linux-kernel - Re: [PATCH v2] drm/bridge: anx7625: Fix not correct get property counts

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Date:   Fri, 11 Mar 2022 12:36:09 +0100
From:   Sam Ravnborg <sam@...nborg.org>
To:     Xin Ji <xji@...logixsemi.com>
Cc:     Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, qwen@...logixsemi.com,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        mliu@...logixsemi.com, hsinyi@...omium.org, bliang@...logixsemi.com
Subject: Re: [PATCH v2] drm/bridge: anx7625: Fix not correct get property
 counts

Hi Xin.

On Fri, Mar 11, 2022 at 06:35:25PM +0800, Xin Ji wrote:
> The property length which returns from "of_get_property", it means array
> bytes count if the property has prefix as "/bits/ 8". The driver should
> call function "of_property_read_u8_array" to get correct array value.
> 
> Fixes: fd0310b6fe7d ("drm/bridge: anx7625: add MIPI DPI input feature")
> Signed-off-by: Xin Ji <xji@...logixsemi.com>
> 
> ---
> V1 -> V2: Fix Sam comment, use of_property_read_u8_array to get array
> value
> ---
>  drivers/gpu/drm/bridge/analogix/anx7625.c | 8 ++++----
>  drivers/gpu/drm/bridge/analogix/anx7625.h | 4 ++--
>  2 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/analogix/anx7625.c b/drivers/gpu/drm/bridge/analogix/anx7625.c
> index c6a9a02ed762..628cbf769141 100644
> --- a/drivers/gpu/drm/bridge/analogix/anx7625.c
> +++ b/drivers/gpu/drm/bridge/analogix/anx7625.c
> @@ -1598,8 +1598,8 @@ static int anx7625_get_swing_setting(struct device *dev,
>  			num_regs = DP_TX_SWING_REG_CNT;
>  
>  		pdata->dp_lane0_swing_reg_cnt = num_regs;
> -		of_property_read_u32_array(dev->of_node, "analogix,lane0-swing",
> -					   pdata->lane0_reg_data, num_regs);
> +		of_property_read_u8_array(dev->of_node, "analogix,lane0-swing",
> +					  pdata->lane0_reg_data, num_regs);

The current implementation do a two step approach. First is find the
number of elements and then read the elements.
The number of elements is only used to limit what is read.

I suggest to use:

of_property_read_u8_array(dev->of_node, "analogix,lane0-swing",
			  pdata->lane0_reg_data, DP_TX_SWING_REG_CNT);

Then you a guaranteed to read at maximum DP_TX_SWING_REG_CNT entries.
And as the number of elements is not stored anywhere that should be fine.

This looks simpler and matches what we for example do in
drivers/gpu/drm/arm/malidp_drv.c - the only user in gpu/ of
of_property_read_u8_array().


	Sam