On Wed, 27 Jun 2007, Jeff Garzik wrote:
>
> Such would be a diagnostic that would trigger on valid SCSI commands, when the
> user is doing nothing wrong and the system can indeed complete the command
> just fine. Additionally, this is moving us in the direction of what the IDE
> driver has apparently been doing.
Indeed. At least the IDE CD-ROM driver does
if ((rq->data_len & 15) || (addr & mask))
info->dma = 0;
where the "mask" is the dma_alignment mask. So it requires the length to
be a multiple of 16, and also requires a certain alignment of the data
(which defaults to 32 bytes for some reason I cannot for the life of me
remember).
The generic BIO layer does that DMA alignment check too when mapping user
pages.
Linus
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