linux-kernel - Re: [PATCH v2] PM: runtime: Return properly from rpm_resume() if dev->power.needs_force

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Date:   Tue, 27 Sep 2022 15:46:21 +0800
From:   Liu Ying <victor.liu@....com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-imx@....com,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2] PM: runtime: Return properly from rpm_resume() if
 dev->power.needs_force_resume flag is set

On Mon, 2022-09-26 at 11:47 +0200, Ulf Hansson wrote:
> On Fri, 23 Sept 2022 at 17:23, Liu Ying <victor.liu@....com> wrote:
> > On Fri, 2022-09-23 at 15:48 +0200, Ulf Hansson wrote:
> > > On Fri, 23 Sept 2022 at 14:47, Liu Ying <victor.liu@....com> wrote:
> > > > After a device transitions to sleep state through it's system
> > > > suspend
> > > > callback pm_runtime_force_suspend(), the device's driver may still
> > > > try
> > > > to do runtime PM for the device(runtime suspend first and then
> > > > runtime
> > > > resume) although runtime PM is disabled by that callback.  The
> > > > runtime
> > > > PM operations would not touch the device effectively and the device
> > > > is
> > > > assumed to be resumed through it's system resume callback
> > > > pm_runtime_force_resume().
> > > 
> > > This sounds like a fragile use case to me. In principle you want to
> > > allow the device to be runtime resumed/suspended, after the device
> > > has
> > > already been put into a low power state through the regular system
> > > suspend callback. Normally it seems better to prevent this from
> > > happening, completely.
> > 
> > Not sure if we really may prevent this from happening completely.
> > 
> > > That said, in this case, I wonder if a better option would be to
> > > point
> > > ->suspend_late() to pm_runtime_force_suspend() and ->resume_early()
> > > to
> > > pm_runtime_force_resume(), rather than using the regular
> > > ->suspend|resume() callbacks. This should avoid the problem, I think,
> > > no?
> > 
> > I thought about this and it actually works for my particular
> > panel-simple case.  What worries me is that the device(DRM device in my
> > case) which triggers the runtime PM operations may also use
> > ->suspend_late/resume_early() callbacks for whatever reasons, hence no
> > fixed order to suspend/resume the two devices(like panel device and DRM
> > device).
> > 
> > Also, not sure if there is any sequence issue by using the
> > ->suspend_late/resume_early() callbacks in the panel-simple driver,
> > since it's written for quite a few display panels which may work with
> > various DRM devices - don't want to break any of them.
> 
> What you are describing here, is the classical problem we have with
> suspend/resume ordering of devices.
> 
> There are in principle two ways to solve this.
> 1. If it makes sense, the devices might be assigned as parent/child.
> 2. If it's more a consumer/supplier thing, we can add a device-link
> between them.

I thought about the two ways for my particular panel-simple case and
the first impression is that it's not straightforward to use them. For
DSI panels(with DRM_MODE_CONNECTOR_DSI connector type), it looks like
panel device's parent is DSI host device(set in mipi_dsi_device_alloc()
). For other types of panels, like DPI panels, many show up in device
tree as child-node of root node and connect a display controller or a
display bridge through OF graph.  Seems that DRM architecture level
lacks some sort of glue code to use the two ways.

Regards,
Liu Ying 

> 
> In this way, the PM core can guarantee that the order becomes correct.
> 
> Kind regards
> Uffe