On 07/16/2015 09:05 PM, Robert Jarzmik wrote:
Lars-Peter Clausen <lars@xxxxxxxxxx> writes:
On 07/16/2015 03:08 PM, Vinod Koul wrote:
On Mon, Jul 13, 2015 at 01:00:54PM +0200, Lars-Peter Clausen wrote:
Actually there has never been a recommendation for slave cases. It came from async
API and slave users should never have used it, untill now.
That doesn't reflect reality. The fast majority slave API users set
the flag and some DMAengine drivers expect it to be set.
That is simply wrong and we need to fix that
Could you explain why you think that it is wrong?
I think we might have different understandings of what exactly the
DMA_CTRL_ACK flag means.
This is documented now
* DMA_CTRL_ACK
- If set, the transfer can be reused after being completed.
- There is a guarantee the transfer won't be freed until it is acked
by async_tx_ack().
- As a consequence, if a device driver wants to skip the dma_map_sg()
and
dma_unmap_sg() in between 2 transfers, because the DMA'd data wasn't used,
it can resubmit the transfer right after its completion.
Any user who doesnt want this interpretation should be fixed, any driver not
doing this should be fixed :)
That would be every user and every driver. This documentation was added as part
of Robert's series, but does not agree, with either other documentation of the
flag, e.g. in include/linux/dmaengine.h, nor with any of the users. The
documentation needs to be fixed, not the other way around.
Might it be possible that I implemented code and documentation with the inverted
logic ? Or said differently, if I had written :
- If *clear*, the transfer can be reused after being completed.
And inverted the test in virt-dma.c, would it be better, Vinod ?
I'm asking because in the code I read :
- include/linux/dmaengine.h:
if clear, the descriptor cannot be reused until the client
acknowledges receipt.
- drivers/dma/virt-dma.c
If set, the transfer can be reused after being completed.
If I invert my DMA_CTRL_ACK documentation and code logic in virt-dma.c, would
that fit the old meaning, and would that still be acceptable by Vinod ?
It would certainly bring the two closer and fix most of the fallout we are
seeing at the moment.
But I still believe that support for re-usable descriptors deserve their own
flag. Drivers which support DMA_CTRL_ACK wont automatically be able to
support re-usable descriptors. The assumption currently is that each
descriptor is submitted exactly once. The dmaengine documentation even
states that submit() needs to be called right after device_prep_*() since
some driver might take a lock in the first and release it in the second.
This is something that will obviously not work with re-usable descriptors.
- Lars
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