linux-kernel - Re: [PATCH v4] media: mtk-vpu: Ensure alignment of 8 for DTCM buffer

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Date:   Mon, 18 Oct 2021 10:16:54 +0900
From:   Alexandre Courbot <acourbot@...omium.org>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Dafna Hirschfeld <dafna.hirschfeld@...labora.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel@...labora.com,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Dafna Hirschfeld <dafna3@...il.com>,
        Tiffany Lin <tiffany.lin@...iatek.com>,
        Andrew-CT Chen <andrew-ct.chen@...iatek.com>,
        minghsiu.tsai@...iatek.com, houlong.wei@...iatek.com,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>
Subject: Re: [PATCH v4] media: mtk-vpu: Ensure alignment of 8 for DTCM buffer

Hi Hans!

On Mon, Oct 4, 2021 at 6:37 PM Hans Verkuil <hverkuil@...all.nl> wrote:
>
> On 20/09/2021 19:04, Dafna Hirschfeld wrote:
> > From: Alexandre Courbot <acourbot@...omium.org>
> >
> > When running memcpy_toio:
> > memcpy_toio(send_obj->share_buf, buf, len);
> > it was found that errors appear if len is not a multiple of 8:
> >
> > [58.350841] mtk-mdp 14001000.rdma: processing failed: -22
>
> Why do errors appear? Is that due to a HW bug? Some other reason?

MTK folks would be the best placed to answer this, but since the
failure is reported by the firmware I'd suspect either a firmware or
hardware limitation.

>
> >
> > This patch ensures the copy of a multiple of 8 size by calling
> > round_up(len, 8) when copying
> >
> > Fixes: e6599adfad30 ("media: mtk-vpu: avoid unaligned access to DTCM buffer.")
> > Signed-off-by: Alexandre Courbot <acourbot@...omium.org>
> > Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
> > Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@...labora.com>
> > Reviewed-by: Houlong Wei <houlong.wei@...iatek.com>
> > ---
> > changes since v3:
> > 1. multile -> multiple
> > 2. add inline doc
> >
> > changes since v2:
> > 1. do the extra copy only if len is not multiple of 8
> >
> > changes since v1:
> > 1. change sign-off-by tags
> > 2. change values to memset
> >
> >  drivers/media/platform/mtk-vpu/mtk_vpu.c | 15 ++++++++++++++-
> >  1 file changed, 14 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/media/platform/mtk-vpu/mtk_vpu.c b/drivers/media/platform/mtk-vpu/mtk_vpu.c
> > index ec290dde59cf..1df031716c8f 100644
> > --- a/drivers/media/platform/mtk-vpu/mtk_vpu.c
> > +++ b/drivers/media/platform/mtk-vpu/mtk_vpu.c
> > @@ -349,7 +349,20 @@ int vpu_ipi_send(struct platform_device *pdev,
> >               }
> >       } while (vpu_cfg_readl(vpu, HOST_TO_VPU));
> >
> > -     memcpy_toio(send_obj->share_buf, buf, len);
> > +     /*
> > +      * when copying data to the vpu hardware, the memcpy_toio operation must copy
> > +      * a multiple of 8. Otherwise the processing fails
>
> Same here: it needs to explain why the processing fails.
>
> > +      */
> > +     if (len % 8 != 0) {
> > +             unsigned char data[SHARE_BUF_SIZE];
>
> Wouldn't it be more robust if you say:
>
>                 unsigned char data[sizeof(send_obj->share_buf)];

Definitely yes.

>
> I also think that the SHARE_BUF_SIZE define needs a comment stating that it must be a
> multiple of 8, otherwise unexpected things can happen.
>
> You also noticed that the current SHARE_BUF_SIZE define is too low, but I saw
> no patch correcting this. Shouldn't that be fixed as well?

AFAICT the firmware expects this exact size on its end, so I don't
believe it can be changed that easily. But maybe someone from MTK can
prove me wrong.

Cheers,
Alex.