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

Geert Uytterhoeven
2025-09-03T12:18:07Z

Hi Tommaso,

Thanks for your patch!

I don't understand why you included this patch in a series with clock
patches. AFAIUC, there is no dependency. Am I missing something?

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.

> 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).

> 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.

> +}
> +
> +static int rz_dmac_runtime_resume(struct device *dev)
> +{
> + struct rz_dmac *dmac = dev_get_drvdata(dev);
> +
> + return reset_control_deassert(dmac->rstc);

Shouldn't this reinitialize some registers?
For now that indeed doesn't matter, as reset is only deasserted
from .probe(), before any register initialization.

> +}
> +
> +static int rz_dmac_resume(struct device *dev)
> +{
> + struct rz_dmac *dmac = dev_get_drvdata(dev);
> + int ret;
> +
> + ret = pm_runtime_force_resume(dev);
> + if (ret)
> + return ret;
> +
> + rz_dmac_chan_init_all(dmac);
> +
> + return 0;
> +}
> +
> +static const struct dev_pm_ops rz_dmac_pm_ops = {
> + /*
> + * TODO for system sleep/resume:
> + * - Wait for the current transfer to complete and stop the device,
> + * - Resume transfers, if any.
> + */
> + NOIRQ_SYSTEM_SLEEP_PM_OPS(pm_runtime_force_suspend, rz_dmac_resume)
> + RUNTIME_PM_OPS(rz_dmac_runtime_suspend, rz_dmac_runtime_resume, NULL)
> +};
> +
> static const struct of_device_id of_rz_dmac_match[] = {
> { .compatible = "renesas,r9a09g057-dmac", },
> { .compatible = "renesas,rz-dmac", },
> @@ -1068,6 +1110,7 @@ MODULE_DEVICE_TABLE(of, of_rz_dmac_match);
>
> static struct platform_driver rz_dmac_driver = {
> .driver = {
> + .pm = pm_ptr(&rz_dmac_pm_ops),
> .name = "rz-dmac",
> .of_match_table = of_rz_dmac_match,
> },

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