linux-kernel - Re: [PATCH] drm/dp_mst: Fix drm_dp_send_dpcd

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Date:   Mon, 27 Apr 2020 09:58:06 -0400
From:   Sean Paul <sean@...rly.run>
To:     Lyude Paul <lyude@...hat.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Benjamin Gaignard <benjamin.gaignard@...com>,
        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>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/dp_mst: Fix drm_dp_send_dpcd_write() return code

On Fri, Apr 24, 2020 at 3:07 PM Lyude Paul <lyude@...hat.com> wrote:
>
> drm_dp_mst_wait_tx_reply() returns > 1 if time elapsed in
> wait_event_timeout() before check_txmsg_state(mgr, txmsg) evaluated to
> true. However, we make the mistake of returning this time from
> drm_dp_send_dpcd_write() on success instead of returning the number of
> bytes written - causing spontaneous failures during link probing:
>
> [drm:drm_dp_send_link_address [drm_kms_helper]] *ERROR* GUID check on
> 10:01 failed: 3975
>
> Yikes! So, fix this by returning the number of bytes written on success
> instead.
>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> Fixes: cb897542c6d2 ("drm/dp_mst: Fix W=1 warnings")
> Cc: Benjamin Gaignard <benjamin.gaignard@...com>
> Cc: Sean Paul <sean@...rly.run>

Reviewed-by: Sean Paul <sean@...rly.run>

> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 03a1496f6120..21dc78cb4ba6 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -3436,8 +3436,12 @@ static int drm_dp_send_dpcd_write(struct drm_dp_mst_topology_mgr *mgr,
>         drm_dp_queue_down_tx(mgr, txmsg);
>
>         ret = drm_dp_mst_wait_tx_reply(mstb, txmsg);
> -       if (ret > 0 && txmsg->reply.reply_type == DP_SIDEBAND_REPLY_NAK)
> -               ret = -EIO;
> +       if (ret > 0) {
> +               if (txmsg->reply.reply_type == DP_SIDEBAND_REPLY_NAK)
> +                       ret = -EIO;
> +               else
> +                       ret = size;
> +       }
>
>         kfree(txmsg);
>  fail_put:
> --
> 2.25.3
>