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

[Dafna Hirschfeld]

On 18.10.21 04:16, Alexandre Courbot wrote:
> Hi Hans!
>
> On Mon, Oct 4, 2021 at 6:37 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
> > On 20/09/2021 19:04, Dafna Hirschfeld wrote:
> > > From: Alexandre Courbot <acourbot@xxxxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxxxx>
> > > Signed-off-by: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>
> > > Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@xxxxxxxxxxxxx>
> > > Reviewed-by: Houlong Wei <houlong.wei@xxxxxxxxxxxx>
> > > ---
> > > 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);
        }
Thanks,
Dafna
> > 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.