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@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > Signed-off-by: Tejas Upadhyay <tejasu@xxxxxxxxxx>
> > > Signed-off-by: Michal Simek <michal.simek@xxxxxxxxxx>
> > > Signed-off-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> >
> > ...
> >
> > > +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
