linux-kernel - Re: [PATCH] drivers/amba: add reset control to primecell probe

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Message-ID: <CAL_Jsq+PRKGwdozr3VECpk2ugrOuWd4CYnRSR7ChyPOKgheYkw@mail.gmail.com>
Date:   Fri, 2 Aug 2019 08:37:02 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Dinh Nguyen <dinguyen@...nel.org>
Cc:     devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>,
        Dinh Nguyen <dinh.nguyen@...el.com>
Subject: Re: [PATCH] drivers/amba: add reset control to primecell probe

On Thu, Aug 1, 2019 at 12:44 PM Dinh Nguyen <dinguyen@...nel.org> wrote:
>
> From: Dinh Nguyen <dinh.nguyen@...el.com>
>
> The primecell controller on some SoCs, i.e. SoCFPGA, is held in reset by
> default. Until recently, the DMA controller was brought out of reset by the
> bootloader(i.e. U-Boot). But a recent change in U-Boot, the peripherals that
> are not used are held in reset and are left to Linux to bring them out of
> reset.

You can fix this in the kernel, but any versions before this change
will remain broken. IMO, the u-boot change should be reverted because
it is breaking an ABI (though not a good one).

> Add a mechanism for getting the reset property and de-assert the primecell
> module from reset if found. This is a not a hard fail if the reset property
> is not present in the device tree node, so the driver will continue to probe.

I think this belongs in the AMBA bus code, not the DT code, as that is
where we already have clock control code for similar reasons.

>
> Signed-off-by: Dinh Nguyen <dinguyen@...nel.org>
> ---
>  drivers/of/platform.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> index 7801e25e6895..d8945705313d 100644
> --- a/drivers/of/platform.c
> +++ b/drivers/of/platform.c
> @@ -21,6 +21,7 @@
>  #include <linux/of_irq.h>
>  #include <linux/of_platform.h>
>  #include <linux/platform_device.h>
> +#include <linux/reset.h>
>
>  const struct of_device_id of_default_bus_match_table[] = {
>         { .compatible = "simple-bus", },
> @@ -229,6 +230,7 @@ static struct amba_device *of_amba_device_create(struct device_node *node,
>         struct amba_device *dev;
>         const void *prop;
>         int i, ret;
> +       struct reset_control *rstc;
>
>         pr_debug("Creating amba device %pOF\n", node);
>
> @@ -270,6 +272,18 @@ static struct amba_device *of_amba_device_create(struct device_node *node,
>                 goto err_free;
>         }
>
> +       /*
> +        * reset control of the primecell block is optional
> +        * and will not fail if the reset property is not found.
> +        */
> +       rstc = of_reset_control_get_exclusive(node, "dma");

'dma' doesn't sound very generic.

> +       if (!IS_ERR(rstc)) {
> +               reset_control_deassert(rstc);
> +               reset_control_put(rstc);
> +       } else {
> +               pr_debug("amba: reset control not found\n");
> +       }
> +
>         ret = amba_device_add(dev, &iomem_resource);
>         if (ret) {
>                 pr_err("amba_device_add() failed (%d) for %pOF\n",
> --
> 2.20.0
>