linux-kernel - Re: [PATCH v1 2/2] drm/tegra: Support disabled CONFIG

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Message-ID: <5b0876be-54c7-0033-4a2f-f0dac8fab7c4@gmail.com>
Date:   Fri, 15 Dec 2017 23:54:31 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Lucas Stach <dev@...xeye.de>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] drm/tegra: Support disabled CONFIG_PM

On 15.12.2017 23:25, Lucas Stach wrote:
> Am Freitag, den 15.12.2017, 01:45 +0300 schrieb Dmitry Osipenko:
>> On 15.12.2017 00:41, Lucas Stach wrote:
>>> Am Montag, den 11.12.2017, 18:26 +0300 schrieb Dmitry Osipenko:
>>>> On 11.12.2017 17:27, Thierry Reding wrote:
>>>>> On Mon, Dec 11, 2017 at 04:53:56PM +0300, Dmitry Osipenko
>>>>> wrote:
>>>>>> On 11.12.2017 13:13, Thierry Reding wrote:
>>>>>>> On Mon, Dec 11, 2017 at 02:19:44AM +0300, Dmitry Osipenko
>>>>>>> wrote:
>>>>>>>> Add manual HW power management to drivers probe/remove in
>>>>>>>> order to
>>>>>>>> not fail in a case of runtime power management being
>>>>>>>> disabled
>>>>>>>> in kernel
>>>>>>>> config.
>>>>>>>>
>>>>>>>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>>>>>>>> ---
>>>>>>>>  drivers/gpu/drm/tegra/dc.c   | 164
>>>>>>>> +++++++++++++++++++++++++++----------------
>>>>>>>>  drivers/gpu/drm/tegra/dsi.c  | 138
>>>>>>>> +++++++++++++++++++++--
>>>>>>>> -------------
>>>>>>>>  drivers/gpu/drm/tegra/hdmi.c |  90 ++++++++++++++++-----
>>>>>>>> ---
>>>>>>>>  drivers/gpu/drm/tegra/sor.c  | 103 +++++++++++++++++--
>>>>>>>> ----
>>>>>>>> ----
>>>>>>>>  4 files changed, 310 insertions(+), 185 deletions(-)
>>>>>>>
>>>>>>> I think that's the wrong way around. We unconditionally
>>>>>>> select
>>>>>>> PM on
>>>>>>> 64-bit ARM already, and I think we should do the same on
>>>>>>> 32-bit 
>>>>>>> ARM.
>>>>>>> There's really no excuse not to enable runtime PM these
>>>>>>> days.
>>>>>>
>>>>>> What is the rational behind enabling PM unconditionally? It
>>>>>> is
>>>>>> actually a very
>>>>>> useful debug feature when there is something wrong with the
>>>>>> PM.
>>>>>> It looks like
>>>>>> Tegra DRM driver is the only driver on Tegra that doesn't
>>>>>> work
>>>>>> properly with PM
>>>>>> being disabled. Please, let's just fix it.
>>>>>
>>>>> What's useful about disabling PM? The problem with allowing !PM
>>>>> is
>>>>> that
>>>>> it adds one more combination that needs to be build- and
>>>>> runtime
>>>>> tested.
>>>>
>>>> As I already stated, disabling PM is very useful for debugging
>>>> when
>>>> system hangs
>>>> unexpectedly. I found it very helpful several times.
>>>
>>> This assumes that the bootloader/firmware left the power domains
>>> powered up. Without PM_GENERIC_DOMAINS, which depends on CONFIG_PM
>>> the
>>> kernel has no means to control the state of the power domains.
>>> Probe
>>> deferral based on the power domain will also not work, so driver
>>> may
>>> probe and try to access power-gated devices, leading to system
>>> hangs in
>>> the common case.
>>
>> Pre-186 Tegra's do not use generic PM domains, but a custom API.
>> Meanwhile T186
>> always has CONFIG_PM enabled.
> 
> This is incorrect since a38045121bf4 (soc/tegra: pmc: Add generic PM
> domain support), i.e. kernel 4.7.

Power domains are defined only by T210/T186 in DT and both SoC's are ARM64, all
ARM32 SoC's use custom API. Moreover T210 doesn't define powerdomain for DC in
DT, so for DC it uses legacy API.

BTW, I'll drop this patch since Thierry isn't positive about having !CONFIG_PM.