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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Mon, 15 Nov 2021 11:21:30 +0100
Message-ID: <CAK8P3a1Fu11-e0CK2of8u3ebdjom84UKuXhBKi5FUs5ZPPdOVA@mail.gmail.com>
Subject: Re: [PATCH 08/11] dmaengine: xilinx_dpdma: stop using slave_id field
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     Vinod Koul <vkoul@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Andy Gross <agross@kernel.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Baolin Wang <baolin.wang7@gmail.com>,
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        Chunyan Zhang <zhang.lyra@gmail.com>,
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On Mon, Nov 15, 2021 at 10:14 AM Laurent Pinchart
<laurent.pinchart@ideasonboard.com> 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'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;

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

Thanks,

       Arnd