Re: [PATCH 05/11] x86, pci, acpi: Move arch-agnostic MMCONFIG (aka ECAM) and ACPI code out of arch/x86/ directory

[Tomasz Nowicki]

On 14.09.2015 11:37, Lorenzo Pieralisi wrote:
> On Fri, Sep 11, 2015 at 01:35:36PM +0100, Tomasz Nowicki wrote:
> > On 11.09.2015 13:20, Lorenzo Pieralisi wrote:
>
> [...]
>
> > > > > With that in place using raw_pci_write/read or the generic accessors
> > > > > becomes almost identical, with code requiring the pci_bus to be
> > > > > created using the generic accessors and ACPICA using the raw version.
> > > > >
> > > > > I might be missing something, so apologies if that's the case.
> > > >
> > > > Actually, I think you showed me the right direction :) Here are some
> > > > conclusions/comments/concerns. Please correct me if I am wrong:
> > > >
> > > > 1. We need raw_pci_write/read accessors (based on ECAM) for ARM64 too
> > > > but only up to the point where buses are enumerated. From that point on,
> > > > we should reuse generic accessors from access.c file, right?
> > >
> > > Well, I still have not figured out whether on arm64 the raw accessors
> > > required by ACPICA make sense.
> > >
> > > So either arm64 relies on the generic MCFG based raw read and writes
> > > or we define the global raw read and writes as empty (ie x86 overrides
> > > them anyway).
> > >
> > > I will get back to you on this.
> > >
> > > > 2. For ARM64 ACPI PCI, we can use generic accessors right away, .map_bus
> > > > would call common code part (pci_dev_base()). The only thing that worry
> > > > me is fact that MCFG regions are RCU list so it needs rcu_read_lock()
> > > > for the .map_bus (mcfg lookup) *and* read/write operation.
> > >
> > > Do you mean the address look-up and the mmio operation should be carried
> > > out atomically right ?
> > Yes.
>
> We can wrap the calls pci_generic_read/write() within a function and
> add rcu_read_lock()/unlock() around them, eg:
>
> int pci_generic_config_read_rcu()
> {
> 	rcu_read_lock();
> 	pci_generic_config_read(...);
> 	rcu_read_unlock();
> }
It looks good to me, thanks for suggestion.

> Honestly it seems the RCU API is needed just because config space
> can be also accessed by raw_ accessors in ACPICA code, that's the only
> reason I see to protect the config structs against config space
> removal (basically config entries are removed only when the host
> bridge is released if I read the code correctly, and the only way
> this can happen concurrently is having ACPICA code reusing the
> same config space but accessing it with no pci_bus struct attached
> to it, by just using the (segment, bus, dev, fn) tuple).
Right.

Side note:
MCFG region can be removed from the pci_mmcfg_list list only if it has 
been "hot added" there. Which means that PCI host bridge specified 
configuration base address (_CBA) different than those from MCFG static 
table e.g.:

DSDT.asl:
Device (PCI0) {
    Name (_HID, EISAID ("PNP0A03"))
    [...]
    Name (_CBA, 0xB0000000)
    [...]
}

But pci_mmcfg_list elements coming from static MCFG table cannot be 
removed, hence they are living there for ever.

Thanks,
Tomasz
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/