On Wed, 2008-01-09 at 11:10 +0900, Tejun Heo wrote: > James Bottomley wrote: > > ATA requires that all DMA transfers begin and end on word boundaries. > > Because of this, a large amount of machinery grew up in ide to adjust > > scatterlists on this basis. However, as of 2.5, the block layer has a > > dma_alignment variable which ensures both the beginning and length of a > > DMA transfer are aligned on the dma_alignment boundary. Although the > > block layer does adjust the beginning of the transfer to ensure this > > happens, it doesn't actually adjust the length, it merely makes sure > > that space is allocated for transfers beyond the declared length. The > > upshot of this is that scatterlists may be padded to any size between > > the actual length and the length adjusted to the dma_alignment safely > > knowing that memory is allocated in this region. > > > > Right at the moment, SCSI takes the default dma_aligment which is on a > > 512 byte boundary. Note that this aligment only applies to transfers > > coming in from user space. However, since all kernel allocations are > > automatically aligned on a minimum of 32 byte boundaries, it is safe to > > adjust them in this manner as well. > > > > Signed-off-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> > > > > --- > > > > This also fixes a current panic in libsas with SATAPI devices. I was > > trying to trace a bad interaction between the libata padding and the new > > sglist code which was the root cause, and ended up concluding that the > > whole libata padding thing was redundant. > > > > In a future improvement, I think we can relax SCSI dma_alignemnt (it's > > causing almost every user space command to be copied instead of directly > > passed through) and allow devices and transports to build up their own > > requirements instead. > > Great but this intersects with currently queued and pending changes for > libata-dev#upstream and needs to be merged. That's OK, I can do the merge .. that's what git is for. > Also, DMA alignment at > block layer isn't enough for ATA. ATA needs drain buffers for ATAPI > commands with variable length response. :-( OK, where is this in the libata code? The dma_pad size is only 4 bytes, so this drain, I assume is only a word long? Given the word alignment requirements of ATA doesn't this still mean it's only draining up to the word boundary anyway (so the code is still correct)? James - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
