Re: [PATCH 00/51] treewide: Switch to __pm_runtime_put_autosuspend()

Richard Fitzgerald <rf@xxxxxxxxxxxxxxxxxxxxx> · Wed, 9 Oct 2024 13:48:15 +0100

On 08/10/2024 7:24 pm, Rafael J. Wysocki wrote:
On Tue, Oct 8, 2024 at 12:35 AM Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:

On Tue, 8 Oct 2024 at 00:25, Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:

Hi Ulf,

On Tue, Oct 08, 2024 at 12:08:24AM +0200, Ulf Hansson wrote:
On Mon, 7 Oct 2024 at 20:49, Laurent Pinchart wrote:
On Fri, Oct 04, 2024 at 04:38:36PM +0200, Ulf Hansson wrote:
On Fri, 4 Oct 2024 at 11:41, Sakari Ailus wrote:

Hello everyone,

This set will switch the users of pm_runtime_put_autosuspend() to
__pm_runtime_put_autosuspend() while the former will soon be re-purposed
to include a call to pm_runtime_mark_last_busy(). The two are almost
always used together, apart from bugs which are likely common. Going
forward, most new users should be using pm_runtime_put_autosuspend().

Once this conversion is done and pm_runtime_put_autosuspend() re-purposed,
I'll post another set to merge the calls to __pm_runtime_put_autosuspend()
and pm_runtime_mark_last_busy().

That sounds like it could cause a lot of churns.

Why not add a new helper function that does the
pm_runtime_put_autosuspend() and the pm_runtime_mark_last_busy()
things? Then we can start moving users over to this new interface,
rather than having this intermediate step?

I think the API would be nicer if we used the shortest and simplest
function names for the most common use cases. Following
pm_runtime_put_autosuspend() with pm_runtime_mark_last_busy() is that
most common use case. That's why I like Sakari's approach of repurposing
pm_runtime_put_autosuspend(), and introducing
__pm_runtime_put_autosuspend() for the odd cases where
pm_runtime_mark_last_busy() shouldn't be called.

Okay, so the reason for this approach is because we couldn't find a
short and descriptive name that could be used in favor of
pm_runtime_put_autosuspend(). Let me throw some ideas at it and maybe
you like it - or not. :-)

I like the idea at least :-)

I don't know what options you guys discussed, but to me the entire
"autosuspend"-suffix isn't really that necessary in my opinion. There
are more ways than calling pm_runtime_put_autosuspend() that triggers
us to use the RPM_AUTO flag for rpm_suspend(). For example, just
calling pm_runtime_put() has the similar effect.

To be honest, I'm lost there. pm_runtime_put() calls
__pm_runtime_idle(RPM_GET_PUT | RPM_ASYNC), while
pm_runtime_put_autosuspend() calls __pm_runtime_suspend(RPM_GET_PUT |
RPM_ASYNC | RPM_AUTO).

__pm_runtime_idle() ends up calling rpm_idle(), which may call
rpm_suspend() - if it succeeds to idle the device. In that case, it
tags on the RPM_AUTO flag in the call to rpm_suspend(). Quite similar
to what is happening when calling pm_runtime_put_autosuspend().

Right.

For almost everybody, except for a small bunch of drivers that
actually have a .runtime_idle() callback, pm_runtime_put() is
literally equivalent to pm_runtime_put_autosuspend().

So really the question is why anyone who doesn't provide a
.runtime_idle() callback bothers with using this special
pm_runtime_put_autosuspend() thing,

Because they are following the documentation? It says:

"Drivers should call pm_runtime_mark_last_busy() to update this field
after carrying out I/O, typically just before calling
pm_runtime_put_autosuspend()."

and

"In order to use autosuspend, subsystems or drivers must call
pm_runtime_use_autosuspend() (...), and thereafter they should use the
various `*_autosuspend()` helper functions instead of the non#
autosuspend counterparts"

So the documentation says I should be using pm_runtime_put_autosuspend()
instead of pm_runtime_put().

Seems unfair to criticise people for following the documentation.