linux-kernel - Re: [PATCH] PM: runtime: Simplify __rpm_get

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Date:   Tue, 24 Jan 2023 15:07:14 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH] PM: runtime: Simplify __rpm_get_callback()

On Tue, Jan 24, 2023 at 12:20 PM Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
>
> Hi Bjorn,
>
> On Thu, Dec 15, 2022 at 7:23 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > From: Bjorn Helgaas <bhelgaas@...gle.com>
> >
> > Simplify __rpm_get_callback() slightly by returning as soon as the return
> > value is known.  No functional change intended.
> >
> > Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
>
> Thanks for your patch, which is now commit 650bdddb6b311705 ("PM:
> runtime: Simplify __rpm_get_callback()") in pm/linux-next.
>
> > --- a/drivers/base/power/runtime.c
> > +++ b/drivers/base/power/runtime.c
> > @@ -20,8 +20,7 @@ typedef int (*pm_callback_t)(struct device *);
> >
> >  static pm_callback_t __rpm_get_callback(struct device *dev, size_t cb_offset)
> >  {
> > -       pm_callback_t cb;
> > -       const struct dev_pm_ops *ops;
> > +       const struct dev_pm_ops *ops = NULL;
> >
> >         if (dev->pm_domain)
> >                 ops = &dev->pm_domain->ops;
> > @@ -31,18 +30,14 @@ static pm_callback_t __rpm_get_callback(struct device *dev, size_t cb_offset)
> >                 ops = dev->class->pm;
> >         else if (dev->bus && dev->bus->pm)
> >                 ops = dev->bus->pm;
> > -       else
> > -               ops = NULL;
> >
> >         if (ops)
> > -               cb = *(pm_callback_t *)((void *)ops + cb_offset);
> > -       else
> > -               cb = NULL;
> > +               return *(pm_callback_t *)((void *)ops + cb_offset);
>
> This is a change in behavior in case the callback turns out to be NULL:
>   - before, it would fall back to the driver-specific callback below,
>   - after, it always returns NULL.

Good point and sorry for missing this!

> >
> > -       if (!cb && dev->driver && dev->driver->pm)
> > -               cb = *(pm_callback_t *)((void *)dev->driver->pm + cb_offset);
> > +       if (dev->driver && dev->driver->pm)
> > +               return *(pm_callback_t *)((void *)dev->driver->pm + cb_offset);
> >
> > -       return cb;
> > +       return NULL;
> >  }
>

Something like the patch below (modulo gmail-induced whitespace
breakage) should restore the previous behavior if I'm not mistaken:

---
 drivers/base/power/runtime.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

Index: linux-pm/drivers/base/power/runtime.c
===================================================================
--- linux-pm.orig/drivers/base/power/runtime.c
+++ linux-pm/drivers/base/power/runtime.c
@@ -31,8 +31,13 @@ static pm_callback_t __rpm_get_callback(
     else if (dev->bus && dev->bus->pm)
         ops = dev->bus->pm;

-    if (ops)
-        return *(pm_callback_t *)((void *)ops + cb_offset);
+    if (ops) {
+        pm_callback_t cb;
+
+        cb = *(pm_callback_t *)((void *)ops + cb_offset);
+        if (cb)
+            return cb;
+    }

     if (dev->driver && dev->driver->pm)
         return *(pm_callback_t *)((void *)dev->driver->pm + cb_offset);