Re: [PATCH v2 00/41] drm: Analog TV Improvements

[Noralf Trønnes]

Den 07.09.2022 18.44, skrev Noralf Trønnes:
> 
> 
> Den 07.09.2022 12.36, skrev Stefan Wahren:
>> Hi Maxime,
>>
>> Am 05.09.22 um 16:57 schrieb Maxime Ripard:
>>> On Fri, Sep 02, 2022 at 01:28:16PM +0200, Noralf Trønnes wrote:
>>>>
>>>> Den 01.09.2022 21.35, skrev Noralf Trønnes:
>>>>>
>>>>> I have finally found a workaround for my kernel hangs.
>>>>>
>>>>> Dom had a look at my kernel and found that the VideoCore was fine, and
>>>>> he said this:
>>>>>
>>>>>> That suggests cause of lockup was on arm side rather than VC side.
>>>>>>
>>>>>> But it's hard to diagnose further. Once you've had a peripheral not
>>>>>> respond, the AXI bus locks up and no further operations are possible.
>>>>>> Usual causes of this are required clocks being stopped or domains
>>>>>> disabled and then trying to access the hardware.
>>>>>>
>>>>> So when I got this on my 64-bit build:
>>>>>
>>>>> [  166.702171] SError Interrupt on CPU1, code 0x00000000bf000002 --
>>>>> SError
>>>>> [  166.702187] CPU: 1 PID: 8 Comm: kworker/u8:0 Tainted: G        W
>>>>>      5.19.0-rc6-00096-gba7973977976-dirty #1
>>>>> [  166.702200] Hardware name: Raspberry Pi 4 Model B Rev 1.1 (DT)
>>>>> [  166.702206] Workqueue: events_freezable_power_
>>>>> thermal_zone_device_check
>>>>> [  166.702231] pstate: 200000c5 (nzCv daIF -PAN -UAO -TCO -DIT -SSBS
>>>>> BTYPE=--)
>>>>> [  166.702242] pc : regmap_mmio_read32le+0x10/0x28
>>>>> [  166.702261] lr : regmap_mmio_read+0x44/0x70
>>>>> ...
>>>>> [  166.702606]  bcm2711_get_temp+0x58/0xb0 [bcm2711_thermal]
>>>>>
>>>>> I wondered if that reg read was stalled due to a clock being stopped.
>>>>>
>>>>> Lo and behold, disabling runtime pm and keeping the vec clock running
>>>>> all the time fixed it[1].
>>>>>
>>>>> I don't know what the problem is, but at least I can now test this
>>>>> patchset.
>>>>>
>>>>> [1] https://gist.github.com/notro/23b984e7fa05cfbda2db50a421cac065
>>>>>
>>>> It turns out I didn't have to disable runtime pm:
>>>> https://gist.github.com/notro/0adcfcb12460b54e54458afe11dc8ea2
>>> If the bcm2711_thermal IP needs that clock to be enabled, it should grab
>>> a reference itself, but it looks like even the device tree binding
>>> doesn't ask for one.
>> The missing clock in the device tree binding is expected, because
>> despite of the code there is not much information about the BCM2711
>> clock tree. But i'm skeptical that the AVS IP actually needs the VEC
>> clock, i think it's more likely that the VEC clock parent is changed and
>> that cause this issue. I could take care of the bcm2711 binding & driver
>> if i know which clock is really necessary.
> 
> Seems you're right, keeping the parent always enabled is enough:
> 
> 	clk_prepare_enable(clk_get_parent(vec->clock)); // pllc_per
> 
> I tried enabling just the grandparent clock as well, but that didn't help.
> 
> Without the clock hack it seems the hang occurs when switching between
> NTSC and PAL, at most I've been able to do that 4-5 times before it hangs.
> 
> For a while it looked like fbdev/fbcon had a play in this, but then I
> realised that it just gave me a NTSC mode to start from and to go back
> to when qutting modetest.
> 

I've looked some more into this problem and I see that downstream is
using a firmware clock for vec:

clk: Move vec clock to clk-raspberrypi
https://github.com/raspberrypi/linux/pull/4639

If I do the same my problem goes away.

It's interesting to note that on downstream 5.10.103-v7l+ #1530,
pllc_per is enabled even if tvout is not enabled:

$ sudo cat /sys/kernel/debug/clk/pllc_per/regdump
cm = 0x00000000
a2w = 0x00000004 (disable bit(8) is not set)

It's when mainline vc4_vec disables this vec parent clock that the crash
occurs.

Sidenote: Another downstream fw clock change with a vec reference[1]:


Another issue not related to the clock crash problem:

I assumed that unloading the vc4 module would release the clocks, but
this didn't happen.

When I looked at it I remembered that there's a catch in the DRM unplug
machinery when it comes to unloading a driver and the DRM disable hooks.

static void vc4_drm_unbind(struct device *dev)
{
	struct drm_device *drm = dev_get_drvdata(dev);

	drm_dev_unplug(drm);
	drm_atomic_helper_shutdown(drm);
}

Here the drm_device is first marked as unplugged and then the pipeline
is disabled. Since vc4_vec_encoder_disable() is protected by
drm_dev_enter() the VEC is not disabled, clocks are not released and PM
is left on.

In the drivers that I have written where the hardware is not expected to
have gone away on device unbind (SPI), I've just left out the
drm_dev_enter() check in the disable hook.

Noralf.

[1] https://github.com/raspberrypi/linux/pull/4706