linux-kernel - Re: [PATCH] drm/bridge: anx7625: Fix edid_read break case in sp_tx_edid

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Date:   Thu, 18 Nov 2021 08:52:03 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Hsin-Yi Wang <hsinyi@...omium.org>
Cc:     Robert Foss <robert.foss@...aro.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Nicolas Boichat <drinkcat@...omium.org>,
        Pi-Hsun Shih <pihsun@...omium.org>,
        Jonas Karlman <jonas@...boo.se>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org,
        Neil Armstrong <narmstrong@...libre.com>,
        linux-kernel@...r.kernel.org,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Tzung-Bi Shih <tzungbi@...gle.com>,
        Xin Ji <xji@...logixsemi.com>
Subject: Re: [PATCH] drm/bridge: anx7625: Fix edid_read break case in
 sp_tx_edid_read()

On Fri, Nov 12, 2021 at 07:24:33PM +0800, Hsin-Yi Wang wrote:
> edid_read() was assumed to return 0 on success. After
> 7f16d0f3b8e2("drm/bridge: anx7625: Propagate errors from sp_tx_rst_aux()"),
> the function can return >= 0 for successful case. Fix the g_edid_break
> condition in sp_tx_edid_read().
> 
> Fixes: 7f16d0f3b8e2("drm/bridge: anx7625: Propagate errors from sp_tx_rst_aux()")
> Signed-off-by: Hsin-Yi Wang <hsinyi@...omium.org>
> Reviewed-by: Robert Foss <robert.foss@...aro.org>
> ---
>  drivers/gpu/drm/bridge/analogix/anx7625.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/bridge/analogix/anx7625.c b/drivers/gpu/drm/bridge/analogix/anx7625.c
> index 1a871f6b6822ee..392203b576042b 100644
> --- a/drivers/gpu/drm/bridge/analogix/anx7625.c
> +++ b/drivers/gpu/drm/bridge/analogix/anx7625.c
> @@ -826,7 +826,7 @@ static int sp_tx_edid_read(struct anx7625_data *ctx,
>  				g_edid_break = edid_read(ctx, offset,
>  							 pblock_buf);
>  
> -				if (g_edid_break)
> +				if (g_edid_break < 0)
>  					break;

g_edid_break is declared u8 and thus never < 0. The break; statement
doesn't indicate an error but that a break was encountered, and the
value of g_edid_break == 1 is used elsewhere in the function to indicate
that condition.  It also doesn't break out of the outer loop, but only
out of the switch statement.

With this patch in place, g_edid_break will have a more or less random
value after an error, and the behavior of this function will be undefined.

Guenter