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

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Date:   Mon, 19 Dec 2016 19:18:54 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Appana Durga Kedareswara Rao <appana.durga.rao@...inx.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 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.

> > Now that you catch busy channels with a software flag, do you still need
> > the xilinx_dma_is_running() and xilinx_dma_is_idle() checks ? Three
> > different checks for the same or very similar conditions are confusing,
> > if you really need them you should document clearly how they differ.
> 
> Will remove the xilinx_dma_is_running and xilinx_dmais_idle() checks and
> will use Chan->idle check across all the IP's. Will fix it v2...
> 
> > > @@ -1110,6 +1116,7 @@ static void xilinx_vdma_start_transfer(struct
> > > xilinx_dma_chan *chan) vdma_desc_write(chan, XILINX_DMA_REG_VSIZE,
> > > last->hw.vsize);
> > > 
> > >  	}
> > > 
> > > +	chan->idle = false;
> > 
> > (and this too)
> 
> Will fix in v2...

-- 
Regards,

Laurent Pinchart