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

[Krzysztof Kozlowski <k.kozlowski@samsung.com>]

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.

I dunno why... maybe someone from DRM could share some thoughts?


Best regards,
Krzysztof