linux-kernel - Re: [PATCH 4/4] drm/sun4i: dsi: Remove incorrect use of runtime PM

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Date:   Fri, 14 Feb 2020 20:37:52 -0600
From:   Samuel Holland <samuel@...lland.org>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Chen-Yu Tsai <wens@...e.org>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 4/4] drm/sun4i: dsi: Remove incorrect use of runtime PM

Maxime,

First, sorry for the tone in my previous message. I wrote it too hastily.

> On 2/11/20 2:26 AM, Maxime Ripard wrote:
>> On Tue, Feb 11, 2020 at 01:28:58AM -0600, Samuel Holland wrote:
>>> 3) The driver relies on being suspended when sun6i_dsi_encoder_enable()
>>>    is called. The resume callback has a comment that says:
>>>
>>>       Some part of it can only be done once we get a number of
>>>       lanes, see sun6i_dsi_inst_init
>>>
>>>    And then part of the resume callback only runs if dsi->device is not
>>>    NULL (that is, if sun6i_dsi_attach() has been called). However, as
>>>    the above call graph shows, the resume callback is guaranteed to be
>>>    called before sun6i_dsi_attach(); it is called before child devices
>>>    get their drivers attached.
>>
>> Isn't it something that has been changed by your previous patch though?
> 
> No. Before the previous patch, sun6i_dsi_bind() requires sun6i_dsi_attach() to
> have been called first. So either the panel driver is not loaded, and issue #2
> happens, or the panel driver is loaded, and you get the following modification
> to the above call graph:
> 
>    mipi_dsi_host_register()
>       ...
>          __device_attach()
>             pm_runtime_get_sync(dev->parent) -> Causes resume
>             bus_for_each_drv()
>                __device_attach_driver()
>                   [panel probe function]
>                      mipi_dsi_attach()
>                         sun6i_dsi_attach()
>             pm_runtime_put(dev->parent) -> Async idle request
>    component_add()
>       ...
>          sun6i_dsi_bind()
>       ...
>          sun6i_dsi_encoder_enable()
>             pm_runtime_get_sync() -> Cancels idle request
> 
> And because `dev->power.runtime_status == RPM_ACTIVE` still, the callback is
> *not* run. Either way you have the same problem.

While the scenario I described is possible (since an unbounded amount of other
work could be queued to pm_wq), I did more testing without these patches, and I
could never trigger it. No matter what combination of module/built-in drivers I
used, there was always enough time between mipi_dsi_host_register() and
sun6i_dsi_encoder_enable() for the device to be suspended. So in practice
sun6i_dsi_inst_init() was always called during boot.

>>>    Therefore, part of the controller initialization will only run if the
>>>    device is suspended between the calls to mipi_dsi_host_register() and
>>>    component_add() (which ends up calling sun6i_dsi_encoder_enable()).
>>>    Again, as shown by the above call graph, this is not the case. It
>>>    appears that the controller happens to work because it is still
>>>    initialized by the bootloader.
>>
>> We don't have any bootloader support for MIPI-DSI, so no, that's not it.

You are correct here. sun6i_dsi_inst_init() was indeed being called at boot. So
my commit log is wrong.

>>>    Because the connector is hardcoded to always be connected, the
>>>    device's runtime PM reference is not dropped until system suspend,
>>>    when sun4i_drv_drm_sys_suspend() ends up calling
>>>    sun6i_dsi_encoder_disable(). However, that is done as a system sleep
>>>    PM hook, and at that point the system PM core has already taken
>>>    another runtime PM reference, so sun6i_dsi_runtime_suspend() is
>>>    not called. Likewise, by the time the PM core releases its reference,
>>>    sun4i_drv_drm_sys_resume() has already re-enabled the encoder.
>>>
>>>    So after system suspend and resume, we have *still never called*
>>>    sun6i_dsi_inst_init(), and now that the rest of the display pipeline
>>>    has been reset, the DSI host is unable to communicate with the panel,
>>>    causing VBLANK timeouts.
>>
>> Either way, I guess just moving the pm_runtime_enable call to
>> sun6i_dsi_attach will fix this, right? We don't really need to have
>> the DSI controller powered up before that time anyway.
> 
> No. It would solve issue #2 (only if the previous patch is
> applied), but not issue #3.
> 
> Regardless of when runtime PM is enabled, sun6i_dsi_runtime_suspend() will not
> be called until the device's usage count drops to 0. And as long as a panel is
> bound, the controller's usage count will be >0, *even during system suspend*
> while the encoder is turned off.
> 
> Before the previous patch, the usage count would never drop to 0 under *any*
> circumstance.

FWIW,
Samuel