[PATCH 2/3] scsi: use dma_get_cache_alignment() as minimum DMA alignment

[Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>]

From: Huacai Chen <chenhc-h23VmSynlr/QT0dZR+AlfA@public.gmane.org>

In non-coherent DMA mode, kernel uses cache flushing operations to maintain
I/O coherency, so scsi's block queue should be aligned to the value
returned by dma_get_cache_alignment().  Otherwise, If a DMA buffer and a
kernel structure share a same cache line, and if the kernel structure has
dirty data, cache_invalidate (no writeback) will cause data corruption.

Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Signed-off-by: Huacai Chen <chenhc-h23VmSynlr/QT0dZR+AlfA@public.gmane.org>
[hch: rebased and updated the comment and changelog]
Signed-off-by: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
---
 drivers/scsi/scsi_lib.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 1cbc497e00bd..00742c50cd44 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -2148,11 +2148,13 @@ void __scsi_init_queue(struct Scsi_Host *shost, struct request_queue *q)
 		q->limits.cluster = 0;
 
 	/*
-	 * set a reasonable default alignment on word boundaries: the
-	 * host and device may alter it using
-	 * blk_queue_update_dma_alignment() later.
+	 * Set a reasonable default alignment:  The larger of 32-byte (dword),
+	 * which is a common minimum for HBAs, and the minimum DMA alignment,
+	 * which is set by the platform.
+	 *
+	 * Devices that require a bigger alignment can increase it later.
 	 */
-	blk_queue_dma_alignment(q, 0x03);
+	blk_queue_dma_alignment(q, max(4, dma_get_cache_alignment()) - 1);
 }
 EXPORT_SYMBOL_GPL(__scsi_init_queue);
 
-- 
2.14.2