Re: [PATCH v3 2/2] vfio: platform: Add generic DT reset support

[Geert Uytterhoeven]

Hi Eric,

On Fri, Apr 13, 2018 at 10:52 AM, Auger Eric <eric.auger@xxxxxxxxxx> wrote:
> On 12/04/18 18:02, Geert Uytterhoeven wrote:
>> On Thu, Apr 12, 2018 at 4:10 PM, Philipp Zabel <p.zabel@xxxxxxxxxxxxxx> wrote:
>>> On Thu, 2018-04-12 at 15:12 +0200, Geert Uytterhoeven wrote:
>>>> On Thu, Apr 12, 2018 at 2:36 PM, Sinan Kaya <okaya@xxxxxxxxxxxxxx> wrote:
>>>>> On 4/12/2018 7:49 AM, Auger Eric wrote:
>>>>>> On 12/04/18 13:32, Geert Uytterhoeven wrote:
>>>>>>> On Thu, Apr 12, 2018 at 12:31 PM, Auger Eric <eric.auger@xxxxxxxxxx> wrote:
>>>>>>>> On 11/04/18 11:15, Geert Uytterhoeven wrote:
>>>>>>>>> Vfio-platform requires 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,
>>>>>>>>> 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@xxxxxxxxx>
>>>>>>>>> Reviewed-by: Philipp Zabel <p.zabel@xxxxxxxxxxxxxx>
>>>>>>>>> --- a/drivers/vfio/platform/vfio_platform_common.c
>>>>>>>>> +++ b/drivers/vfio/platform/vfio_platform_common.c
>>>>>>>>> @@ -127,8 +130,16 @@ 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 = of_reset_control_get_exclusive(vdev->device->of_node, NULL);
>>>>>>>>
>>>>>>>> Shouldn't we prefer the top level reset_control_get_exclusive()?
>>>>>>>
>>>>>>> I guess that should work, too.
>>>>>>>
>>>>>>>> To make sure about the exclusive/shared terminology, does
>>>>>>>> get_reset_control_get_exclusive() check we have an exclusive wire
>>>>>>>> between this device and the reset controller?
>>>>>>>
>>>>>>> AFAIU, the "exclusive" means that only a single user can obtain access to
>>>>>>> the reset, and it does not guarantee that we have an exclusive wire between
>>>>>>> the device and the reset controller.
>>>>>>>
>>>>>>> The latter depends on the SoC's reset topology. If a reset wire is shared
>>>>>>> by multiple devices (e.g. resets shared by PWM or Display Unit devices on
>>>>>>> R-Car SoCs), exporting a subset of these devices to a guest is a bad idea,
>>>>>>> indeed.
>>>>>>
>>>>>> So who's going to check this assigned device will not trigger a reset of
>>>>>> other non assigned devices sharing the same reset controller?
>>>
>>> If the reset control is requested as exclusive, any other driver
>>> requesting the same reset control will fail (or this reset_control_get
>>> will fail, whichever comes last).
>>>
>>>>> I like the direction in general. I was hoping that you'd call it of_reset_control
>>>>> rather than reset_control.
>>>>
>>>> You mean vfio_platform_device.of_reset_control?
>>>> If we switch to reset_control_get_exclusive(), that doesn't make much sense...
>>>>
>>>>> Is there anything in the OF spec about what to expect from DT's reset?
>>>>
>>>> Documentation/devicetree/bindings/reset/reset.txt says:
>>>>
>>>> "A word on where to place reset signal consumers in device tree: It is possible
>>>>  in hardware for a reset signal to affect multiple logically separate HW blocks
>>>>  at once. In this case, it would be unwise to represent this reset signal in
>>>>  the DT node of each affected HW block, since if activated, an unrelated block
>>>>  may be reset. Instead, reset signals should be represented in the DT node
>>>>  where it makes most sense to control it; this may be a bus node if all
>>>>  children of the bus are affected by the reset signal, or an individual HW
>>>>  block node for dedicated reset signals. The intent of this binding is to give
>>>>  appropriate software access to the reset signals in order to manage the HW,
>>>>  rather than to slavishly enumerate the reset signal that affects each HW
>>>>  block."
>>>
>>> This was written in 2012, and unfortunately the DT binding documentation
>>> does not inform hardware development, and has not been updated since.
>>>
>>> There's generally two kinds of reset uses:
>>> - either to bring a device into a known state at a given point in time,
>>>   which is often done using a timed assert/deassert sequence,
>>> - or just to park a device while not in active use (must deassert any
>>>   time before use, may or may not assert any time after use)
>>>
>>> For the former case, the above paragraph makes a lot of sense, because
>>> when it is necessary to reset a device that shares the reset line with
>>> another, either choice between disturbing the other device, or not
>>> being able to reset when necessary, is a bad one. The reset controller
>>> framework supports those use cases via the reset_control_get_exclusive
>>> function variants.
>>>
>>> But for the latter case, there is absolutely no need to forbid sharing
>>> reset lines among multiple devices, as deassertion/assertion can just be
>>> handled reference counted, like clocks or power management. The reset
>>> controller framework supports those use cases via the
>>> reset_control_get_shared function variants.
>>>
>>> The case we are talking about here is the first one.
>>
>> Except that vfio-platform wants to reset the device before and after its
>> use by the guest, for isolation reasons, which does cause a major
>> disturbance in case of a shared reset.
>
> Do we have any guarantee that drivers whose device are sharing the reset
> signal will have got the reset control when the vfio-platform driver
> calls reset_control_get_exclusive(). In such a case vfio-platform
> reset_control_get_exclusive() will fail and this is what we want.
> Otherwise it is unsafe, right. Doesn't this assumption look a little risky?

No we don't: most drivers don't use reset_control at all.
I think on the Renesas SoCs only USB and Ethernet PHY (which is BTW external,
and thus not covered by the on-SoC reset controller) do.
A few more users may be added in the future.
But all module resets are described in DT.

Gr{oetje,eeting}s,

                        Geert

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

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