Re: [PATCH 08/11] dmaengine: xilinx_dpdma: stop using slave_id field

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

Hi Arnd,

On Mon, Nov 15, 2021 at 11:21:30AM +0100, Arnd Bergmann wrote:
> On Mon, Nov 15, 2021 at 10:14 AM Laurent Pinchart wrote:
> > On Mon, Nov 15, 2021 at 09:54:00AM +0100, Arnd Bergmann wrote:
> > > @@ -1285,11 +1287,13 @@ static int xilinx_dpdma_config(struct dma_chan *dchan,
> > >       spin_lock_irqsave(&chan->lock, flags);
> > >
> > >       /*
> > > -      * Abuse the slave_id to indicate that the channel is part of a video
> > > -      * group.
> > > +      * Abuse the peripheral_config to indicate that the channel is part
> >
> > Is it still an abuse, or is this now the right way to pass custom data
> > to the DMA engine driver ?
> 
> It doesn't make the driver any more portable, but it's now being
> more explicit about it. As far as I can tell, this is the best way
> to pass data that cannot be expressed through the regular interfaces
> in DT and the dmaengine API.
> 
> Ideally there would be a generic way to pass this flag, but I couldn't
> figure out what this is actually doing, or whether there is a better
> way. Maybe Vinod has an idea.

I don't think we need a generic API in this case. The DMA engine is
specific to the display device, I don't foresee a need to mix-n-match.

> I'll change s/Abuse/Use/ for the moment until I get a definite answer.
> 
> > > +      * of a video group.
> > >        */
> > > -     if (chan->id <= ZYNQMP_DPDMA_VIDEO2)
> > > -             chan->video_group = config->slave_id != 0;
> > > +     pconfig = config->peripheral_config;
> >
> > This could be moved to the variable declaration above, up to you.
> 
> I considered that but since it doesn't fit in a normal 80-column
> line, it seemed best to do it here.
> 
> > > +     if (chan->id <= ZYNQMP_DPDMA_VIDEO2 &&
> > > +         config->peripheral_size == sizeof(*pconfig))
> >
> > Silently ignoring a size mismatch isn't nice. Could we validate the size
> > at the beginning of the function and return an error ?
> 
> Yes, good idea. Since this would mean a bug in another driver,
> I'll add a WARN_ON() as well to make it clear which driver caused it.
> This is what I have now, let me know if you have any further suggestions:
> 
>         /*
>          * Use the peripheral_config to indicate that the channel is part
>          * of a video group. This requires matching use of the custom
>          * structure in each driver.
>          */
>         pconfig = config->peripheral_config;
>         if (WARN_ON(config->peripheral_size != 0 &&
>                     config->peripheral_size != sizeof(*pconfig)))
>                 return -EINVAL;

How about

 	if (WARN_ON(config->peripheral_config &&
		    config->peripheral_size != sizeof(*pconfig)))

> 
>         spin_lock_irqsave(&chan->lock, flags);
>         if (chan->id <= ZYNQMP_DPDMA_VIDEO2 &&
>             config->peripheral_size == sizeof(*pconfig))

And here you can test pconfig != NULL.

>                 chan->video_group = pconfig->video_group;
>         spin_unlock_irqrestore(&chan->lock, flags);
> 
>         return 0;
> 
> > With these issues addressed,
> >
> > Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

-- 
Regards,

Laurent Pinchart