From mboxrd@z Thu Jan 1 00:00:00 1970 From: dan.j.williams@intel.com (Dan Williams) Date: Tue, 24 May 2011 14:03:30 -0700 Subject: [PATCH] dmaengine: Add API documentation for slave dma usage In-Reply-To: <1306253171.30236.9.camel@vkoul-udesk3> References: <1306238657-30089-1-git-send-email-vinod.koul@intel.com> <1306253171.30236.9.camel@vkoul-udesk3> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Tue, May 24, 2011 at 9:06 AM, Koul, Vinod wrote: > On Tue, 2011-05-24 at 17:40 +0200, Per Forlin wrote: >> Hi Vinod, >> >> On Tue, May 24, 2011 at 2:04 PM, Koul, Vinod wrote: >> > From: Vinod Koul >> > >> > Signed-off-by: Vinod Koul >> > --- >> > ?Documentation/dma-slave-api.txt | ? 74 +++++++++++++++++++++++++++++++++++++++ >> > ?1 files changed, 74 insertions(+), 0 deletions(-) >> > ?create mode 100644 Documentation/dma-slave-api.txt >> I suggest putting this in subsection of dmaengine.txt instead. >> dmaengine.txt would be the natural place to look at. > Agreed, that would make it easier for people to find > >> > +4. Submit the transaction(s) and wait for callback notification when slave API >> > +is 3 above returns, the non NULL value would imply a "descriptor" for the >> > +transaction. These transaction(s) would need to be submitted which pushes them >> > +into queue in DMA driver. If DMA is idle then the first descriptor submit will >> > +be pushed to DMA and subsequent ones will be queued. On completion of the DMA >> > +operation the next in queue is submitted and a tasklet triggered. The tasklet >> > +would then call the client driver completion callback routine for notification, >> > +if set. >> > + >> Does submit really start the transfer as well? My interpretation of >> submit is that is only adds desc to a pending queue. When calling >> issue_pending all these descs will be schedule for DMA transfer. Calls >> to submit after this point will added to the pending queue again and >> not be issued until calling issue_pending once more. > For slave dma devices, submit() is used to start the transaction if the > channel is idle. If its already doing a transaction then it will queue > it up and submit once cureent excuting one is completed. It is not > required to call issue_pending once more. > I am not sure if this is true for non slave usage, Dan would that be > correct for you as well? No, ->submit() is just an "add this descriptor to the chain" operation, and ->issue_pending() is always required to make sure the everything submitted previously is actively executing. This was a holdover from the very first dmaengine implementation where, for efficiency reasons, it could save mmio writes by batching the issuing of requests. -- Dan