linux-kernel - Re: [PATCH] drm/bridge: ti-sn65dsi86: Fix output polarity setting bug

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Date:   Mon, 28 Nov 2022 17:13:50 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Qiqi Zhang <eddy.zhang@...k-chips.com>
Cc:     andrzej.hajda@...el.com, neil.armstrong@...aro.org,
        robert.foss@...aro.org, Laurent.pinchart@...asonboard.com,
        jonas@...boo.se, jernej.skrabec@...il.com, airlied@...il.com,
        daniel@...ll.ch, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Subject: Re: [PATCH] drm/bridge: ti-sn65dsi86: Fix output polarity setting bug

Hi,

On Fri, Nov 25, 2022 at 2:54 AM Qiqi Zhang <eddy.zhang@...k-chips.com> wrote:
>
> According to the description in ti-sn65dsi86's datasheet:
>
> CHA_HSYNC_POLARITY:
> 0 = Active High Pulse. Synchronization signal is high for the sync
> pulse width. (default)
> 1 = Active Low Pulse. Synchronization signal is low for the sync
> pulse width.
>
> CHA_VSYNC_POLARITY:
> 0 = Active High Pulse. Synchronization signal is high for the sync
> pulse width. (Default)
> 1 = Active Low Pulse. Synchronization signal is low for the sync
> pulse width.
>
> We should only set these bits when the polarity is negative.
> Signed-off-by: Qiqi Zhang <eddy.zhang@...k-chips.com>
> ---
>  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> index 3c3561942eb6..eb24322df721 100644
> --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> @@ -931,9 +931,9 @@ static void ti_sn_bridge_set_video_timings(struct ti_sn65dsi86 *pdata)
>                 &pdata->bridge.encoder->crtc->state->adjusted_mode;
>         u8 hsync_polarity = 0, vsync_polarity = 0;
>
> -       if (mode->flags & DRM_MODE_FLAG_PHSYNC)
> +       if (mode->flags & DRM_MODE_FLAG_NHSYNC)
>                 hsync_polarity = CHA_HSYNC_POLARITY;
> -       if (mode->flags & DRM_MODE_FLAG_PVSYNC)
> +       if (mode->flags & DRM_MODE_FLAG_NVSYNC)
>                 vsync_polarity = CHA_VSYNC_POLARITY;

Looks right to me.

Reviewed-by: Douglas Anderson <dianders@...omium.org>

I've never seen the polarity matter for any eDP panels I've worked
with, which presumably explains why this was wrong for so long. As far
as I can tell, it's been wrong since the start. Probably you should
have:

Fixes: a095f15c00e2 ("drm/bridge: add support for sn65dsi86 bridge driver")

I put this on a sc7180-trogdor-lazor device and it didn't make
anything worse. Since the sync polarity never mattered to begin with,
I guess this isn't a surprise. ...so I guess that's a weak tested-by:

Tested-by: Douglas Anderson <dianders@...omium.org>

I'm happy to land this patch, but sounds like we're hoping to get
extra testing so I'll hold off for now.

-Doug