linux-kernel - Re: [PATCH] drm/panel: sitronix-st7789v: fix sync flags for t28cp45tn89

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Message-ID: <5cac35a3-80f4-43ab-b8f8-7b9217740083@mailbox.org>
Date: Thu, 25 Sep 2025 15:53:05 +0200
From: Marek Vasut <marek.vasut@...lbox.org>
To: Sebastian Reichel <sre@...nel.org>
Cc: Neil Armstrong <neil.armstrong@...aro.org>,
 Jessica Zhang <jessica.zhang@....qualcomm.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Michael Riesch <michael.riesch@...labora.com>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 kernel@...labora.com,
 Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
Subject: Re: [PATCH] drm/panel: sitronix-st7789v: fix sync flags for
 t28cp45tn89

On 9/25/25 2:48 PM, Sebastian Reichel wrote:

Hello Sebastian,

> On Thu, Sep 25, 2025 at 02:15:41AM +0200, Marek Vasut wrote:
>> On 9/24/25 11:46 PM, Sebastian Reichel wrote:
>>> From: Sebastian Reichel <sebastian.reichel@...labora.com>
>>>
>>> I planned to set the polarity of horizontal and vertical sync, but
>>> accidentally described vertical sync twice with different polarity
>>> instead.
>>>
>>> Note, that there is no functional change, because the driver only
>>> makes use of DRM_MODE_FLAG_P[HV]SYNC to divert from the default
>>> active-low polarity.
>>>
>>> Reported-by: Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
>>> Closes: https://lore.kernel.org/all/20250923132616.GH20765@pendragon.ideasonboard.com/
>>> Fixes: a411558cc143 ("drm/panel: sitronix-st7789v: add Inanbo T28CP45TN89 support")
>>> Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>
>>> ---
>>>    drivers/gpu/drm/panel/panel-sitronix-st7789v.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/panel/panel-sitronix-st7789v.c b/drivers/gpu/drm/panel/panel-sitronix-st7789v.c
>>> index 04d91929eedda092b966b8cffdef5b267748f190..dedf0a390a88dd45a8179e2d22e872128c87cfda 100644
>>> --- a/drivers/gpu/drm/panel/panel-sitronix-st7789v.c
>>> +++ b/drivers/gpu/drm/panel/panel-sitronix-st7789v.c
>>> @@ -261,7 +261,7 @@ static const struct drm_display_mode t28cp45tn89_mode = {
>>>    	.vtotal = 320 + 8 + 4 + 4,
>>>    	.width_mm = 43,
>>>    	.height_mm = 57,
>>> -	.flags = DRM_MODE_FLAG_PVSYNC | DRM_MODE_FLAG_NVSYNC,
>>> +	.flags = DRM_MODE_FLAG_NHSYNC | DRM_MODE_FLAG_PVSYNC,
>> Is this panel somehow special with its NHSYNC / PVSYNC , compared to the
>> other supported panels, which all use PHSYNC / PVSYNC ? I would expect all
>> of these DSI TCON to use the same polarity, how come this one needs NHSYNC ?
> 
> I wrote this based on reverse engineering incl. quite a bit try-and-error and
> don't have very good data about the panel. Using DRM_MODE_FLAG_PHSYNC like all
> the other panels results in garbage data on the display.
Interesting.

Can you please add that information as a code comment in V2 and also add 
to V2 my

Reviewed-by: Marek Vasut <marek.vasut+renesas@...lbox.org>

Thank you