linux-kernel - Re: [PATCH] drm/panel: Add prepare_prev

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Message-ID: <6c0dd9fd-5d8e-537c-804f-7a03d5899a07@linaro.org>
Date:   Mon, 14 Aug 2023 10:01:27 +0200
From:   neil.armstrong@...aro.org
To:     Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Jessica Zhang <quic_jesszhan@...cinc.com>,
        Sam Ravnborg <sam@...nborg.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Douglas Anderson <dianders@...omium.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Rob Clark <robdclark@...il.com>
Cc:     quic_parellan@...cinc.com, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/panel: Add prepare_prev_first flag to Visionox
 VTDR6130

Hi Abhinav,

On 10/08/2023 18:26, Abhinav Kumar wrote:
> Hi Neil
> 
> On 8/3/2023 10:19 AM, Jessica Zhang wrote:
>>
>>
>> On 7/31/2023 6:00 AM, Neil Armstrong wrote:
>>> Hi,
>>>
>>> On 26/07/2023 00:56, Jessica Zhang wrote:
>>>> Due to a recent introduction of the pre_enable_prev_first bridge flag [1],
>>>> the panel driver will be probed before the DSI is enabled, causing the
>>>> DCS commands to fail to send.
>>>>
>>>> Ensure that DSI is enabled before panel probe by setting the
>>>> prepare_prev_first flag for the panel.
>>>
>>> Well this is specific to MSM DSI driver, it's not related at all to the panel.
>>
> 
> I dont fully agree this is a MSM DSI driver specific thing.
> 
> If the panel can send its commands in its enable() callback, then this flag need not be set.
> 
> When a panel sends its DCS commands in its pre_enable() callback, any DSI controller will need to be ON before that otherwise DCS commands cannot be sent.
> 
> With this in mind, may I know why is this a MSM change and not a panel change?
> 
> As per my discussion with Dmitry during the last sync up, we were aligned on this expectation.

As of today, only the MSM DSI driver expects panels to have prepare_prev_first because it's the first
one calling pre_enable() before the DSI controller to be on, all other DSI drivers I know
still enables the DSI controller in mode_set() and thus can send commands in pre_enable() which
is a loose way to map the pre-video state for DSI panels...

A panel driver should not depend on features of a DSI controller, which is the case here
with this patch. Today's expectation is to send DSI commands in pre_enable() then when enabled
expect to be in video mode when enable() is called.

The main reason is because some DSI controllers cannot send LP commands after switching
to video mode (allwinner for example), so we must take this into account.

For v6.6, I don't see other solutions than reverting 9e15123eca79 (reverting won't regress anything,
because now it regresses also other panels on MSM platforms) and try to find a proper solution for v6.7...

Neil

> 
> Thanks
> 
> Abhinav
> 
>> Hi Neil,
>>
>> I think there might be some confusion caused by the commit message -- instead of "enabled before panel probe", it should be "enabled before panel pre_enable()" as the panel on commands are sent during prepare(), which is matched to bridge pre_enable().
>>
>> IIRC the general rule is that the panel driver should set the prepare_prev_first flag if the on commands are sent during pre_enable(), so I'll keep the code change but correct the commit message if that's ok with you.
>>
>> Thanks,
>>
> 
>> Jessica Zhang
>>
>>>
>>> Neil
>>>
>>>>
>>>> [1] commit 4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first to alter bridge init order")

It's not the right commit that cause regression here, it's :

9e15123eca79 drm/msm/dsi: Stop unconditionally powering up DSI hosts at modeset

>>>>
>>>> Fixes: 2349183d32d8 ("drm/panel: add visionox vtdr6130 DSI panel driver")
>>>> Signed-off-by: Jessica Zhang <quic_jesszhan@...cinc.com>
>>>> ---
>>>>   drivers/gpu/drm/panel/panel-visionox-vtdr6130.c | 1 +
>>>>   1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>>> index bb0dfd86ea67..e1363e128e7e 100644
>>>> --- a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>>> +++ b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>>> @@ -296,6 +296,7 @@ static int visionox_vtdr6130_probe(struct mipi_dsi_device *dsi)
>>>>       dsi->format = MIPI_DSI_FMT_RGB888;
>>>>       dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_NO_EOT_PACKET |
>>>>                 MIPI_DSI_CLOCK_NON_CONTINUOUS;
>>>> +    ctx->panel.prepare_prev_first = true;
>>>>       drm_panel_init(&ctx->panel, dev, &visionox_vtdr6130_panel_funcs,
>>>>                  DRM_MODE_CONNECTOR_DSI);
>>>>
>>>> ---
>>>> base-commit: 28a5c036b05fc5c935cc72d76abd3589825ea9cd
>>>> change-id: 20230717-visionox-vtdr-prev-first-e00ae02eec9f
>>>>
>>>> Best regards,
>>>