Re: [PATCH] drm/panel: Add prepare_prev_first flag to Visionox VTDR6130

From: Jessica Zhang
Date: Fri Aug 25 2023 - 14:38:54 EST



On 8/21/2023 3:01 AM, neil.armstrong@xxxxxxxxxx wrote:
Hi Maxime,

On 21/08/2023 10:17, Maxime Ripard wrote:
Hi,

On Fri, Aug 18, 2023 at 10:25:48AM +0200, neil.armstrong@xxxxxxxxxx wrote:
On 17/08/2023 20:35, Dmitry Baryshkov wrote:
On 16/08/2023 10:51, neil.armstrong@xxxxxxxxxx wrote:
Sending HS commands will always work on any controller, it's all
about LP commands. The Samsung panels you listed only send HS
commands so they can use prepare_prev_first and work on any
controllers.

I think there is some misunderstanding there, supported by the
description of the flag.

If I remember correctly, some hosts (sunxi) can not send DCS
commands after enabling video stream and switching to HS mode, see
[1]. Thus, as you know, most of the drivers have all DSI panel setup
commands in drm_panel_funcs::prepare() /
drm_bridge_funcs::pre_enable() callbacks, not paying attention
whether these commands are to be sent in LP or in HS mode.

Previously DSI source drivers could power on the DSI link either in
mode_set() or in pre_enable() callbacks, with mode_set() being the
hack to make panel/bridge drivers to be able to send commands from
their prepare() / pre_enable() callbacks.

With the prev_first flags being introduced, we have established that
DSI link should be enabled in DSI host's pre_enable() callback and
switched to HS mode (be it command or video) in the enable()
callback.

So far so good.

It seems coherent, I would like first to have a state of all DSI host
drivers and make this would actually work first before adding the
prev_first flag to all the required panels.

This is definitely what we should do in an ideal world, but at least for
sunxi there's no easy way for it at the moment. There's no documentation
for it and the driver provided doesn't allow this to happen.

Note that I'm not trying to discourage you or something here, I'm simply
pointing out that this will be something that we will have to take into
account. And it's possible that other drivers are in a similar
situation.

Unfortunately this change is not fully backwards-compatible. This
requires that all DSI panels sending commands from prepare() should
have the prepare_prev_first flag. In some sense, all such patches
might have Fixes: 5ea6b1702781 ("drm/panel: Add prepare_prev_first
flag to drm_panel").

This kind of migration should be done *before* any possible
regression, not the other way round.

If all panels sending commands from prepare() should have the
prepare_prev_first flag, then it should be first, check for
regressions then continue.

<snip>


I understand, but this patch doesn't qualify as a fix for
9e15123eca79 and is too late to be merged in drm-misc-next for
v6.6, and since 9e15123eca79 actually breaks some support it
should be reverted (+ deps) since we are late in the rc cycles.

If we go this way, we can never reapply these patches. There will be
no guarantee that all panel drivers are completely converted. We
already have a story without an observable end -
DRM_BRIDGE_ATTACH_NO_CONNECTOR.

I don't understand this point, who would block re-applying the patches ?

The migration to DRM_BRIDGE_ATTACH_NO_CONNECTOR was done over multiple
Linux version and went smoothly because we reverted regressing patches
and restarted when needed, I don't understand why we can't do this
here aswell.

I'd consider that the DSI driver is correct here and it is about the
panel drivers that require fixes patches. If you care about the
particular Fixes tag, I have provided one several lines above.

Unfortunately it should be done in the other way round, prepare for
migration, then migrate,

I mean if it's a required migration, then it should be done and I'll
support it from both bridge and panel PoV.

So, first this patch has the wrong Fixes tag, and I would like a
better explanation on the commit message in any case. Then I would
like to have an ack from some drm-misc maintainers before applying it
because it fixes a patch that was sent via the msm tree thus per the
drm-misc rules I cannot apply it via the drm-misc-next-fixes tree.

Sorry, it's not clear to me what you'd like our feedback on exactly?

So let me resume the situation:

- pre_enable_prev_first was introduced in [1]
- some panels made use of pre_enable_prev_first
- Visionox VTDR6130 was enabled on SM8550 systems and works on v6.5 kernels and before
- patch [2] was introduced on MSM DRM tree, breaking VTDR6130 on SM8550 systems (and probably other Video mode panels on Qcom platforms)
- this fix was sent late, and is now too late to be merged via drm-misc-next

Hi Neil and Maxime,

I agree with Neil that 9e15123eca79 was the commit that introduced the issue (since it changed the MSM DSI host behavior).

However, I'm not too keen on simply reverting that patch because

1) it's not wrong to have the dsi_power_on in pre_enable. Arguably, it actually makes more sense to power on DSI host in pre_enable than in modeset (since modeset is meant for setting the bridge mode), and

2) I think it would be good practice to keep specific bridge chip checks out of the DSI host driver.


That being said, what do you think about setting the default value of prepare_prev_first to true (possibly in panel_bridge_attach)?

It seems to me that most panel drivers send DCS commands during pre_enable, so maybe it would make more sense to power on DSI host before panel enable() by default. Any panel that needs DSI host to be powered on later could then explicitly set the flag to false in their respective drivers.

Thanks,

Jessica Zhang



I do not consider it's the right way to fix regression caused by [2]
I consider [2] should be reverted, panels migrated to pre_enable_prev_first when needed, tested and the [2] applied again

I have no objection about [2] and it should be done widely over the whole DSI controllers
and DSI Video panels.

I also object about the Fixes tag of this patch, which is wrong, and Dmitry considers [1]
should be used but it's even more wrong since [2] really caused the regression.

And if [2] was to correct one to use, it was pushed via the MSM tree so it couldn't be
applied via drm-misc-next-fixes, right ?

[1] 4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first to alter bridge init order")
[2] 9e15123eca79 ("drm/msm/dsi: Stop unconditionally powering up DSI hosts at modeset")

Thanks,
Neil


Maxime