Re: [PATCH V6 3/5] PCI: thunder-pem: Allow to probe PEM-specific register range for ACPI case

[Duc Dang]

On Wed, Sep 21, 2016 at 11:04 AM, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> On Wed, Sep 21, 2016 at 03:05:49PM +0100, Lorenzo Pieralisi wrote:
>> On Tue, Sep 20, 2016 at 02:17:44PM -0500, Bjorn Helgaas wrote:
>> > On Tue, Sep 20, 2016 at 04:09:25PM +0100, Ard Biesheuvel wrote:
>>
>> [...]
>>
>> > > None of these platforms can be fixed entirely in software, and given
>> > > that we will not be adding quirks for new broken hardware, we should
>> > > ask ourselves whether having two versions of a quirk, i.e., one for
>> > > broken hardware + currently shipping firmware, and one for the same
>> > > broken hardware with fixed firmware is really an improvement over what
>> > > has been proposed here.
>> >
>> > We're talking about two completely different types of quirks:
>> >
>> >   1) MCFG quirks to use memory-mapped config space that doesn't quite
>> >      conform to the ECAM model in the PCIe spec, and
>> >
>> >   2) Some yet-to-be-determined method to describe address space
>> >      consumed by a bridge.
>> >
>> > The first two patches of this series are a nice implementation for 1).
>> > The third patch (ThunderX-specific) is one possibility for 2), but I
>> > don't like it because there's no way for generic software like the
>> > ACPI core to discover these resources.
>>
>> Ok, so basically this means that to implement (2) we need to assign
>> some sort of _HID to these quirky PCI bridges (so that we know what
>> device they represent and we can retrieve their _CRS). I take from
>> this discussion that the goal is to make sure that all non-config
>> resources have to be declared through _CRS device objects, which is
>> fine but that requires a FW update (unless we can fabricate ACPI
>> devices and corresponding _CRS in the kernel whenever we match a
>> given MCFG table signature).
>
> All resources consumed by ACPI devices should be declared through
> _CRS.  If you want to fabricate ACPI devices or _CRS via kernel
> quirks, that's fine with me.  This could be triggered via MCFG
> signature, DMI info, host bridge _HID, etc.
>
>> We discussed this already and I think we should make a decision:
>>
>> http://lists.infradead.org/pipermail/linux-arm-kernel/2016-March/414722.html
>>
>> > > > I'd like to step back and come up with some understanding of how
>> > > > non-broken firmware *should* deal with this issue.  Then, if we *do*
>> > > > work around this particular broken firmware in the kernel, it would be
>> > > > nice to do it in a way that fits in with that understanding.
>> > > >
>> > > > For example, if a companion ACPI device is the preferred solution, an
>> > > > ACPI quirk could fabricate a device with the required resources.  That
>> > > > would address the problem closer to the source and make it more likely
>> > > > that the rest of the system will work correctly: /proc/iomem could
>> > > > make sense, things that look at _CRS generically would work (e.g,
>> > > > /sys/, an admittedly hypothetical "lsacpi", etc.)
>> > > >
>> > > > Hard-coding stuff in drivers is a point solution that doesn't provide
>> > > > any guidance for future platforms and makes it likely that the hack
>> > > > will get copied into even more drivers.
>> > > >
>> > >
>> > > OK, I see. But the guidance for future platforms should be 'do not
>> > > rely on quirks', and what I am arguing here is that the more we polish
>> > > up this code and make it clean and reusable, the more likely it is
>> > > that will end up getting abused by new broken hardware that we set out
>> > > to reject entirely in the first place.
>> > >
>> > > So of course, if the quirk involves claiming resources, let's make
>> > > sure that this occurs in the cleanest and most compliant way possible.
>> > > But any factoring/reuse concerns other than for the current crop of
>> > > broken hardware should be avoided imo.
>> >
>> > If future hardware is completely ECAM-compliant and we don't need any
>> > more MCFG quirks, that would be great.
>>
>> Yes.
>>
>> > But we'll still need to describe that memory-mapped config space
