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

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Date:   Wed, 30 Nov 2022 13:45:51 +0800
From:   Qiqi Zhang <eddy.zhang@...k-chips.com>
To:     tomi.valkeinen@...asonboard.com, dianders@...omium.org
Cc:     Laurent.pinchart@...asonboard.com, airlied@...il.com,
        andrzej.hajda@...el.com, daniel@...ll.ch,
        dri-devel@...ts.freedesktop.org, jernej.skrabec@...il.com,
        jonas@...boo.se, linux-kernel@...r.kernel.org,
        neil.armstrong@...aro.org, robert.foss@...aro.org
Subject: Re: [PATCH] drm/bridge: ti-sn65dsi86: Fix output polarity setting bug

Hi,

on Nov. 29, 2022, 11:45 a.m. Tomi wrote:
>On 29/11/2022 03:13, Doug Anderson wrote:
>> 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
>
>Afaik, DP doesn't have sync polarity as such (neither does DSI), and the
>sync polarity is just "metadata". So if you're in full-DP domain, I
>don't see why it would matter. I guess it becomes relevant when you
>convert from DP to some other bus format.

Just like Tomi said, the wrong polarity worked fine on my eDP panel(LP079QX1)
and standard DP monitor, I didn't notice the polarity configuration problem
here until my customer used the following solution and got a abnormal display:
GPU->mipi->eDP->DP->lvds->panel.

>> 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")

Doug you mean I need to update my commit message? It's my first time using
kernel list and I'm a little confused about this.

>>
>> 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.
>
>Looks fine to me and works for me with my DP monitor.
>
>Reviewed-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>

-Eddy