Re: [PATCH v4 3/6] dmaengine: Add support for repeating transactions

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Vinod,

On Thu, May 14, 2020 at 11:53:44PM +0530, Vinod Koul wrote:
> On 13-05-20, 19:59, Laurent Pinchart wrote:
> > DMA engines used with displays perform 2D interleaved transfers to read
> > framebuffers from memory and feed the data to the display engine. As the
> > same framebuffer can be displayed for multiple frames, the DMA
> > transactions need to be repeated until a new framebuffer replaces the
> > current one. This feature is implemented natively by some DMA engines
> > that have the ability to repeat transactions and switch to a new
> > transaction at the end of a transfer without any race condition or frame
> > loss.
> > 
> > This patch implements support for this feature in the DMA engine API. A
> > new DMA_PREP_REPEAT transaction flag allows DMA clients to instruct the
> > DMA channel to repeat the transaction automatically until one or more
> > new transactions are issued on the channel (or until all active DMA
> > transfers are explicitly terminated with the dmaengine_terminate_*()
> > functions). A new DMA_REPEAT transaction type is also added for DMA
> > engine drivers to report their support of the DMA_PREP_REPEAT flag.
> > 
> > The DMA_PREP_REPEAT flag is currently supported for interleaved
> > transactions only. Its usage can easily be extended to cover more
> > transaction types simply by adding an appropriate check in the
> > corresponding dmaengine_prep_*() function.
> > 
> > Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > ---
> > If this approach is accepted I can send a new version that updates
> > documentation in Documentation/driver-api/dmaengine/, and extend support
> > of DMA_PREP_REPEAT to the other transaction types, if desired already.
> > 
> >  include/linux/dmaengine.h | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> > index 64461fc64e1b..9fa00bdbf583 100644
> > --- a/include/linux/dmaengine.h
> > +++ b/include/linux/dmaengine.h
> > @@ -61,6 +61,7 @@ enum dma_transaction_type {
> >  	DMA_SLAVE,
> >  	DMA_CYCLIC,
> >  	DMA_INTERLEAVE,
> > +	DMA_REPEAT,
> >  /* last transaction type for creation of the capabilities mask */
> >  	DMA_TX_TYPE_END,
> >  };
> > @@ -176,6 +177,11 @@ struct dma_interleaved_template {
> >   * @DMA_PREP_CMD: tell the driver that the data passed to DMA API is command
> >   *  data and the descriptor should be in different format from normal
> >   *  data descriptors.
> > + * @DMA_PREP_REPEAT: tell the driver that the transaction shall be automatically
> > + *  repeated when it ends if no other transaction has been issued on the same
> > + *  channel. If other transactions have been issued, this transaction completes
> > + *  normally. This flag is only applicable to interleaved transactions and is
> > + *  ignored for all other transaction types.
> >   */
> >  enum dma_ctrl_flags {
> >  	DMA_PREP_INTERRUPT = (1 << 0),
> > @@ -186,6 +192,7 @@ enum dma_ctrl_flags {
> >  	DMA_PREP_FENCE = (1 << 5),
> >  	DMA_CTRL_REUSE = (1 << 6),
> >  	DMA_PREP_CMD = (1 << 7),
> > +	DMA_PREP_REPEAT = (1 << 8),
> 
> Thanks for sending this. I think this is a good proposal which Peter
> made for solving this issue and it has great merits, but this is
> incomplete.
> 
> DMA_PREP_REPEAT|RELOAD should only imply repeating of transactions,
> nothing else. I would like to see APIs having explicit behaviour, so let
> us also add another flag DMA_PREP_LOAD_NEXT|NEW to indicate that the
> next transactions will replace the current one when submitted after calling
> .issue_pending().
> 
> Also it makes sense to explicitly specify when the transaction should be
> reloaded. Rather than make a guesswork based on hardware support, we
> should specify the EOB/EOT in these flags as well.
> 
> Next is callback notification mechanism and when it should be invoked.
> EOT is today indicated by DMA_PREP_INTERRUPT, EOB needs to be added.
> 
> So to summarize your driver needs to invoke
> DMA_PREP_REPEAT|DMA_PREP_LOAD_NEXT|DMA_LOAD_EOT|DMA_PREP_INTERRUPT
> specifying that the transactions are repeated untill next one pops up
> and replaced at EOT with callbacks being invoked at EOT boundaries.

Are you *serious* ? I feel trapped in a cross-over of Groundhog Day and
Brazil.

> @Peter, did I miss anything else in this..? Please send the patch for
> this (to start with just the headers so that Laurent can start
> using them) and detailed patch with documentation as follow up, I trust
> you two can coordinate :)

I won't call that coordination, no. If you want to design something
absurd that's your call, not mine, I don't want to get involved.

-- 
Regards,

Laurent Pinchart