Re: [PATCH v3 3/4] PCI/ACPI: Add Broadcom bcm2711 MCFG quirk

[Jeremy Linton]

Hi,

On 10/5/21 5:31 PM, Bjorn Helgaas wrote:
> On Tue, Oct 05, 2021 at 10:43:32AM -0500, Jeremy Linton wrote:
> > On 10/5/21 10:10 AM, Bjorn Helgaas wrote:
> > > On Thu, Aug 26, 2021 at 02:15:56AM -0500, Jeremy Linton wrote:
> > > > Now that there is a bcm2711 quirk, it needs to be enabled when the
> > > > MCFG is missing. Use an ACPI namespace _DSD property
> > > > "linux-ecam-quirk-id" as an alternative to the MCFG OEM.
> > > >
> > > > Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
> > > > Acked-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
> > > > Acked-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> > > > ---
> > > >    drivers/acpi/pci_mcfg.c | 17 +++++++++++++++++
> > > >    1 file changed, 17 insertions(+)
> > > >
> > > > diff --git a/drivers/acpi/pci_mcfg.c b/drivers/acpi/pci_mcfg.c
> > > > index 53cab975f612..04c517418365 100644
> > > > --- a/drivers/acpi/pci_mcfg.c
> > > > +++ b/drivers/acpi/pci_mcfg.c
> > > > @@ -169,6 +169,9 @@ static struct mcfg_fixup mcfg_quirks[] = {
> > > >    	ALTRA_ECAM_QUIRK(1, 13),
> > > >    	ALTRA_ECAM_QUIRK(1, 14),
> > > >    	ALTRA_ECAM_QUIRK(1, 15),
> > > > +
> > > > +	{ "bc2711", "", 0, 0, MCFG_BUS_ANY, &bcm2711_pcie_ops,
> > > > +	  DEFINE_RES_MEM(0xFD500000, 0xA000) },
> > > >    };
> > > >    static char mcfg_oem_id[ACPI_OEM_ID_SIZE];
> > > > @@ -198,8 +201,22 @@ static void pci_mcfg_apply_quirks(struct acpi_pci_root *root,
> > > >    	u16 segment = root->segment;
> > > >    	struct resource *bus_range = &root->secondary;
> > > >    	struct mcfg_fixup *f;
> > > > +	const char *soc;
> > > >    	int i;
> > > > +	/*
> > > > +	 * This may be a machine with a PCI/SMC conduit, which means it doesn't
> > > > +	 * have an MCFG. Use an ACPI namespace definition instead.
> > > > +	 */
> > > > +	if (!fwnode_property_read_string(acpi_fwnode_handle(root->device),
> > > > +					 "linux-ecam-quirk-id", &soc)) {
> > > > +		if (strlen(soc) != ACPI_OEM_ID_SIZE)
> > >
> > >  From a reviewing perspective, it's not obvious why soc must be exactly
> > > ACPI_OEM_ID_SIZE.  Does that imply space-padding in the DT string or
> > > something?
> >
> > This is at the moment an ACPI only DSD, and it must follow the MADT OEM_ID
> > format for now because we are effectively just overriding that field. The
> > rest of the code in this module is just treating it as a fixed 6 bytes.
> >
> > > Is there any documentation for this DT property?
> >
> > Its not a DT property, and its unclear since its linux only if it
> > belongs in previously linked ACPI registry.
>
> Oh, right, it comes from a _DSD.
>
> > > Also not obvious why strlen() is safe here.  I mean, I looked a couple
> > > levels deep in fwnode_property_read_string(), but whatever guarantees
> > > null termination is buried pretty deep.
> >
> > I've not tracked down who, if anyone other than the AML compiler is
> > guaranteeing a null. The spec says something to the effect "Most other
> > string, however,  are of variable-length and are automatically null
> > terminated by the compiler". Not sure if that helps any.
>
> Doesn't help for me.  The PCI core shouldn't go in the weeds no matter
> what junk we might get from an AML compiler.  Maybe
> fwnode_property_read_string() guarantees null termination, but it's
> not documented and not easy to verify.
>
> I think a strncpy() here might be better.  Not sure it's worthwhile to
> emit a specific error message for the wrong length.

I think we went around about this a bit, but yes strncpy() is exactly 
what we want because the rest of the code assumes 6 non-null terminated 
characters and strncpy won't terminate it if its longer. OTOH, i'm not 
sure we really want shorter strings padded with nulls either.

strncpy() is easy though, so sure. <shrug>


> > > It seems a little weird to use an MCFG quirk mechanism when there's no
> > > MCFG at all on this platform.
> >
> > Well its just a point to hook in a CFG space quirk, and since that
> > is what most of the MCFG quirks are, it seemed reasonable to reuse
> > it rather than recreate it.
>
> Yeah, it's ugly no matter how we slice it.  The pci_no_msi()
> especially has nothing to do with ECAM at all.  But I don't know how
> to identify this thing for a quirk.  PNP0A08 devices really rely on
> ECAM or a system firmware config accessor.
>
> > > > +			dev_err(&root->device->dev, "ECAM quirk should be %d characters\n",
> > > > +				ACPI_OEM_ID_SIZE);
> > > > +		else
> > > > +			memcpy(mcfg_oem_id, soc, ACPI_OEM_ID_SIZE);
> > > > +	}
> > > > +
> > > >    	for (i = 0, f = mcfg_quirks; i < ARRAY_SIZE(mcfg_quirks); i++, f++) {
> > > >    		if (pci_mcfg_quirk_matches(f, segment, bus_range)) {
> > > >    			if (f->cfgres.start)
> > > > --
> > > > 2.31.1