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

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Message-ID: <1d509eea-37ef-bfd1-cfe7-0a204d8c4bd4@collabora.com>
Date:   Mon, 29 Nov 2021 16:39:07 +0200
From:   Dafna Hirschfeld <dafna.hirschfeld@...labora.com>
To:     Alexandre Courbot <acourbot@...omium.org>,
        Hans Verkuil <hverkuil@...all.nl>
Cc:     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 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);
         }
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.
>