Re: [PATCH] serial: 8250_mtk: Simplify clock sequencing and runtime PM

[Chen-Yu Tsai]

On Tue, Jun 6, 2023 at 5:36 PM AngeloGioacchino Del Regno
<angelogioacchino.delregno@xxxxxxxxxxxxx> wrote:
>
> Il 06/06/23 11:17, Chen-Yu Tsai ha scritto:
> > The 8250_mtk driver's runtime PM support has some issues:
> >
> > - The bus clock is enabled (through runtime PM callback) later than a
> >    register write
> > - runtime PM resume callback directly called in probe, but no
> >    pm_runtime_set_active() call is present
> > - UART PM function calls the callbacks directly, _and_ calls runtime
> >    PM API
> > - runtime PM callbacks try to do reference counting, adding yet another
> >    count between runtime PM and clocks
> >
> > This fragile setup worked in a way, but broke recently with runtime PM
> > support added to the serial core. The system would hang when the UART
> > console was probed and brought up.
> >
> > Tony provided some potential fixes [1][2], though they were still a bit
> > complicated. The 8250_dw driver, which the 8250_mtk driver might have
> > been based on, has a similar structure but simpler runtime PM usage.
> >
> > Simplify clock sequencing and runtime PM support in the 8250_mtk driver.
> > Specifically, the clock is acquired enabled and assumed to be active,
> > unless toggled through runtime PM suspend/resume. Reference counting is
> > removed and left to the runtime PM core. The serial pm function now
> > only calls the runtime PM API.
> >
> > [1] https://lore.kernel.org/linux-serial/20230602092701.GP14287@xxxxxxxxxxx/
> > [2] https://lore.kernel.org/linux-serial/20230605061511.GW14287@xxxxxxxxxxx/
> >
> > Fixes: 84a9582fd203 ("serial: core: Start managing serial controllers to enable runtime PM")
> > Suggested-by: Tony Lindgren <tony@xxxxxxxxxxx>
> > Signed-off-by: Chen-Yu Tsai <wenst@xxxxxxxxxxxx>
>
> You're both cleaning this up and solving a critical issue and I completely agree
> about doing that.
>
> I can imagine what actually fixes the driver, but still, is it possible to split
> this commit in two?
> One that solves the issue, one that performs the much needed cleanups.
>
> If it's not possible, then we can leave this commit as it is... and if the problem
> about splitting is the Fixes tag... well, we don't forcefully need it: after all,
> issues started arising after runtime PM support for 8250 landed and before that the
> driver technically worked, even though it was fragile.

The pure fix would look like what Tony posted [1]. However it would add stuff
that isn't strictly needed after the cleanup. Doing it in one patch results
in less churn. Think of it another way: it's a nice cleanup that just so
happens to fix a regression.

As for the fixes tag, it's there so other people potentially doing backports
of the 8250 runtime PM work can spot this followup fix.

ChenYu

[1] https://lore.kernel.org/linux-serial/20230605061511.GW14287@xxxxxxxxxxx/

> Thanks,
> Angelo
>
> > ---
> >   drivers/tty/serial/8250/8250_mtk.c | 50 ++++++------------------------
> >   1 file changed, 10 insertions(+), 40 deletions(-)
> >
> > diff --git a/drivers/tty/serial/8250/8250_mtk.c b/drivers/tty/serial/8250/8250_mtk.c
> > index aa8e98164d68..74da5676ce67 100644
> > --- a/drivers/tty/serial/8250/8250_mtk.c
> > +++ b/drivers/tty/serial/8250/8250_mtk.c
> > @@ -431,12 +431,7 @@ static int __maybe_unused mtk8250_runtime_suspend(struct device *dev)
> >       while
> >               (serial_in(up, MTK_UART_DEBUG0));
> >
> > -     if (data->clk_count == 0U) {
> > -             dev_dbg(dev, "%s clock count is 0\n", __func__);
> > -     } else {
> > -             clk_disable_unprepare(data->bus_clk);
> > -             data->clk_count--;
> > -     }
> > +     clk_disable_unprepare(data->bus_clk);
> >
> >       return 0;
> >   }
> > @@ -444,19 +439,8 @@ static int __maybe_unused mtk8250_runtime_suspend(struct device *dev)
> >   static int __maybe_unused mtk8250_runtime_resume(struct device *dev)
> >   {
> >       struct mtk8250_data *data = dev_get_drvdata(dev);
> > -     int err;
> >
> > -     if (data->clk_count > 0U) {
> > -             dev_dbg(dev, "%s clock count is %d\n", __func__,
> > -                     data->clk_count);
> > -     } else {
> > -             err = clk_prepare_enable(data->bus_clk);
> > -             if (err) {
> > -                     dev_warn(dev, "Can't enable bus clock\n");
> > -                     return err;
> > -             }
> > -             data->clk_count++;
> > -     }
> > +     clk_prepare_enable(data->bus_clk);
> >
> >       return 0;
> >   }
> > @@ -465,14 +449,12 @@ static void
> >   mtk8250_do_pm(struct uart_port *port, unsigned int state, unsigned int old)
> >   {
> >       if (!state)
> > -             if (!mtk8250_runtime_resume(port->dev))
> > -                     pm_runtime_get_sync(port->dev);
> > +             pm_runtime_get_sync(port->dev);
> >
> >       serial8250_do_pm(port, state, old);
> >
> >       if (state)
> > -             if (!pm_runtime_put_sync_suspend(port->dev))
> > -                     mtk8250_runtime_suspend(port->dev);
> > +             pm_runtime_put_sync_suspend(port->dev);
> >   }
> >
> >   #ifdef CONFIG_SERIAL_8250_DMA
> > @@ -504,7 +486,7 @@ static int mtk8250_probe_of(struct platform_device *pdev, struct uart_port *p,
> >               return 0;
> >       }
> >
> > -     data->bus_clk = devm_clk_get(&pdev->dev, "bus");
> > +     data->bus_clk = devm_clk_get_enabled(&pdev->dev, "bus");
> >       if (IS_ERR(data->bus_clk))
> >               return PTR_ERR(data->bus_clk);
> >
> > @@ -587,25 +569,16 @@ static int mtk8250_probe(struct platform_device *pdev)
> >
> >       platform_set_drvdata(pdev, data);
> >
> > -     pm_runtime_enable(&pdev->dev);
> > -     err = mtk8250_runtime_resume(&pdev->dev);
> > -     if (err)
> > -             goto err_pm_disable;
> > -
> >       data->line = serial8250_register_8250_port(&uart);
> > -     if (data->line < 0) {
> > -             err = data->line;
> > -             goto err_pm_disable;
> > -     }
> > +     if (data->line < 0)
> > +             return data->line;
> >
> >       data->rx_wakeup_irq = platform_get_irq_optional(pdev, 1);
> >
> > -     return 0;
> > -
> > -err_pm_disable:
> > -     pm_runtime_disable(&pdev->dev);
> > +     pm_runtime_set_active(&pdev->dev);
> > +     pm_runtime_enable(&pdev->dev);
> >
> > -     return err;
> > +     return 0;
> >   }
> >
> >   static int mtk8250_remove(struct platform_device *pdev)
> > @@ -619,9 +592,6 @@ static int mtk8250_remove(struct platform_device *pdev)
> >       pm_runtime_disable(&pdev->dev);
> >       pm_runtime_put_noidle(&pdev->dev);
> >
> > -     if (!pm_runtime_status_suspended(&pdev->dev))
> > -             mtk8250_runtime_suspend(&pdev->dev);
> > -
> >       return 0;
> >   }
> >
>
>