mask:  the memory boundary mask
 **/
void blk_queue_virt_boundary(struct request_queue *q, unsigned long mask)
{
        q->limits.virt_boundary_mask = mask;

        /*
         * Devices that require a virtual boundary do not support scatter/gather
         * I/O natively, but instead require a descriptor list entry for each
         * page (which might not be idential to the Linux PAGE_SIZE).  Because
         * of that they are not limited by our notion of "segment size".
         */
        if (mask)
                q->limits.max_segment_size = UINT_MAX;
}
EXPORT_SYMBOL(blk_queue_virt_boundary);

/**
 * blk_queue_dma_alignment - set dma length and memory alignment
 * @q:     the request queue for the device
 * @mask:  alignment mask
 *
 * description:
 *    set required memory and length alignment for direct dma transactions.
 *    this is used when building direct io requests for the queue.
 *
 **/
void blk_queue_dma_alignment(struct request_queue *q, int mask)
{
        q->dma_alignment = mask;
}
EXPORT_SYMBOL(blk_queue_dma_alignment);

/**
 * blk_queue_update_dma_alignment - update dma length and memory alignment
 * @q:     the request queue for the device
 * @mask:  alignment mask
 *
 * description:
 *    update required memory and length alignment for direct dma transactions.
 *    If the requested alignment is larger than the current alignment, then
 *    the current queue alignment is updated to the new value, otherwise it
 *    is left alone.  The design of this is to allow multiple objects
 *    (driver, device, transport etc) to set their respective
 *    alignments without having them interfere.
 *
 **/
void blk_queue_update_dma_alignment(struct request_queue *q, int mask)
{
        BUG_ON(mask > PAGE_SIZE);

        if (mask > q->dma_alignment)
                q->dma_alignment = mask;
}
EXPORT_SYMBOL(blk_queue_update_dma_alignment);

/**
 * blk_set_queue_depth - tell the block layer about the device queue depth
 * @q:          the request queue for the device
 * @depth:              queue depth
 *
 */
void blk_set_queue_depth(struct request_queue *q, unsigned int depth)
{
        q->queue_depth = depth;
        rq_qos_queue_depth_changed(q);
}
EXPORT_SYMBOL(blk_set_queue_depth);

/**
 * blk_queue_write_cache - configure queue's write cache
 * @q:          the request queue for the device
 * @wc:         write back cache on or off
 * @fua:        device supports FUA writes, if true
 *
 * Tell the block layer about the write cache of @q.
 */
void blk_queue_write_cache(struct request_queue *q, bool wc, bool fua)
{
        if (wc)
                blk_queue_flag_set(QUEUE_FLAG_WC, q);
        else
                blk_queue_flag_clear(QUEUE_FLAG_WC, q);
        if (fua)
                blk_queue_flag_set(QUEUE_FLAG_FUA, q);
        else
                blk_queue_flag_clear(QUEUE_FLAG_FUA, q);

        wbt_set_write_cache(q, test_bit(QUEUE_FLAG_WC, &q->queue_flags));
}
EXPORT_SYMBOL_GPL(blk_queue_write_cache);

/**
 * blk_queue_required_elevator_features - Set a queue required elevator features
 * @q:          the request queue for the target device
 * @features:   Required elevator features OR'ed together
 *
 * Tell the block layer that for the device controlled through @q, only the
 * only elevators that can be used are those that implement at least the set of
 * features specified by @features.
 */
void blk_queue_required_elevator_features(struct request_queue *q,
                                          unsigned int features)
{
        q->required_elevator_features = features;
}
EXPORT_SYMBOL_GPL(blk_queue_required_elevator_features);

/**
 * blk_queue_can_use_dma_map_merging - configure queue for merging segments.
 * @q:          the request queue for the device
 * @dev:        the device pointer for dma
 *
 * Tell the block layer about merging the segments by dma map of @q.
 */
bool blk_queue_can_use_dma_map_merging(struct request_queue *q,
                                       struct device *dev)
{
        unsigned long boundary = dma_get_merge_boundary(dev);

        if (!boundary)
                return false;

        /* No need to update max_segment_size. see blk_queue_virt_boundary() */
        blk_queue_virt_boundary(q, boundary);

        return true;
}
EXPORT_SYMBOL_GPL(blk_queue_can_use_dma_map_merging);

static int __init blk_settings_init(void)
{
        blk_max_low_pfn = max_low_pfn - 1;
        blk_max_pfn = max_pfn - 1;
        return 0;
}
subsys_initcall(blk_settings_init);