RFC: top level compatibles for virtual platforms

[Yoder Stuart-B08248]

> -----Original Message-----
> From: Wood Scott-B07421
> Sent: Monday, July 11, 2011 1:05 PM
> To: Yoder Stuart-B08248
> Cc: Wood Scott-B07421; Tabi Timur-B04825; Grant Likely; Benjamin Herrenschmidt; Gala Kumar-
> B11780; Alexander Graf; linuxppc-dev at ozlabs.org
> Subject: Re: RFC: top level compatibles for virtual platforms
> 
> On Mon, 11 Jul 2011 12:41:20 -0500
> Yoder Stuart-B08248 <B08248 at freescale.com> wrote:
> 
> >
> >
> > > -----Original Message-----
> > > From: Wood Scott-B07421
> > > Sent: Monday, July 11, 2011 11:24 AM
> > > To: Tabi Timur-B04825
> > > Cc: Yoder Stuart-B08248; Grant Likely; Benjamin Herrenschmidt; Gala
> > > Kumar-B11780; Wood Scott- B07421; Alexander Graf;
> > > linuxppc-dev at ozlabs.org
> > > Subject: Re: RFC: top level compatibles for virtual platforms
> > >
> > > On Mon, 11 Jul 2011 10:45:47 -0500
> > > Timur Tabi <timur at freescale.com> wrote:
> > >
> > > > >> Also, if these are KVM creations, shouldn't there be a "kvm" in
> > > > >> the compatible string somewhere?
> > > > >
> > > > > There is nothing KVM specific about these platforms.  Any
> > > > > hypervisor could create a similar virtual machine.
> > > >
> > > > True, but I think we're on a slippery slope, here.  Virtualization
> > > > allows us to create "virtual platforms" that are not well defined.
> > > > Linux requires a unique compatible string for each platform.
> > >
> > > The device tree is supposed to describe the hardware (virtual or
> > > otherwise), not just supply what Linux wants.  Perhaps there simply
> > > shouldn't be a toplevel compatible if there's nothing appropriate to describe there -- and
> fix whatever issues Linux has with that.
> >
> > But there is a concept in Linux of a platform 'machine':
> 
> So have a Linux "machine" that is used when no other one matches.  That doesn't justify making
> something up in the device tree.
> 
> > define_machine(p4080_ds) {
> >         .name                   = "P4080 DS",
> >         .probe                  = p4080_ds_probe,
> >         .setup_arch             = corenet_ds_setup_arch,
> >         .init_IRQ               = corenet_ds_pic_init,
> > #ifdef CONFIG_PCI
> >         .pcibios_fixup_bus      = fsl_pcibios_fixup_bus,
> > #endif
> >         .get_irq                = mpic_get_coreint_irq,
> >         .restart                = fsl_rstcr_restart,
> >         .calibrate_decr         = generic_calibrate_decr,
> >         .progress               = udbg_progress,
> > };
> >
> > Right now p4080_ds_probe needs something to match on to determine
> > whether this is the machine type.   How would it work if
> > there was no top level compatible to match on?   Some
> > platforms (e.g. e500v2-type) need mpc85xx_ds_pic_init(), others need
> > corenet_ds_pic_init().
> 
> Just because Linux does it that way now doesn't mean it needs to.  The interrupt controller
> has a compatible property.  Match on it like any other device.  You can find which one is the
> root interrupt controller by looking for nodes with the interrupt-controller property that
> doesn't have an explicit interrupt-parent (or an interrupts property?  seems to be a conflict
> between ePAPR and the original interrupt mapping document).

This may be the right long term thing to do, but restructuring
how Linux powerpc platforms work is a bigger effort.  I was looking
for an incremental improvement over what we do now, which is pass
a compatible of MPC8544DS and P4080DS for these virtual platforms.

However, they _are_ compatible with MPC8544DS and P4080DS so maybe
leaving the compatible string alone is ok for now.

Stuart