• James Bottomley's avatar
    libata: eliminate the home grown dma padding in favour of · dde20207
    James Bottomley authored
    that provided by the block layer
    
    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.
    
    tj: * Adjusting sg after padding is done in block layer.  Make libata
          set queue alignment correctly for ATAPI devices and drop broken
          sg mangling from ata_sg_setup().
        * Use request->raw_data_len for ATAPI transfer chunk size.
        * Killed qc->raw_nbytes.
        * Separated out killing qc->n_iter.
    Signed-off-by: default avatarJames Bottomley <James.Bottomley@HansenPartnership.com>
    Signed-off-by: default avatarTejun Heo <htejun@gmail.com>
    Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
    dde20207
libata.h 44.2 KB