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

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Date:   Mon, 6 Dec 2021 16:23:25 +0900
From:   Alexandre Courbot <acourbot@...omium.org>
To:     Dafna Hirschfeld <dafna.hirschfeld@...labora.com>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        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,
        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

On Mon, Nov 29, 2021 at 11:39 PM Dafna Hirschfeld
<dafna.hirschfeld@...labora.com> wrote:
>
>
>
> On 18.10.21 04:16, Alexandre Courbot wrote:
> > 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.
>
> won't it actually be better to implement it like this:
> (assuming len is always multiply of 4 - which I think it must be since access must be 4 aligned)
>
>         void __iomem *to = obj->share_buf;
>
>          if (len % 8 != 0) {
>                  memcpy_toio(to, buf, len - 4);
>                  to += len - 4;
>                  buf += len - 4;
>                  writel_relaxed(*(u32 *)buf, to);
>          } else {
>                  memcpy_toio(obj->share_buf, buf, len);
>          }

Not sure if avoiding that stack allocation is worth the extra
complexity and requirement for len being a multiple of 4. Also I'd
like to test it on real hardware to confirm it is indeed ok.