linux-kernel - Re: [PATCH] drm/dp: Fix bogus DPCD version check in drm_dp_read_downstream

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Date:   Mon, 10 May 2021 21:58:17 +0300
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Lyude Paul <lyude@...hat.com>
Cc:     dri-devel@...ts.freedesktop.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/dp: Fix bogus DPCD version check in
 drm_dp_read_downstream_info()

On Fri, May 07, 2021 at 05:42:09PM -0400, Lyude Paul wrote:
> Ville pointed this out to me when fixing some issues in
> drm_dp_read_downstream_info() - the DPCD version check here is bogus as
> there's no DisplayPort versions prior to 1.0. The original code from i915
> that this was extracted from actually did:
> 
>   dpcd[DP_DPCD_REV] == DP_DPCD_REV_10
> 
> Which is correct, and somehow got missed when extracting this function. So
> let's fix this. Note that as far as I'm aware, I don't think this fixes any
> actual issues users are hitting.
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> Cc: Ville Syrjälä <ville.syrjala@...ux.intel.com>

Reviewed-by: Ville Syrjälä <ville.syrjala@...ux.intel.com>

> ---
>  drivers/gpu/drm/drm_dp_helper.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 0f84df8798ab..55b53df6ce34 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -677,7 +677,7 @@ int drm_dp_read_downstream_info(struct drm_dp_aux *aux,
>  	memset(downstream_ports, 0, DP_MAX_DOWNSTREAM_PORTS);
>  
>  	/* No downstream info to read */
> -	if (!drm_dp_is_branch(dpcd) || dpcd[DP_DPCD_REV] < DP_DPCD_REV_10)
> +	if (!drm_dp_is_branch(dpcd) || dpcd[DP_DPCD_REV] == DP_DPCD_REV_10)
>  		return 0;
>  
>  	/* Some branches advertise having 0 downstream ports, despite also advertising they have a
> -- 
> 2.30.2

-- 
Ville Syrjälä
Intel