Re: [PATCH 00/05] PM: Runtime PM v13 for Platform Devices 20090807

Alan Stern
2009-08-07T16:17:56Z

On Sat, 8 Aug 2009, Magnus Damm wrote:

> On Fri, Aug 7, 2009 at 11:32 PM, Alan Stern<stern@rowland.harvard.edu> wrote:
> > On Fri, 7 Aug 2009, Magnus Damm wrote:
> >
> >> PM: Runtime PM v13 for Platform Devices 20090807
> >>
> >> [PATCH 01/05] PM: Runtime PM v13 - add dev_pm_ops helpers
> >
> > This patch doesn't do anything much, besides reverting a change I asked
> > Rafael to make.  I don't see how it helps platform-specific code do
> > anything.
>
> It's helping different bus types to implement the runtime part of the
> dev_pm_ops in a consistent way. I suggest that all bus types should
> return -ENOSYS if the callback is missing.

Doesn't Rafael's code already do this?

> And they can do so by using
> the helper functions. The change is not platform specific, but my
> latest SuperH platform Runtime PM prototype makes use of it.
>
> The latest SuperH specific Runtime PM implementation require
> dev_pm_ops even though there is no work to be done for the driver. The
> code works in a sort of opt-in way at this point, so callbacks are
> explicitly required. I'd like us to standardize on this behaviour if
> possible, so runtime pm enabled platform drivers can be shared between
> different platform bus implementations. So my LCDC platform driver
> will work fine on both SuperH SoCs and ARM SoCs.

I still don't see the connection. Why are helper functions useful?

> Sorry for reverting your change, but I couldn't see any clear benefit
> with the v11->v13 lock-drop-inside-the-if-case change. Isn't it just
> avoiding dropping the lock in the uncommon error case? Maybe I'm
> misunderstanding.

Basically you are right, except for one thing: The error case might not
be so uncommon. That's the benefit.

Rafael, along these lines, I suspect we might not want to go into an
error state if a runtime suspend fails because there is no callback
function. Returning -ENOSYS to the caller is fine, but leave
runtime_error set to 0. Maybe do the same for runtime resume.

> >> [PATCH 02/05] PM: Runtime PM v13 - let bus-less devices succeed
> >
> > This could be added without 01/05.  But why do you want it?  Busless
> > devices don't have PM runtime callbacks, so whether the core thinks the
> > callbacks succeed or not doesn't make any difference.
> >
> > You say that "Runtime suspend and resume of devices on the platform bus
> > is impossible without this change", but you don't explain why -- or why
> > the patch makes runtime suspend and resume of these devices possible.
>
> Right now, in the standard upstream kernel all platform devices get
> assigned a parent device unless one exists are registration time. The
> shared parent device is parent-less. Since the Runtime PM code resumes
> the parent before the child, the resume operation will fail because
> there is no dev_pm_ops for the shared parent.

Not if the parent is disabled for runtime PM, which it is in this case,
right?

> So I wonder which way that is the best to allow resuming platform
> devices. Patch [02/05] is one way, but maybe there are more elegant
> ways to handle it? Should the platform code be modified instead? If
> so, how? I suppose root hubs for USB may have a similar issue, no?

If necessary, I would suggest adding appropriate dummy runtime PM
routines for that catch-all parent device. But it may not be
necessary.

> >> [PATCH 03/05] PM: Runtime PM v13 - add debug printouts
> >
> > This looks good.
>
> Thanks. Maybe it's a good plan to add similar printouts to other
> functions as well?

Perhaps so. When writing similar code for USB I found that lots of
debugging printouts were needed all over the place, to get everything
working right. Once it was working, they were a nuisance.

Alan Stern