linux-kernel - Re: [PATCH] dmaengine: add verification of DMA

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Date:   Mon, 30 May 2022 10:06:39 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Dave Jiang <dave.jiang@...el.com>, Vinod <vkoul@...nel.org>
Cc:     Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        dmaengine <dmaengine@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dmaengine: add verification of DMA_INTERRUPT capability
 for dmatest

Hi Dave, Vinod,

On Wed, Apr 13, 2022 at 12:58 AM Dave Jiang <dave.jiang@...el.com> wrote:
> Looks like I forgot to add DMA_INTERRUPT cap setting to the idxd driver and
> dmatest is still working regardless of this mistake. Add an explicit check
> of DMA_INTERRUPT capability for dmatest to make sure the DMA device being used
> actually supports interrupt before the test is launched and also that the
> driver is programmed correctly.
>
> Signed-off-by: Dave Jiang <dave.jiang@...el.com>

Thanks for your patch, which is now commit a8facc7b988599f8
("dmaengine: add verification of DMA_INTERRUPT capability for
dmatest") upstream.

> --- a/drivers/dma/dmatest.c
> +++ b/drivers/dma/dmatest.c
> @@ -675,10 +675,16 @@ static int dmatest_func(void *data)
>         /*
>          * src and dst buffers are freed by ourselves below
>          */
> -       if (params->polled)
> +       if (params->polled) {
>                 flags = DMA_CTRL_ACK;
> -       else
> -               flags = DMA_CTRL_ACK | DMA_PREP_INTERRUPT;
> +       } else {
> +               if (dma_has_cap(DMA_INTERRUPT, dev->cap_mask)) {
> +                       flags = DMA_CTRL_ACK | DMA_PREP_INTERRUPT;
> +               } else {
> +                       pr_err("Channel does not support interrupt!\n");
> +                       goto err_pq_array;
> +               }
> +       }
>
>         ktime = ktime_get();
>         while (!(kthread_should_stop() ||
> @@ -906,6 +912,7 @@ static int dmatest_func(void *data)

Shimoda-san reports that this commit breaks dmatest on rcar-dmac.
Like most DMA engine drivers, rcar-dmac does not set the DMA_INTERRUPT
capability flag, hence dmatest now fails to start:

    dmatest: Channel does not support interrupt!

To me, it looks like the new check is bogus, as I believe it confuses
two different concepts:

  1. Documentation/driver-api/dmaengine/provider.rst says:

       - DMA_INTERRUPT

         - The device is able to trigger a dummy transfer that will
           generate periodic interrupts

  2. In non-polled mode, dmatest sets DMA_PREP_INTERRUPT.
     include/linux/dmaengine.h says:

       * @DMA_PREP_INTERRUPT - trigger an interrupt (callback) upon
completion of
       *  this transaction

As dmatest uses real transfers, I think it does not depend on
the ability to use interrupts from dummy transfers.

Do you agree?
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