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

Jeremy Linton
2021-10-05T23:32:43Z

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@arm.com>
>>>> Acked-by: Florian Fainelli <f.fainelli@gmail.com>
>>>> Acked-by: Bjorn Helgaas <bhelgaas@google.com>
>>>> ---
>>>> 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
>>>>
>>