linux-kernel - Re: [PATCH] media: rcar_drif: Use dma_request_chan() instead dma_request_slave

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Message-ID: <CAMuHMdXU9_WY54iU5DY6UHZHmyaR06pW7X0nnyCWHA=R3GtcJA@mail.gmail.com>
Date:   Mon, 20 Jan 2020 13:36:10 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Peter Ujfalusi <peter.ujfalusi@...com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Ramesh Shanmugasundaram <rashanmu@...il.com>,
        Vinod <vkoul@...nel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH] media: rcar_drif: Use dma_request_chan() instead dma_request_slave_channel()

Hi Peter,

On Mon, Jan 20, 2020 at 1:09 PM Peter Ujfalusi <peter.ujfalusi@...com> wrote:
> On 20/01/2020 14.05, Geert Uytterhoeven wrote:
> > On Tue, Dec 17, 2019 at 11:41 AM Peter Ujfalusi <peter.ujfalusi@...com> wrote:
> >> dma_request_slave_channel() is a wrapper on top of dma_request_chan()
> >> eating up the error code.
> >>
> >> By using dma_request_chan() directly the driver can support deferred
> >> probing against DMA.
> >>
> >> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
> >
> > Reviewed-by: Geert Uytterhoeven <geert+renesas@...der.be>
> >
> > One comment below.
> >
> >> --- a/drivers/media/platform/rcar_drif.c
> >> +++ b/drivers/media/platform/rcar_drif.c
> >> @@ -275,10 +275,10 @@ static int rcar_drif_alloc_dmachannels(struct rcar_drif_sdr *sdr)
> >>         for_each_rcar_drif_channel(i, &sdr->cur_ch_mask) {
> >>                 struct rcar_drif *ch = sdr->ch[i];
> >>
> >> -               ch->dmach = dma_request_slave_channel(&ch->pdev->dev, "rx");
> >> -               if (!ch->dmach) {
> >> +               ch->dmach = dma_request_chan(&ch->pdev->dev, "rx");
> >> +               if (IS_ERR(ch->dmach)) {
> >>                         rdrif_err(sdr, "ch%u: dma channel req failed\n", i);
> >
> > Now there is an error code, you might (1) want to print it, and (2) only
> > do so when it is not due to probe deferral:
> >
> >         if (PTR_ERR(ch->dmach) != -EPROBE_DEFER)
> >                 rdrif_err(sdr, "ch%u: dma channel req failed %pe\n",
> > i, ch->dmach);
>
> Yes, this is true.
>
> >
> >> -                       ret = -ENODEV;
> >> +                       ret = PTR_ERR(ch->dmach);
>
> if (ret != -EPROBE_DEFER)
>         rdrif_err(sdr, "ch%u: dma channel req failed (%d)\n", i, ret);
>
> might be simpler.

Sure, checking ret is simpler.
But printing ch->dmach means you can use the new %pe format specifier
to pretty-print the error code.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds