Re: [RFC PATCH v3 2/2] ARM64/PCI: Start using quirks handling for ACPI based PCI host controller

[Christopher Covington]

Hi Duc,

On 06/20/2016 05:42 AM, Lorenzo Pieralisi wrote:
> On Fri, Jun 17, 2016 at 02:37:02PM -0700, Duc Dang wrote:
>> On Thu, Jun 16, 2016 at 10:48 AM, Lorenzo Pieralisi
>> <lorenzo.pieralisi@xxxxxxx> wrote:
>>> On Wed, Jun 15, 2016 at 11:34:11AM -0400, Christopher Covington wrote:
>>>> From: Tomasz Nowicki <tn@xxxxxxxxxxxx>
>>>>
>>>> pci_generic_ecam_ops is used by default. Since there are platforms
>>>> which have non-compliant ECAM space we need to overwrite these
>>>> accessors prior to PCI buses enumeration. In order to do that
>>>> we call pci_mcfg_get_ops to retrieve pci_ecam_ops structure so that
>>>> we can use proper PCI config space accessors and bus_shift.
>>>>
>>>> pci_generic_ecam_ops is still used for platforms free from quirks.
>>>>
>>>> Signed-off-by: Tomasz Nowicki <tn@xxxxxxxxxxxx>
>>>> ---
>>>>  arch/arm64/kernel/pci.c | 7 ++++---
>>>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/kernel/pci.c b/arch/arm64/kernel/pci.c
>>>> index 94cd43c..a891bda 100644
>>>> --- a/arch/arm64/kernel/pci.c
>>>> +++ b/arch/arm64/kernel/pci.c
>>>> @@ -139,6 +139,7 @@ pci_acpi_setup_ecam_mapping(struct acpi_pci_root *root)
>>>>       struct pci_config_window *cfg;
>>>>       struct resource cfgres;
>>>>       unsigned int bsz;
>>>> +     struct pci_ecam_ops *ops;
>>>>
>>>>       /* Use address from _CBA if present, otherwise lookup MCFG */
>>>>       if (!root->mcfg_addr)
>>>> @@ -150,12 +151,12 @@ pci_acpi_setup_ecam_mapping(struct acpi_pci_root *root)
>>>>               return NULL;
>>>>       }
>>>>
>>>> -     bsz = 1 << pci_generic_ecam_ops.bus_shift;
>>>> +     ops = pci_mcfg_get_ops(root);
>>>> +     bsz = 1 << ops->bus_shift;
>>>>       cfgres.start = root->mcfg_addr + bus_res->start * bsz;
>>>>       cfgres.end = cfgres.start + resource_size(bus_res) * bsz - 1;
>>>>       cfgres.flags = IORESOURCE_MEM;
>>>> -     cfg = pci_ecam_create(&root->device->dev, &cfgres, bus_res,
>>>> -                           &pci_generic_ecam_ops);
>>>> +     cfg = pci_ecam_create(&root->device->dev, &cfgres, bus_res, ops);
>>>
>>> Arnd pointed this out already, I think that's the only pending question
>>> here.
>>>
>>> pci_ecam_create() maps ECAM space for config regions retrieved from
>>> the MCFG, which are *supposed* to be ECAM compliant.
>>>
>>> Do we think that's *always* correct/safe regardless of the kind
>>> of quirk we are currently fixing up ?
>>>
>>> Or we do think that configuration space regions should come from
>>> a different resource declared in the ACPI namespace if the regions
>>> are not MCFG/ECAM compliant (ie config space is not defined through
>>> MCFG at all - possibly through a _CRS method for a vendor specific
>>> _HID under the PNP0A03 node ?)
>>
>> Hi Lorenzo,
>>
>> For X-Gene: the ECAM space is used to access the configuration space
>> of PCIe devices, with additional help from controller register to
>> specify the bus, device and function number. Below is the RFC patch
>> that implements ECAM fixup for X-Gene PCIe controller on top of this
>> RFC ECAM quirk v3 for your and others reference.
> 
> Yes, you have an additional resource in the PNP0A03 _CRS to describe
> your register that is a deliberate abuse of the ACPI standard in
> that the _CRS is meant to describe resources that are passed on
> to secondary buses

A potential alternative came up in an off-list discussion: Would it be
better to hard code the information in the quirk workaround than look it
up from a repurposed ACPI resource?

Supporting quirk workarounds for early, non-compliant hardware is
helpful and perhaps necessary for bootstrapping the ecosystem in a
timely manner. But we don't really want to provide an expandable or
reusable interface that would make it easy for new hardware to remain
non-compliant.

Regards,
Cov

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