Re: [PATCH v8 11/34] gpu: host1x: Add runtime PM and OPP support

[Thierry Reding <thierry.reding@gmail.com>]

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On Tue, Aug 17, 2021 at 02:04:38PM +0200, Ulf Hansson wrote:
> On Tue, 17 Aug 2021 at 03:30, Dmitry Osipenko <digetx@gmail.com> wrote:
> >
> > Add runtime PM and OPP support to the Host1x driver. It's required for
> > enabling system-wide DVFS and supporting dynamic power management using
> > a generic power domain. For the starter we will keep host1x always-on
> > because dynamic power management require a major refactoring of the driver
> > code since lot's of code paths will need the RPM handling and we're going
> > to remove some of these paths in the future. Host1x doesn't consume much
> > power so it is good enough, we at least need to resume Host1x in order
> > to initialize the power state.
> >
> > Tested-by: Peter Geis <pgwipeout@gmail.com> # Ouya T30
> > Tested-by: Paul Fertser <fercerpav@gmail.com> # PAZ00 T20
> > Tested-by: Nicolas Chauvet <kwizart@gmail.com> # PAZ00 T20 and TK1 T124
> > Tested-by: Matt Merhar <mattmerhar@protonmail.com> # Ouya T30
> > Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > ---
> 
> [...]
> 
> > +
> >  static int host1x_probe(struct platform_device *pdev)
> >  {
> >         struct host1x *host;
> > @@ -394,6 +423,10 @@ static int host1x_probe(struct platform_device *pdev)
> >         /* set common host1x device data */
> >         platform_set_drvdata(pdev, host);
> >
> > +       err = devm_tegra_core_dev_init_opp_table_simple(&pdev->dev);
> > +       if (err)
> > +               return err;
> > +
> >         host->regs = devm_ioremap_resource(&pdev->dev, regs);
> >         if (IS_ERR(host->regs))
> >                 return PTR_ERR(host->regs);
> > @@ -423,12 +456,9 @@ static int host1x_probe(struct platform_device *pdev)
> >                 return err;
> >         }
> >
> > -       host->rst = devm_reset_control_get(&pdev->dev, "host1x");
> > -       if (IS_ERR(host->rst)) {
> > -               err = PTR_ERR(host->rst);
> > -               dev_err(&pdev->dev, "failed to get reset: %d\n", err);
> > +       err = host1x_get_resets(host);
> > +       if (err)
> >                 return err;
> > -       }
> >
> >         err = host1x_iommu_init(host);
> >         if (err < 0) {
> > @@ -443,22 +473,10 @@ static int host1x_probe(struct platform_device *pdev)
> >                 goto iommu_exit;
> >         }
> >
> > -       err = clk_prepare_enable(host->clk);
> > -       if (err < 0) {
> > -               dev_err(&pdev->dev, "failed to enable clock\n");
> > -               goto free_channels;
> > -       }
> > -
> > -       err = reset_control_deassert(host->rst);
> > -       if (err < 0) {
> > -               dev_err(&pdev->dev, "failed to deassert reset: %d\n", err);
> > -               goto unprepare_disable;
> > -       }
> > -
> 
> Removing the clk_prepare_enable() and reset_control_deassert() from
> host1x_probe(), might not be a good idea. See more about why, below.
> 
> >         err = host1x_syncpt_init(host);
> >         if (err) {
> >                 dev_err(&pdev->dev, "failed to initialize syncpts\n");
> > -               goto reset_assert;
> > +               goto free_channels;
> >         }
> >
> >         err = host1x_intr_init(host, syncpt_irq);
> > @@ -467,10 +485,14 @@ static int host1x_probe(struct platform_device *pdev)
> >                 goto deinit_syncpt;
> >         }
> >
> > -       host1x_debug_init(host);
> > +       pm_runtime_enable(&pdev->dev);
> >
> > -       if (host->info->has_hypervisor)
> > -               host1x_setup_sid_table(host);
> > +       /* the driver's code isn't ready yet for the dynamic RPM */
> > +       err = pm_runtime_resume_and_get(&pdev->dev);
> 
> If the driver is being built with the CONFIG_PM Kconfig option being
> unset, pm_runtime_resume_and_get() will return 0 to indicate success -
> and without calling the ->runtime_resume() callback.
> In other words, the clock will remain gated and the reset will not be
> deasserted, likely causing the driver to be malfunctioning.
> 
> If the driver isn't ever being built with CONFIG_PM unset, feel free
> to ignore my above comments.
> 
> Otherwise, if it needs to work both with and without CONFIG_PM being
> set, you may use the following pattern in host1x_probe() to deploy
> runtime PM support:
> 
> "Enable the needed resources to probe the device"
> pm_runtime_get_noresume()
> pm_runtime_set_active()
> pm_runtime_enable()
> 
> "Before successfully completing probe"
> pm_runtime_put()

We made a conscious decision a few years ago to have ARCH_TEGRA select
PM on both 32-bit and 64-bit ARM, specifically to avoid the need to do
this dance (though there are still a few drivers left that do this, I
think).

So I think this should be unnecessary. Unless perhaps if the sysfs PM
controls have any influence on this. As far as I know, as long as the
PM kconfig option is enabled, the sysfs control only influence the
runtime behaviour (i.e. setting the sysfs PM control to "on" is going
to force runtime PM to be resumed) but there's no way to disable
runtime PM altogether via sysfs that would make the above necessary.

Thierry

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