linux-kernel - Re: [PATCH v4 3/3] dmaengine: sf-pdma: Get number of channel by device tree

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Date:   Tue, 25 Jan 2022 13:08:18 +0800
From:   Zong Li <zong.li@...ive.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Rob Herring <robh+dt@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Bin Meng <bin.meng@...driver.com>,
        Green Wan <green.wan@...ive.com>, Vinod <vkoul@...nel.org>,
        dmaengine <dmaengine@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v4 3/3] dmaengine: sf-pdma: Get number of channel by
 device tree

On Fri, Jan 21, 2022 at 6:29 PM Zong Li <zong.li@...ive.com> wrote:
>
> On Fri, Jan 21, 2022 at 4:33 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> >
> > Hi Zong, Palmer,
> >
> > On Fri, Jan 21, 2022 at 3:21 AM Zong Li <zong.li@...ive.com> wrote:
> > > On Fri, Jan 21, 2022 at 2:52 AM Palmer Dabbelt <palmer@...belt.com> wrote:
> > > > On Sun, 16 Jan 2022 17:35:28 PST (-0800), zong.li@...ive.com wrote:
> > > > > It currently assumes that there are always four channels, it would
> > > > > cause the error if there is actually less than four channels. Change
> > > > > that by getting number of channel from device tree.
> > > > >
> > > > > For backwards-compatible, it uses the default value (i.e. 4) when there
> > > > > is no 'dma-channels' information in dts.
> > > >
> > > > Some of the same wording issues here as those I pointed out in the DT
> > > > bindings patch.
> > > >
> > > > > Signed-off-by: Zong Li <zong.li@...ive.com>
> >
> > > > > --- a/drivers/dma/sf-pdma/sf-pdma.c
> > > > > +++ b/drivers/dma/sf-pdma/sf-pdma.c
> > > > > @@ -482,9 +482,7 @@ static void sf_pdma_setup_chans(struct sf_pdma *pdma)
> > > > >  static int sf_pdma_probe(struct platform_device *pdev)
> > > > >  {
> > > > >       struct sf_pdma *pdma;
> > > > > -     struct sf_pdma_chan *chan;
> > > > >       struct resource *res;
> > > > > -     int len, chans;
> > > > >       int ret;
> > > > >       const enum dma_slave_buswidth widths =
> > > > >               DMA_SLAVE_BUSWIDTH_1_BYTE | DMA_SLAVE_BUSWIDTH_2_BYTES |
> > > > > @@ -492,13 +490,21 @@ static int sf_pdma_probe(struct platform_device *pdev)
> > > > >               DMA_SLAVE_BUSWIDTH_16_BYTES | DMA_SLAVE_BUSWIDTH_32_BYTES |
> > > > >               DMA_SLAVE_BUSWIDTH_64_BYTES;
> > > > >
> > > > > -     chans = PDMA_NR_CH;
> > > > > -     len = sizeof(*pdma) + sizeof(*chan) * chans;
> > > > > -     pdma = devm_kzalloc(&pdev->dev, len, GFP_KERNEL);
> > > > > +     pdma = devm_kzalloc(&pdev->dev, sizeof(*pdma), GFP_KERNEL);
> > > > >       if (!pdma)
> > > > >               return -ENOMEM;
> > > > >
> > > > > -     pdma->n_chans = chans;
> > > > > +     ret = of_property_read_u32(pdev->dev.of_node, "dma-channels",
> > > > > +                                &pdma->n_chans);
> > > > > +     if (ret) {
> > > > > +             dev_notice(&pdev->dev, "set number of channels to default value: 4\n");
> > > > > +             pdma->n_chans = PDMA_MAX_NR_CH;
> > > > > +     }
> > > > > +
> > > > > +     if (pdma->n_chans > PDMA_MAX_NR_CH) {
> > > > > +             dev_err(&pdev->dev, "the number of channels exceeds the maximum\n");
> > > > > +             return -EINVAL;
> > > >
> > > > Can we get away with just using only the number of channels the driver
> > > > actually supports?  ie, just never sending an op to the channels above
> > > > MAX_NR_CH?  That should leave us with nothing to track.
> >
> > In theory we can...
> >
> > > It might be a bit like when pdma->n_chans is bigger than the maximum,
> > > set the pdma->chans to PDMA_MAX_NR_CH, then we could ensure that we
> > > don't access the channels above the maximum. If I understand
> > > correctly, I gave the similar thought in the thread of v2 patch, and
> > > there are some discussions on that, but this way seems to lead to
> > > hard-to-track problems.
> >
> > ... but that would mean that when a new variant appears that supports
> > more channels, no error is printed, and people might not notice
> > immediately that the higher channels are never used.
> >
>
> I guess people might need to follow the dt-bindings, so they couldn't
> specify the number of channels to the value which is more than
> maximum. But as you mentioned, if people don't notice that and specify
> it more than maximum,  they wouldn't be aware that the higher channels
> are never used. It seems to me that we could keep returning the error
> there, or show a warning message and use PDMA_MAX_NR_CH in that
> situation, both looks good to me.
>

Hi all, thank you for the review, I'd like to prepare the next version
patch, if current implementation of this part is ok to you, I will
keep it in the next version. Please let me know if anything can be
improved. Thanks

> > 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