litmus-rt.git - The LITMUS^RT kernel.

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author	Robert P. J. Day <rpjday@mindspring.com>	2007-10-17 02:29:32 -0400
committer	Linus Torvalds <torvalds@woody.linux-foundation.org>	2007-10-17 11:42:56 -0400
commit	b311e921b385b5a848f0806df16276aca1ecae07 (patch)
tree	957947e319b23150ddbbf7db52f9bdd24b6cdba5 /include
parent	d9aacccf457d07d729cfc7bf20ff0c7428357a9f (diff)

Add a "rounddown_pow_of_two" routine to log2.h

To go along with the existing "roundup_pow_of_two" routine, add one for rounding down since that operation appears to crop up on a regular basis in the source tree. [m.kozlowski@tuxland.pl: fix unbalanced parentheses] Signed-off-by: Robert P. J. Day <rpjday@mindspring.com> Signed-off-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Diffstat (limited to 'include')

-rw-r--r--

include/linux/log2.h

1 files changed, 25 insertions, 0 deletions


diff --git a/include/linux/log2.h b/include/linux/log2.h index 1b8a2c1cb0e3..c8cf5e8ef171 100644 --- a/include/linux/log2.h +++ b/include/linux/log2.h
@@ -63,6 +63,15 @@ unsigned long __roundup_pow_of_two(unsigned long n)
63	return 1UL << fls_long(n - 1);	63	return 1UL << fls_long(n - 1);
64	}	64	}
65		65
		66	/*
		67	* round down to nearest power of two
		68	*/
		69	static inline __attribute__((const))
		70	unsigned long __rounddown_pow_of_two(unsigned long n)
		71	{
		72	return 1UL << (fls_long(n) - 1);
		73	}
		74
66	/**	75	/**
67	* ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value	76	* ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
68	* @n - parameter	77	* @n - parameter
@@ -165,4 +174,20 @@ unsigned long __roundup_pow_of_two(unsigned long n)
165	__roundup_pow_of_two(n) \	174	__roundup_pow_of_two(n) \
166	)	175	)
167		176
		177	/**
		178	* rounddown_pow_of_two - round the given value down to nearest power of two
		179	* @n - parameter
		180	*
		181	* round the given value down to the nearest power of two
		182	* - the result is undefined when n == 0
		183	* - this can be used to initialise global variables from constant data
		184	*/
		185	#define rounddown_pow_of_two(n) \
		186	( \
		187	__builtin_constant_p(n) ? ( \
		188	(n == 1) ? 0 : \
		189	(1UL << ilog2(n))) : \
		190	__rounddown_pow_of_two(n) \
		191	)
		192
168	#endif /* _LINUX_LOG2_H */	193	#endif /* _LINUX_LOG2_H */

