Re: [PATCH v2 5/5] dmaengine: sh: rz-dmac: Add runtime PM support

Geert Uytterhoeven
2025-09-04T15:10:55Z

Hi Tommaso,

On Thu, 4 Sept 2025 at 17:02, Tommaso Merciai
<tommaso.merciai.xr@bp.renesas.com> wrote:
> On Wed, Sep 03, 2025 at 02:17:53PM +0200, Geert Uytterhoeven wrote:
> > On Wed, 3 Sept 2025 at 10:28, Tommaso Merciai
> > <tommaso.merciai.xr@bp.renesas.com> wrote:
> > > Enable runtime power management in the rz-dmac driver by adding suspend and
> > > resume callbacks. This ensures the driver can correctly assert and deassert
> >
> > This is not really what this patch does: the Runtime PM-related changes
> > just hide^Wmove reset handling into the runtime callbacks.
>
> Agreed.
>
> > > the reset control and manage power state transitions during suspend and
> > > resume. Adding runtime PM support allows the DMA controller to reduce power
> >
> > (I assume) This patch does fix resuming from _system_ suspend.
> >
> > > consumption when idle and maintain correct operation across system sleep
> > > states, addressing the previous lack of dynamic power management in the
> > > driver.
> >
> > The driver still does not do dynamic power management: you still call
> > pm_runtime_resume_and_get() from the driver's probe() .callback, and
> > call pm_runtime_put() only from the .remove() callback, so the device
> > is powered all the time.
> > To implement dynamic power management, you have to change that,
> > and call pm_runtime_resume_and_get() and pm_runtime_put() from the
> > .device_alloc_chan_resources() resp. .device_free_chan_resources()
> > callbacks (see e.g. drivers/dma/sh/rcar-dmac.c).
>
> Thanks for the hints!
> So following your hints we need to:
>
> - call pm_runtime_get_sync() from rz_dmac_alloc_chan_resources()
> - call pm_runtime_put() from rz_dmac_free_chan_resources()
>
> With that then we can remove pm_runtime_put() from the remove function
> and add this at the end of the probe function.
>
> > > Signed-off-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
> >
> > > --- a/drivers/dma/sh/rz-dmac.c
> > > +++ b/drivers/dma/sh/rz-dmac.c
> > > @@ -437,6 +437,17 @@ static int rz_dmac_xfer_desc(struct rz_dmac_chan *chan)
> > > * DMA engine operations
> > > */
> > >
> > > +static void rz_dmac_chan_init_all(struct rz_dmac *dmac)
> > > +{
> > > + unsigned int i;
> > > +
> > > + rz_dmac_writel(dmac, DCTRL_DEFAULT, CHANNEL_0_7_COMMON_BASE + DCTRL);
> > > + rz_dmac_writel(dmac, DCTRL_DEFAULT, CHANNEL_8_15_COMMON_BASE + DCTRL);
> > > +
> > > + for (i = 0; i < dmac->n_channels; i++)
> > > + rz_dmac_ch_writel(&dmac->channels[i], CHCTRL_DEFAULT, CHCTRL, 1);
> > > +}
> > > +
> > > static int rz_dmac_alloc_chan_resources(struct dma_chan *chan)
> > > {
> > > struct rz_dmac_chan *channel = to_rz_dmac_chan(chan);
> > > @@ -970,10 +981,6 @@ static int rz_dmac_probe(struct platform_device *pdev)
> > > goto err_pm_disable;
> > > }
> > >
> > > - ret = reset_control_deassert(dmac->rstc);
> > > - if (ret)
> > > - goto err_pm_runtime_put;
> > > -
> > > for (i = 0; i < dmac->n_channels; i++) {
> > > ret = rz_dmac_chan_probe(dmac, &dmac->channels[i], i);
> > > if (ret < 0)
> > > @@ -1028,8 +1035,6 @@ static int rz_dmac_probe(struct platform_device *pdev)
> > > channel->lmdesc.base_dma);
> > > }
> > >
> > > - reset_control_assert(dmac->rstc);
> > > -err_pm_runtime_put:
> > > pm_runtime_put(&pdev->dev);
> > > err_pm_disable:
> > > pm_runtime_disable(&pdev->dev);
> > > @@ -1052,13 +1057,50 @@ static void rz_dmac_remove(struct platform_device *pdev)
> > > channel->lmdesc.base,
> > > channel->lmdesc.base_dma);
> > > }
> > > - reset_control_assert(dmac->rstc);
> > > pm_runtime_put(&pdev->dev);
> > > pm_runtime_disable(&pdev->dev);
> > >
> > > platform_device_put(dmac->icu.pdev);
> > > }
> > >
> > > +static int rz_dmac_runtime_suspend(struct device *dev)
> > > +{
> > > + struct rz_dmac *dmac = dev_get_drvdata(dev);
> > > +
> > > + return reset_control_assert(dmac->rstc);
> >
> > Do you really want to reset the device (and thus loose register state)
> > each and every time the device is runtime-suspended? For now it doesn't
> > matter much, but once you implement real dynamic power management,
> > it does.
> > I think the reset handling should be moved to the system suspend/resume
> > callbacks.
>
> Agreed. With above changes maybe we can move all into
> NOIRQ_SYSTEM_SLEEP_PM_OPS(rz_dmac_suspend, rz_dmac_resume)
> With your suggested changes I'm not sure if pm_runtime_ops are really needed.

After these changes, you indeed no longer need any pm_runtime_ops.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds