Alan Cox wrote:
On Mon, 29 Oct 2007 22:17:40 -0600
Robert Hancock <hancockr@xxxxxxx> wrote:
In the sata_nv driver, when running in ADMA mode, we can do 64-bit DMA.
However, when an ATAPI device like a DVD drive is connected, we can't
use ADMA mode, and so we have to abide by the restrictions of a normal
SFF ATA controller and can only do 32-bit DMA. We detect this and try to
set the blk_queue_bounce_limit, blk_queue_segment_boundary and
blk_queue_max_hw_segments to the values corresponding to a normal SFF
controller.
What about the DMA padding buffer from nv_adma_port_start and internal
buffers for commands like request sense that don't come via the request
queue directly.
Indeed we do call ata_port_start from nv_adma_port_start, which calls
dmam_alloc_coherent to allocate the SFF PRD table. Since the DMA mask is
64-bit, this could indeed be allocated above 4GB which would be bad.
I suppose what we could do is just not call ata_port_start there, but
move it into nv_adma_slave_config and call it when going into non-ADMA
mode. We'd have to drop the DMA mask down to 32-bit first as well as
setting blk_queue_bounce_limit though, which is one of my questions, is
this OK to do?
Also it seems nv_adma_use_reg_mode() can decide to send other commands
via the non ADMA interface even for ATA devices. Are we 100% certain it
never decides to let through a command with DMA via the register
interface in this case - what do you see if you instrument the function ?
The only cases where that could happen are for polling DMA commands
(which I presume we never do) or where result taskfile is requested. The
latter could be a problem for ATA passthrough commands using DMA, I
suppose.. Question is what we can do about it.. We have to switch out of
ADMA mode to read a result taskfile. I guess that's not really a problem
unless somebody starts issuing NCQ commands via ATA pass-through. Do we
allow that?
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