James> This is actually unnecessary. We've already had this
James> discussion on the parisc mailing lists (over a year ago, I
James> think).
Fair enough, the only reason I came up with this patch is that I
looked at all unaligned DMA done by USB, and
drivers/usb/storage/transport.c does DMA into sense_buffer.
James> The SCSI subsystem is designed with this type of cache
James> incoherency problem in mind. The Scsi_Cmnd structure has
James> an ownership model to forestall cache line bouncing. The
James> idea is that when you want to dma to/from components of a
James> Scsi_Cmnd, you can only do it when the ownership is
James> SCSI_OWNER_LOW_LEVEL, which guarantees that nothing in the
James> mid layer will touch the command and trigger an incoherency
James> (at least until it times out). The low level driver is
James> supposed to know about the cache incoherency problem, and
James> so avoids touching the structure between cache line
James> invalidation/writeback and DMA completion, thus, with a
James> correctly implemented driver, there should be no
James> possibility of DMA corruption.
Out of curiousity, how do you deal with someone writing to Scsi_Cmnd
_before_ the DMA and having that data lost when pci_map_single() with
PCI_DMA_FROMDEVICE invalidates the cache before writeback?
Best,
Roland
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/