>> > somewhere.  If that's done with PNP0C02 or similar devices (as is done
>> > on my x86 laptop), we'd be all set.
>>
>> I am not sure I understand what you mean here. Are you referring
>> to MCFG regions reported as PNP0c02 resources through its _CRS ?
>
> Yes.  PCI Firmware Spec r3.0, Table 4-2, note 2 says address ranges
> reported via MCFG or _CBA should be reserved by _CRS of a PNP0C02
> device.
>
>> IIUC PNP0C02 is a reservation mechanism, but it does not help us
>> associate its _CRS to a specific PCI host bridge instance, right ?
>
> Gab proposed a hierarchy that *would* associate a PNP0C02 device with
> a PCI bridge:
>
>   Device (PCI1) {
>     Name (_HID, "HISI0080") // PCI Express Root Bridge
>     Name (_CID, "PNP0A03") // Compatible PCI Root Bridge
>     Method (_CRS, 0, Serialized) { // Root complex resources (windows) }
>     Device (RES0) {
>       Name (_HID, "HISI0081") // HiSi PCIe RC config base address
>       Name (_CID, "PNP0C02")  // Motherboard reserved resource
>       Name (_CRS, ResourceTemplate () { ... }
>     }
>   }
>
> That's a possibility.  The PCI Firmware Spec suggests putting RES0 at
> the root (under \_SB), but I don't know why.
>
> Putting it at the root means we couldn't generically associate it with
> a bridge, although I could imagine something like this:
>
>   Device (RES1) {
>     Name (_HID, "HISI0081") // HiSi PCIe RC config base address
>     Name (_CID, "PNP0C02")  // Motherboard reserved resource
>     Name (_CRS, ResourceTemplate () { ... }
>     Method (BRDG) { "PCI1" }  // hand-wavy ASL
>   }
>   Device (PCI1) {
>     Name (_HID, "HISI0080") // PCI Express Root Bridge
>     Name (_CID, "PNP0A03") // Compatible PCI Root Bridge
>     Method (_CRS, 0, Serialized) { // Root complex resources (windows) }
>   }
>
> Where you could search PNP0C02 devices for a cookie that matched the
> host bridge.
>
>> > If we need to work around firmware in the field that doesn't do that,
>> > one possibility is a PNP quirk along the lines of
>> > quirk_amd_mmconfig_area().
>>
>> You mean matching PNP0C01/PNP0c02 and create a resource (that has to
>> hardcoded in a static array in the kernel anyway, there is no way to
>> retrieve it otherwise) in the corresponding PNP quirk handler ?
>
> Right.  On some hardware we can read the resource out of a
> device-specific register, as we do in quirk_intel_mch().  But if
> that's not possible, it would have to be hard-coded.
>
>> And it is not a given we can match against PNP0c01/PNP0c02.
>>
>> So it looks like the only solution is allocating an _HID for each
>> host bridge that is not ECAM compliant to add resources to its _CRS
>> (unless the MCFG quirk does not need any additional data/resource,
>> eg "use different set of PCI accessorsi 32-bit vs byte-access").
>
> It doesn't matter whether it's ECAM-compliant or not.  Any
> memory-mapped config space should be reported via some device's _CRS.
>
> The existing x86 practice is to use PNP0C02 devices for this purpose,
> and I think we should just follow that practice.
>
>> For FW that is immutable I really do not see what we can do apart
>> from hardcoding the non-config resources (consumed by a bridge),
>> somehow.
>
> Right.  Well, I assume you mean we should hard-code "non-window
> resources consumed directly by a bridge".  If firmware in the field is
> broken, we should work around it, and that may mean hard-coding some
> resources.
>
> My point is that the hard-coding should not be buried in a driver
> where it's invisible to the rest of the kernel.  If we hard-code it in
> a quirk that adds _CRS entries, then the kernel will work just like it
> would if the firmware had been correct in the first place.  The
> resource will appear in /sys/devices/pnp*/*/resources and /proc/iomem,
> and if we ever used _SRS to assign or move ACPI devices, we would know
> to avoid the bridge resource.

Hi Bjorn,

Are you suggesting to add code similar to functions in
linux/drivers/pnp/quirks.c to declare/attach the additional resource
that the host need to have when the resource is not in MCFG table?

>
> Bjorn
Regards,
Duc Dang.