Messages in this thread | | | Subject | Re: [PATCH] 2.4 use __dma_buffer in scsi.h | Date | Wed, 12 Jun 2002 22:17:59 -0400 | From | James Bottomley <> |
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> Use __dma_buffer macro to align sense_buffer member of Scsi_Cmnd. [...]
> - unsigned char sense_buffer[SCSI_SENSE_BUFFERSIZE]; /* obtained by REQUEST SENSE > + unsigned char sense_buffer[SCSI_SENSE_BUFFERSIZE] __dma_buffer; /* obtained by REQUEST SENSE
This is actually unnecessary. We've already had this discussion on the parisc mailing lists (over a year ago, I think).
The SCSI subsystem is designed with this type of cache incoherency problem in mind. The Scsi_Cmnd structure has an ownership model to forestall cache line bouncing. The idea is that when you want to dma to/from components of a Scsi_Cmnd, you can only do it when the ownership is SCSI_OWNER_LOW_LEVEL, which guarantees that nothing in the mid layer will touch the command and trigger an incoherency (at least until it times out). The low level driver is supposed to know about the cache incoherency problem, and so avoids touching the structure between cache line invalidation/writeback and DMA completion, thus, with a correctly implemented driver, there should be no possibility of DMA corruption.
Each of the Scsi_Cmnd blocks is individually allocated with kmalloc, so they're guaranteed to be non-interfering when it comes to DMA.
Incidentally, if you're really going to insist on padding the structure, some drivers also use the cmnd element to DMA the command from, so that should be aligned as well. But I think the best course of action is to fix any low level drivers that are not following the cache rules rather than expanding Scsi_Cmnd.
James Bottomley
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