Re: [PATCH v6 4/6] dmaengine: xilinx: dpdma: Add the Xilinx DisplayPort DMA engine driver

From: Vinod Koul <vkoul@kernel.org>
To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>,
	dmaengine@vger.kernel.org, Michal Simek <michal.simek@xilinx.com>,
	Hyun Kwon <hyun.kwon@xilinx.com>,
	Tejas Upadhyay <tejasu@xilinx.com>,
	Satish Kumar Nagireddy <SATISHNA@xilinx.com>
Subject: Re: [PATCH v6 4/6] dmaengine: xilinx: dpdma: Add the Xilinx DisplayPort DMA engine driver
Date: Wed, 15 Jul 2020 12:24:03 +0530	[thread overview]
Message-ID: <20200715065403.GC34333@vkoul-mobl> (raw)
In-Reply-To: <20200711221644.GD5954@pendragon.ideasonboard.com>

On 12-07-20, 01:16, Laurent Pinchart wrote:
> Hi Peter,
> 
> On Thu, Jul 09, 2020 at 04:21:38PM +0300, Peter Ujfalusi wrote:
> > On 08/07/2020 23.19, Laurent Pinchart wrote:
> > > From: Hyun Kwon <hyun.kwon@xilinx.com>
> > > 
> > > The ZynqMP DisplayPort subsystem includes a DMA engine called DPDMA with
> > > 6 DMa channels (4 for display and 2 for audio). This driver exposes the
> > > DPDMA through the dmaengine API, to be used by audio (ALSA) and display
> > > (DRM) drivers for the DisplayPort subsystem.
> > > 
> > > Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
> > > Signed-off-by: Tejas Upadhyay <tejasu@xilinx.com>
> > > Signed-off-by: Michal Simek <michal.simek@xilinx.com>
> > > Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > 
> > ...
> > 
> > > +static void xilinx_dpdma_chan_queue_transfer(struct xilinx_dpdma_chan *chan)
> > > +{
> > > +	struct xilinx_dpdma_device *xdev = chan->xdev;
> > > +	struct xilinx_dpdma_sw_desc *sw_desc;
> > > +	struct xilinx_dpdma_tx_desc *desc;
> > > +	struct virt_dma_desc *vdesc;
> > > +	u32 reg, channels;
> > > +
> > > +	lockdep_assert_held(&chan->lock);
> > > +
> > > +	if (chan->desc.pending)
> > > +		return;
> > > +
> > > +	if (!chan->running) {
> > > +		xilinx_dpdma_chan_unpause(chan);
> > > +		xilinx_dpdma_chan_enable(chan);
> > > +		chan->first_frame = true;
> > > +		chan->running = true;
> > > +	}
> > > +
> > > +	if (chan->video_group)
> > > +		channels = xilinx_dpdma_chan_video_group_ready(chan);
> > > +	else
> > > +		channels = BIT(chan->id);
> > > +
> > > +	if (!channels)
> > > +		return;
> > > +
> > > +	vdesc = vchan_next_desc(&chan->vchan);
> > > +	if (!vdesc)
> > > +		return;
> > > +
> > > +	if (!chan->first_frame && !(vdesc->tx.flags & DMA_PREP_LOAD_EOT)) {
> > > +		/*
> > > +		 * The client forgot to set the DMA_PREP_LOAD_EOT flag. The DMA
> > > +		 * engine API requires the channel to silently ignore the
> > > +		 * descriptor, leaving the client waiting forever for the new
> > > +		 * descriptor to be processed.
> > > +		 */
> > 
> > This hardly going to happen. But if it does, a gentle dev_dbg() might
> > save some time for the user on debugging?

Correct!

Also this is not quite right place for this driver to check this. In
prep_ call this driver should check if the channel is idle and if not
DMA_PREP_LOAD_EOT would be mandatory to be set and return error.

> I think you know my opinion on this already :-) I believe we should have
> designed this API in a way that makes this error impossible, by dropping
> the DMA_PREP_LOAD_EOT flag and considering that the default case. That
> was rejected. Do we now need to work around the problem in drivers ?

Driver can do that easily, API can allow this as some hardware can have
allow this.

Thanks
-- 
~Vinod