Re: [PATCH V3 1/3] vfio, platform: add support for ACPI while detecting the reset driver

[okaya]

On 2016-03-29 07:25, Arnd Bergmann wrote:
> On Tuesday 29 March 2016 06:59:15 okaya@xxxxxxxxxxxxxx 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 <eric.auger@xxxxxxxxxx> (device tree only)
> > >> Tested-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx> (ACPI only)
> > >> Signed-off-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx>
> > >>
> > >
> > >
> > > 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 
problem



> > >
> > > * 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
> > doesnât 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