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

[Lorenzo Pieralisi]

[+ Zhang Rui]

On Thu, Sep 22, 2016 at 05:10:42PM -0500, Bjorn Helgaas wrote:
> On Thu, Sep 22, 2016 at 01:31:01PM -0500, Bjorn Helgaas wrote:
> > On Thu, Sep 22, 2016 at 01:44:46PM +0100, Lorenzo Pieralisi wrote:
> > > On Thu, Sep 22, 2016 at 11:10:13AM +0000, Gabriele Paoloni wrote:
> > > > Hi Lorenzo, Bjorn
> > > > 
> > > > > -----Original Message-----
> > > > > From: Lorenzo Pieralisi [mailto:lorenzo.pieralisi@xxxxxxx]
> > > > > Sent: 22 September 2016 10:50
> > > > > To: Bjorn Helgaas
> > > > > Cc: Ard Biesheuvel; Tomasz Nowicki; David Daney; Will Deacon; Catalin
> > > > > Marinas; Rafael Wysocki; Arnd Bergmann; Hanjun Guo; Sinan Kaya;
> > > > > Jayachandran C; Christopher Covington; Duc Dang; Robert Richter; Marcin
> > > > > Wojtas; Liviu Dudau; Wangyijing; Mark Salter; linux-
> > > > > pci@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; Linaro ACPI
> > > > > Mailman List; Jon Masters; Andrea Gallo; Jeremy Linton; liudongdong
> > > > > (C); Gabriele Paoloni; Jeff Hugo; linux-acpi@xxxxxxxxxxxxxxx; linux-
> > > > > kernel@xxxxxxxxxxxxxxx; Rafael J. Wysocki
> > > > > Subject: Re: [PATCH V6 3/5] PCI: thunder-pem: Allow to probe PEM-
> > > > > specific register range for ACPI case
> > > > > 
> > > > > On Wed, Sep 21, 2016 at 01:04:57PM -0500, Bjorn Helgaas 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.
> > > > > 
> > > > > I think the PNP quirk approach + PNP0c02 resource put forward by Gab
> > > > > is enough.
> > > > 
> > > > Great thanks as we take a final decision I will ask Dogndgong to submit
> > > > another RFC based on this approach
> > > > 
> > > > > 
> > > > > > > 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.
> > > > > 
> > > > > Ok, that's agreed. It goes without saying that since you are quoting
> > > > > the PCI spec, if FW fails to report MCFG regions in a PNP0c02 device
> > > > > _CRS I will consider that a FW bug.
> > > > > 
> > > > > > > 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.o
> > > > > 
> > > > > Ok, I am fine with both and I think we are converging, but the way
> > > > > to solve this problem has to be uniform for all ARM partners (and
> > > > > not only ARM). Two points here:
> > > > > 
> > > > > 1) Adding a device/subdevice allows people to add a _CRS reporting the
> > > > >    non-window bridge resources. Fine. It also allows people to chuck in
> > > > >    there all sorts of _DSD properties to describe their PCI host bridge
> > > > >    as it is done with DT properties (those _DSD can contain eg clocks
> > > > >    etc.), this may be tempting (so that they can reuse the same DT
> > > > >    driver and do not have to update their firmware) but I want to be
> > > > >    clear here: that must not happen. So, a subdevice with a _CRS to
> > > > >    report resources, yes, but it will stop there.
> > > > > 2) It is unclear to me how to formalize the above. People should not
> > > > >    write FW by reading the PCI mailing list, so these guidelines have
> > > > > to
> > > > >    be written, somehow. I do not want to standardize quirks, I want
> > > > >    to prevent random ACPI table content, which is different.
> > > > >    Should I report this to the ACPI spec working group ? If we do
> > > > >    not do that everyone will go solve this problem as they deem fit.
> > > > > 
> > > > 
> > > > Do we really need to formalize this?
> > > > 
> > > > As we discussed in the Linaro call at the moment we have few vendors
> > > > that need quirks and we want to avoid promoting/accepting quirks for
> > > > the future.
> > > > 
> > > > At the time of the call I think we decided to informally accept a set
> > > > of quirks for the current platforms and reject any other quirk coming
> > > > after a certain date/kernel version (this to be decided).
> > > > 
> > > > I am not sure if there is a way to document/formalize a temporary
> > > > exception from the rule...
> > > 
> > > - (1) will be enforced.
> > 
> > I'm not sure it's necessary or possible to enforce a "no future
> > quirks" rule.  For one thing, there's already a pretty strong
> > incentive to avoid quirks: if your hardware doesn't require quirks,
> > it works with OSes already in the field.
> > 
> > MCFG quirks allow us to use the generic ACPI pci_root.c driver even if
> > the hardware doesn't support ECAM quite according to the spec.
> > 
> > PNP0C02 usage is a workaround for the failure of the Consumer/Producer
> > bit.  PNP0C02 quirks compensate for firmware that doesn't describe
> > resource usage accurately.  It's possible the ACPI spec folks could
> > come up with a better Consumer/Producer workaround, if that's needed.
> > Apparently x86 hasn't needed it yet.
> > 
> > If people add _DSD methods for clocks or whatnot, the hardware won't
> > work with the generic pci_root.c driver, so there's already an
> > incentive for avoiding them.  x86 has managed without such methods;
> > arm64 should be able to do the same.
> 
> Re-reading this, I'm afraid my response sounds a little dismissive,
> and I feel like I'm missing some important information.  So I
> apologize if I missed your whole point, Lorenzo.

No you are spot on, I just wanted to emphasize, given that we are
adding an _HID and a subdevice, that developer should not be tempted
to use it to match against a PCI host driver to reuse the DT code,
we should not use the quirk mechanism as a backdoor to re-using DT
drivers in ACPI context.

Anyway, there is a review process to spot these possible misuses,
mine was just a heads-up, quirks will happen, I just do not want
to wreak the standard ACPI PCI firmware model to support them.

Given that there are already PNP0c02 bindings out there where the
PNP0c02 is used as in Gab's example:

https://patchwork.kernel.org/patch/4757111/

I think the only pending question I have is whether we are allowed
to define a PNP0A03 subdevice with a _CRS resource space that is
not contained in its parent _CRS, if we answer this question I
think we are done.

I will raise the PNP0c02 usage issue with the ASWG anyway.

Thanks !
Lorenzo