/* * 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/bootmem.h> /* for max_pfn/max_low_pfn */ #include <linux/gcd.h> #include <linux/lcm.h> #include <linux/jiffies.h> #include <linux/gfp.h> #include "blk.h" unsigned long blk_max_low_pfn; EXPORT_SYMBOL(blk_max_low_pfn); unsigned long blk_max_pfn; /** * blk_queue_prep_rq - set a prepare_request function for queue * @q: queue * @pfn: prepare_request function * * It's possible for a queue to register a prepare_request callback which * is invoked before the request is handed to the request_fn. The goal of * the function is to prepare a request for I/O, it can be used to build a * cdb from the request data for instance. * */ void blk_queue_prep_rq(struct request_queue *q, prep_rq_fn *pfn) { q->prep_rq_fn = pfn; } EXPORT_SYMBOL(blk_queue_prep_rq); /** * blk_queue_unprep_rq - set an unprepare_request function for queue * @q: queue * @ufn: unprepare_request function * * It's possible for a queue to register an unprepare_request callback * which is invoked before the request is finally completed. The goal * of the function is to deallocate any data that was allocated in the * prepare_request callback. * */ void blk_queue_unprep_rq(struct request_queue *q, unprep_rq_fn *ufn) { q->unprep_rq_fn = ufn; } EXPORT_SYMBOL(blk_queue_unprep_rq); /** * blk_queue_merge_bvec - set a merge_bvec function for queue * @q: queue * @mbfn: merge_bvec_fn * * Usually queues have static limitations on the max sectors or segments that * we can put in a request. Stacking drivers may have some settings that * are dynamic, and thus we have to query the queue whether it is ok to * add a new bio_vec to a bio at a given offset or not. If the block device * has such limitations, it needs to register a merge_bvec_fn to control * the size of bio's sent to it. Note that a block device *must* allow a * single page to be added to an empty bio. The block device driver may want * to use the bio_split() function to deal with these bio's. By default * no merge_bvec_fn is defined for a queue, and only the fixed limits are * honored. */ void blk_queue_merge_bvec(struct request_queue *q, merge_bvec_fn *mbfn) { q->merge_bvec_fn = mbfn; } EXPORT_SYMBOL(blk_queue_merge_bvec); void blk_queue_softirq_done(struct request_queue *q, softirq_done_fn *fn) { q->softirq_done_fn = fn; } EXPORT_SYMBOL(blk_queue_softirq_done); void blk_queue_rq_timeout(struct request_queue *q, unsigned int timeout) { q->rq_timeout = timeout; } EXPORT_SYMBOL_GPL(blk_queue_rq_timeout); void blk_queue_rq_timed_out(struct request_queue *q, rq_timed_out_fn *fn) { q->rq_timed_out_fn = fn; } EXPORT_SYMBOL_GPL(blk_queue_rq_timed_out); void blk_queue_lld_busy(struct request_queue *q, lld_busy_fn *fn) { q->lld_busy_fn = fn; } EXPORT_SYMBOL_GPL(blk_queue_lld_busy); /** * 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_integrity_segments = 0; lim->seg_boundary_mask = BLK_SEG_BOUNDARY_MASK; lim->max_segment_size = BLK_MAX_SEGMENT_SIZE; lim->max_sectors = lim->max_hw_sectors = BLK_SAFE_MAX_SECTORS; lim->max_write_same_sectors = 0; lim->max_discard_sectors = 0; lim->discard_granularity = 0; lim->discard_alignment = 0; lim->discard_misaligned = 0; lim->discard_zeroes_data = 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->cluster = 1; } 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->discard_zeroes_data = 1; lim->max_segments = USHRT_MAX; lim->max_hw_sectors = UINT_MAX; lim->max_segment_size = UINT_MAX; lim->max_sectors = UINT_MAX; lim->max_write_same_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 * __bio_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_queue_congestion_threshold(q); q->nr_batching = BLK_BATCH_REQ; blk_set_default_limits(&q->limits); /* * by default assume old behaviour and bounce for any highmem page */ blk_queue_bounce_limit(q, BLK_BOUNCE_HIGH); } 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_limits_max_hw_sectors - set hard and soft limit of max sectors for request * @limits: the queue limits * @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 combined capabilities of I/O * controller and storage device. * * 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_limits_max_hw_sectors(struct queue_limits *limits, unsigned int max_hw_sectors) { if ((max_hw_sectors << 9) < PAGE_CACHE_SIZE) { max_hw_sectors = 1 << (PAGE_CACHE_SHIFT - 9); printk(KERN_INFO "%s: set to minimum %d\n", __func__, max_hw_sectors); } limits->max_hw_sectors = max_hw_sectors; limits->max_sectors = min_t(unsigned int, max_hw_sectors, BLK_DEF_MAX_SECTORS); } EXPORT_SYMBOL(blk_limits_max_hw_sectors); /** * 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: * See description for blk_limits_max_hw_sectors(). **/ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_sectors) { blk_limits_max_hw_sectors(&q->limits, max_hw_sectors); } EXPORT_SYMBOL(blk_queue_max_hw_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_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_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_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_CACHE_SIZE) { max_size = PAGE_CACHE_SIZE; printk(KERN_INFO "%s: set to minimum %d\n", __func__, max_size); } 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_write_same_sectors = min(t->max_write_same_sectors, b->max_write_same_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->max_segments = min_not_zero(t->max_segments, b->max_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) - 1)) { 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(t->io_opt, b->io_opt); t->cluster &= b->cluster; t->discard_zeroes_data &= b->discard_zeroes_data; /* 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; } /* Find lowest common alignment_offset */ t->alignment_offset = lcm(t->alignment_offset, alignment) & (max(t->physical_block_size, t->io_min) - 1); /* 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->discard_granularity = max(t->discard_granularity, b->discard_granularity); t->discard_alignment = lcm(t->discard_alignment, alignment) % t->discard_granularity; } 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_dma_pad - set pad mask * @q: the request queue for the device * @mask: pad mask * * Set 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_dma_pad(struct request_queue *q, unsigned int mask) { q->dma_pad_mask = mask; } EXPORT_SYMBOL(blk_queue_dma_pad); /** * 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


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