linux-kernel - Re: [PATCH/RFC v4 2/2] vfio: platform: Add generic reset controller support

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Date:   Wed, 19 Sep 2018 14:54:31 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Alex Williamson <alex.williamson@...hat.com>,
        KVM list <kvm@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC v4 2/2] vfio: platform: Add generic reset controller support

Hi Eric,

On Wed, Sep 19, 2018 at 2:36 PM Auger Eric <eric.auger@...hat.com> wrote:
> On 9/17/18 6:39 PM, Geert Uytterhoeven wrote:
> > Vfio-platform requires dedicated reset support, provided either by ACPI,
> > or, on DT platforms, by a device-specific reset driver matching against
> > the device's compatible value.
> >
> > On many SoCs, devices are connected to an SoC-internal reset controller.
> > If the reset hierarchy is described in DT using "resets" properties, or
> > in lookup tables in platform code, such devices can be reset in a
> > generic way through the reset controller subsystem.  Hence add support
> > for this, avoiding the need to write device-specific reset drivers for
> > each single device on affected SoCs.
> >
> > Devices that do require a more complex reset procedure can still provide
> > a device-specific reset driver, as that takes precedence.
> >
> > Note that this functionality depends on CONFIG_RESET_CONTROLLER=y, and
> > becomes a no-op (as in: "No reset function found for device") if reset
> > controller support is disabled.
> >
> > Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> > Reviewed-by: Philipp Zabel <p.zabel@...gutronix.de>
> > Reviewed-by: Simon Horman <horms+renesas@...ge.net.au>

> > --- a/drivers/vfio/platform/vfio_platform_common.c
> > +++ b/drivers/vfio/platform/vfio_platform_common.c

> > @@ -128,8 +131,16 @@ static int vfio_platform_get_reset(struct vfio_platform_device *vdev)
> >               vdev->of_reset = vfio_platform_lookup_reset(vdev->compat,
> >                                                       &vdev->reset_module);
> >       }
> > +     if (vdev->of_reset)
> > +             return 0;
> > +
> > +     rstc = reset_control_get_dedicated(vdev->device, NULL);
> > +     if (!IS_ERR(rstc)) {
> > +             vdev->reset_control = rstc;
> > +             return 0;
> > +     }
> >
> > -     return vdev->of_reset ? 0 : -ENOENT;
> > +     return PTR_ERR(rstc);
> This changes the returned value as seen by the user (probe returned
> valud). Can we keep -ENOENT in case of no reset controller found?

On success, it still returns 0.
On failure, it forwards the error from reset_control_get_dedicated(), which
is IMHO better than replacing it by -ENOENT: we try to propagate error
codes as much as possible.  It could e.g. return -EPROBE_DEFER.

Is there anything that relies on the function returning -ENOENT?

> Otherwise looks good to me with the new "dedicated" reset semantics.

Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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