linux-kernel - Re: [PATCH 13/78] staging: media: hantro_drv: use pm_runtime_resume_and

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Date:   Mon, 26 Apr 2021 14:33:27 +0200
From:   Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
To:     Ezequiel Garcia <ezequiel@...labora.com>
Cc:     linuxarm@...wei.com, mauro.chehab@...wei.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH 13/78] staging: media: hantro_drv: use
 pm_runtime_resume_and_get()

Em Sat, 24 Apr 2021 20:23:53 -0300
Ezequiel Garcia <ezequiel@...labora.com> escreveu:

> Hi Mauro,
> 
> On Sat, 2021-04-24 at 08:44 +0200, Mauro Carvalho Chehab wrote:
> > Commit dd8088d5a896 ("PM: runtime: Add pm_runtime_resume_and_get to deal with usage counter")
> > added pm_runtime_resume_and_get() in order to automatically handle
> > dev->power.usage_count decrement on errors.
> > 
> > Use the new API, in order to cleanup the error check logic.
> > 
> > Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
> > ---
> >  drivers/staging/media/hantro/hantro_drv.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/staging/media/hantro/hantro_drv.c b/drivers/staging/media/hantro/hantro_drv.c
> > index 595e82a82728..3147dcbebeb9 100644
> > --- a/drivers/staging/media/hantro/hantro_drv.c
> > +++ b/drivers/staging/media/hantro/hantro_drv.c
> > @@ -155,7 +155,7 @@ static void device_run(void *priv)
> >         ret = clk_bulk_enable(ctx->dev->variant->num_clocks, ctx->dev->clocks);
> >         if (ret)
> >                 goto err_cancel_job;
> > -       ret = pm_runtime_get_sync(ctx->dev->dev);
> > +       ret = pm_runtime_resume_and_get(ctx->dev->dev);
> >         if (ret < 0)
> >                 goto err_cancel_job;
> >    
> 
> Seems this one needs a different fix: err_cancel_job
> will call hantro_job_finish which has a pm_runtime put.

Good point. Thanks for reviewing it!

It sounds that this is a place where the best seems
to keep using pm_runtime_get_sync(), but let's at least add a
comment explaining why it should be kept here. This should
help to avoid people to copy-and-paste the code on situations
where pm_runtime_resume_and_get() should be used instead.

See enclosed patch.

Thanks,
Mauro

[PATCH] media: hantro: document the usage of pm_runtime_get_sync()

Despite other *_get()/*_put() functions, where usage count is
incremented only if not errors, the pm_runtime_get_sync() has
a different behavior, incrementing the counter *even* on
errors.

That's an error prone behavior, as people often forget to
decrement the usage counter.

However, the hantro driver depends on this behavior, as it
will decrement the usage_count unconditionally at the m2m
job finish time, which makes sense.

So, intead of using the pm_runtime_resume_and_get() that
would decrement the counter on error, keep the current
API, but add a documentation explaining the rationale for
keep using pm_runtime_get_sync().

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>

diff --git a/drivers/staging/media/hantro/hantro_drv.c b/drivers/staging/media/hantro/hantro_drv.c
index 595e82a82728..96f940c1c85c 100644
--- a/drivers/staging/media/hantro/hantro_drv.c
+++ b/drivers/staging/media/hantro/hantro_drv.c
@@ -155,6 +155,13 @@ static void device_run(void *priv)
 	ret = clk_bulk_enable(ctx->dev->variant->num_clocks, ctx->dev->clocks);
 	if (ret)
 		goto err_cancel_job;
+
+	/*
+	 * The pm_runtime_get_sync() will increment dev->power.usage_count,
+	 * even on errors. That's the expected behavior here, since the
+	 * hantro_job_finish() function at the error handling code
+	 * will internally call pm_runtime_put_autosuspend().
+	 */
 	ret = pm_runtime_get_sync(ctx->dev->dev);
 	if (ret < 0)
 		goto err_cancel_job;