Re: [RFC 1/2] PM / Domains: Power on domain early during system resume

[Kevin Hilman <khilman@kernel.org>]

Andrzej Hajda <a.hajda@samsung.com> writes:

> On 10/30/2014 08:36 AM, Krzysztof Kozlowski wrote:
>> On śro, 2014-10-29 at 10:46 -0700, Kevin Hilman wrote:
>>> Krzysztof Kozlowski <k.kozlowski@samsung.com> writes:
>>>
>>>> When resuming the system the power domain has to be powered on early so
>>>> any runtime PM aware devices could resume.
>>>>
>>>> This fixes following scenario reproduced on Exynos DRM:
>>>> 1. Power domain is off before suspending the system.
>>>> 2. System is suspended to RAM.
>>>> 3. Resuming starts. The Exynos DRM driver resume callback is called.
>>>> 4. The Exynos DRM driver calls drm_helper_resume_force_mode which turns
>>>>    the screen on by calling exynos_dsi_dpms with DRM_MODE_DPMS_ON.
>>> Dumb Q: if the device (and power domain) were off before (and during)
>>> suspend, why are they being resumed?
>>>
>>> Shouldn't the resume path restore things to the same state they were
>>> before suspend?
>> One could expect that... but the Exynos DRM driver behaves differently
>> (and some other drivers also). In resume method it calls
>> drm_helper_resume_force_mode() which forces restoring mode setting
>> configuration. Apparently setting a mode needs DPMS on:
>> static void exynos_drm_crtc_commit(struct drm_crtc *crtc)
>> {
>> 	...
>> 	exynos_drm_crtc_dpms(crtc, DRM_MODE_DPMS_ON);
>> 	...
>>
>> The previous DPMS status (status during suspend) is completely ignored
>> here.
>
> Suspend callback switches off all connectors (thus all other devs in
> their pipeline) by calling dpms_off,
> in restore callback all devs are restored to their previous state by
> calling appropriate dpms.
> So I guess drm_helper_resume_force_mode() call at the end of resume is
> incorrect.

Though I'm not terribly familiar with DRM, it seems incorrect because I
expect resume to restore the state of things when suspend happened, not
forcibly resume everything.

> On the other side it is present in many other drivers, so I am also
> little bit confused.

Many other DRM drivers?  or other drivers too?

Kevin