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

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Date:   Mon, 4 Oct 2021 11:37:25 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Dafna Hirschfeld <dafna.hirschfeld@...labora.com>,
        linux-media@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Cc:     Alexandre Courbot <acourbot@...omium.org>, kernel@...labora.com,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        dafna3@...il.com, tiffany.lin@...iatek.com,
        andrew-ct.chen@...iatek.com, minghsiu.tsai@...iatek.com,
        houlong.wei@...iatek.com, mchehab@...nel.org,
        matthias.bgg@...il.com
Subject: Re: [PATCH v4] media: mtk-vpu: Ensure alignment of 8 for DTCM buffer

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?

> 
> 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)];

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?

Regards,

	Hans

> +
> +		memset(data + len, 0, sizeof(data) - len);
> +		memcpy(data, buf, len);
> +		memcpy_toio(send_obj->share_buf, data, round_up(len, 8));
> +	} else {
> +		memcpy_toio(send_obj->share_buf, buf, len);
> +	}
> +
>  	writel(len, &send_obj->len);
>  	writel(id, &send_obj->id);
>  
>