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

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Message-ID: <9dc74e23-2f57-7db3-6fb6-4f75ed61575f@redhat.com>
Date:   Wed, 19 Sep 2018 17:31:43 +0200
From:   Auger Eric <eric.auger@...hat.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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 Geert,

On 9/19/18 2:54 PM, Geert Uytterhoeven wrote:
> 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?
None I am aware of actually. I was afraid about compatibility break but
here we would change an errno by another one so maybe that's not a big
deal at that stage of vfio_platform usage?

Thanks

Eric
> 
>> Otherwise looks good to me with the new "dedicated" reset semantics.
> 
> Thanks!
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
>