Re: [PATCH v2 3/3] dmaengine: sf-pdma: Get number of channel by device tree

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Zong,

On Thu, Jan 13, 2022 at 8:26 AM Zong Li <zong.li@sifive.com> wrote:
> On Thu, Jan 13, 2022 at 2:53 PM Zong Li <zong.li@sifive.com> wrote:
> > On Wed, Jan 12, 2022 at 4:28 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > On Tue, Jan 11, 2022 at 9:51 AM Zong Li <zong.li@sifive.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.
> > > >
> > > > Signed-off-by: Zong Li <zong.li@sifive.com>
> > >
> > > Thanks for your patch!
> > >
> > > > --- a/drivers/dma/sf-pdma/sf-pdma.c
> > > > +++ b/drivers/dma/sf-pdma/sf-pdma.c
> > > > @@ -484,21 +484,24 @@ 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;
> > > > +       int len, ret;
> > > >         const enum dma_slave_buswidth widths =
> > > >                 DMA_SLAVE_BUSWIDTH_1_BYTE | DMA_SLAVE_BUSWIDTH_2_BYTES |
> > > >                 DMA_SLAVE_BUSWIDTH_4_BYTES | DMA_SLAVE_BUSWIDTH_8_BYTES |
> > > >                 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;
> > > > +       len = sizeof(*pdma) + sizeof(*chan) * PDMA_MAX_NR_CH;
> > >
> > > Why is the last part added (yes, this is a pre-existing issue)?
> > > struct sf_pdma already contains space for chans[PDMA_MAX_NR_CH].
> > > Either drop the last part, or change sf_pdma.chans[] to a flexible
> > > array member.
> > >
> > > BTW, you can use the struct_size() or flex_array_size() helper
> > > to calculate len.
> >
> > Thanks for your suggestions, let me fix it in the next version.
> >
> > >
> > > >         pdma = devm_kzalloc(&pdev->dev, len, 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;
> > > > +       }
> > > >
> > > >         res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> > > >         pdma->membase = devm_ioremap_resource(&pdev->dev, res);
> > > > @@ -556,7 +559,7 @@ static int sf_pdma_remove(struct platform_device *pdev)
> > > >         struct sf_pdma_chan *ch;
> > > >         int i;
> > > >
> > > > -       for (i = 0; i < PDMA_NR_CH; i++) {
> > > > +       for (i = 0; i < pdma->n_chans; i++) {
> > > >                 ch = &pdma->chans[i];
> > >
> > > If dma-channels in DT > PDMA_NR_CH, this becomes an out-of-bound
> > > access.
> > >
> >
> > Okay, let me get the min() between pdma->chans and PDMA_MAX_NR_CH,
> > please let me know if it isn't good to you.
>
> Please allow me give more details on it, I would compare the value of
> pdma->chans with PDMA_MAX_NR_CH in probe function, and set the
> pdma->chans to PDMA_MAX_NR_CH if the value in DT is bigger than
> PDMA_MAX_NR_CH.

Silently limiting "dma-channels" to PDMA_MAX_NR_CH is not a good idea,
as that may lead to hard-to-track problems.

Basically you have two options:
  1. Just use the value of "dma-channels" if present.
     This has the advantage that it will work automatically with
     future variants that have more channels, but allows the
     developer to trigger memory exhaustion by providing a very large value.
  2. Return -EINVAL if "dma-channels" is larger than PDMA_MAX_NR_CH.
     This is the safest, but requires driver changes for a future variant
      that has more channels.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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