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SubjectRe: [PATCH V3 1/3] vfio, platform: add support for ACPI while detecting the reset driver
On 2016-03-29 07:25, Arnd Bergmann wrote:
> On Tuesday 29 March 2016 06:59:15 wrote:
>> On 2016-03-29 05:25, Arnd Bergmann wrote:
>> > On Monday 28 March 2016 09:35:22 Sinan Kaya wrote:
>> >> The code is using the compatible DT string to associate a reset driver
>> >> with
>> >> the actual device itself. The compatible string does not exist on ACPI
>> >> based systems. HID is the unique identifier for a device driver
>> >> instead.
>> >> The change allows a driver to register with DT compatible string or
>> >> ACPI
>> >> HID and then match the object with one of these conditions.
>> >>
>> >> Rules for loading the reset driver are as follow:
>> >> - ACPI HID needs match for ACPI systems
>> >> - DT compat needs to match for OF systems
>> >>
>> >> Tested-by: Eric Auger <> (device tree only)
>> >> Tested-by: Shanker Donthineni <> (ACPI only)
>> >> Signed-off-by: Sinan Kaya <>
>> >>
>> >
>> >
>> > This really feels wrong for two reasons:
>> >
>> > * device assignment of non-PCI devices is really special and doesn't
>> > seem to make sense on general purpose servers that would be the
>> > target
>> > for ACPI normally
>> Why is it special? Acpi is not equal to pci. Platform devices are
>> first
>> class devices too. Especially, _cls was introduced for this reason.
> It still feels like a hack. The normal design for a server is to have
> all internal devices show up on the PCI host bridge, next to the PCIe
> ports, to have a simple way to manage any device, both internal and
> off-chip. Putting a device on random MMIO registers outside of the
> discoverable buses and have the firmware work around the lack of
> discoverability will always be inferior.

It is a HW implementation choice. Having everything as pci problem has
been already solved. I would vote for it when we had SW pci bridge layer
just to use usb and sata. Not anymore. Especially, _cls solves this

>> >
>> > * If there is indeed a requirement for ACPI to handle something like
>> > this,
>> > it should be part of the ACPI spec, with a well-defined method of
>> > handling
>> > reset, rather than having to add a device specific hack for each
>> > device separately.
>> >
>> I see. Normally, this is done by calling _rst method. AFAIK, Linux
>> doesn’t support _rst. I can check its presence and call it if it is
>> there.
> Yes, that sounds reasonable: In patch 2 where you check for the
> presence of the reset method, just keep the existing logic for
> DT based systems, and use _rst on ACPI based systems instead,
> then you can drop both patches 1 and 3.

I can certainly drop patch #3 and push the reset responsibility to acpi.

I never liked having a fragmented sw design across multiple drivers.

I need something for patch #1. Compatible is a DT property not ACPI.but
then, I won't have a reset driver anymore.

If we think about how vfio pci works, we pass the pci vendor and device
id to new_id file to find out which pci device needs to be pass thru.

I can go to a similar route. This time we pass the object id through
new_id and I call reset method on this object.

Let me know what you think?

> Arnd

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