linux-kernel - RE: [PATCH 1/3] dmaengine: xilinx_dma: Check for channel idle state before submitting dma descriptor

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Date:   Fri, 23 Dec 2016 08:49:07 +0000
From:   Appana Durga Kedareswara Rao <appana.durga.rao@...inx.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC:     "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "vinod.koul@...el.com" <vinod.koul@...el.com>,
        "michal.simek@...inx.com" <michal.simek@...inx.com>,
        Soren Brinkmann <sorenb@...inx.com>,
        "moritz.fischer@...us.com" <moritz.fischer@...us.com>,
        "luis@...ethencourt.com" <luis@...ethencourt.com>,
        "Jose.Abreu@...opsys.com" <Jose.Abreu@...opsys.com>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/3] dmaengine: xilinx_dma: Check for channel idle state
 before submitting dma descriptor

Hi Laurent Pinchart,

	Sorry for the delay in the reply.
	Thanks for the review...
> 
> Hi Kedar,
> 
> On Monday 19 Dec 2016 15:39:43 Appana Durga Kedareswara Rao wrote:
> > Hi Laurent Pinchart,
> >
> > 	Thanks for the review...
> >
> > > > +	if (!chan->idle)
> > > > +		return;
> > >
> > > Don't you need to perform the same check for the DMA and CDMA channels
> ?
> > > If so, shouldn't this be moved to common code ?
> >
> > Will fix it in v2...
> >
> > > There's another problem (not strictly introduced by this patch) I
> > > wanted to mention. The append_desc_queue() function, called from
> > > your tx_submit handler, appends descriptors to the pending_list. The
> > > DMA engine API states that a transfer submitted by tx_submit will
> > > not be executed until
> > > .issue_pending() is called. However, if a transfer is in progress at
> > > tx_submit time, I believe that the IRQ handler, at transfer
> > > completion, will start the next transfer from the pending_list even
> > > if
> > > .issue_pending() hasn't been called for it.
> > >
> > > >  	if (list_empty(&chan->pending_list))
> > > >  		return;
> >
> > If user submits more than h/w limit then that case only driver will
> > process The descriptors from the pending_list for other cases the
> > pending_list will be Empty so driver just returns from there.
> 
> I understand that, but that's not the problem. Your .tx_submit() handler calls
> append_desc_queue() which adds the tx descriptor to the pending_list. If a
> transfer is in progress at that time, I believe the transfer completion IRQ handler
> will take the next descriptor from the pending_list and process it, even though
> issue_pending() hasn't been called for it.
> 

Thanks for the explanation...
Agree will keep this my to-do list...

Regards,
Kedar.