Commit 4f2e2280862a ("RFC: arm: pci: Add PCI cam support to PCI-E ecam driver")

[Pali Rohár]

On Monday 07 February 2022 11:48:29 Alistair Delva wrote:
> On Mon, Feb 7, 2022 at 11:39 AM Pali Rohár <pali at kernel.org> wrote:
> >
> > PING! Could you look at this email?
> >
> > On Thursday 20 January 2022 14:48:34 Pali Rohár wrote:
> > > Hello Alistair!
> > >
> > > On Wednesday 19 January 2022 14:48:21 Alistair Delva wrote:
> > > > Hi Pali,
> > > >
> > > > Sorry for the late reply..
> > > >
> > > > On Thu, Jan 13, 2022 at 4:34 AM Pali Rohár <pali at kernel.org> wrote:
> > > > >
> > > > > Hello!
> > > > >
> > > > > Now I see that you have merged commit 4f2e2280862a ("RFC: arm: pci: Add
> > > > > PCI cam support to PCI-E ecam driver"). It adds some "PCI cam support"
> > > > > with generic DT binding "pci-host-cam-generic".
> > > > >
> > > > > I have tried to find some information about it, but in PCIe
> > > > > specification there is nothing like PCI CAM. And neither in old PCI
> > > > > local bus 2.x or 3.x specs.
> > > >
> > > > I can't really help you with documentation, but "pci-host-cam-generic"
> > > > isn't something we made up, it is the same name used upstream by
> > > > Linux: https://elixir.bootlin.com/linux/latest/source/drivers/pci/controller/pci-host-generic.c#L60
> > >
> > > Ou, I did not know about it.
> > >
> > > > We don't have specs, we just reverse engineered what was happening in
> > > > the crosvm vm manager emulation of this device
> > > > (https://chromium.googlesource.com/chromiumos/platform/crosvm/+/refs/heads/master/aarch64/src/fdt.rs).
> > >
> > > Hm... And could you in Google contact authors of this code (as it is
> > > hosted in Google too) if they could provide specification for it? It
> > > could really help to understand how it is suppose to do...
> 
> I will try, but as noted by Mark, we are not the sole user of this
> device and we certainly don't own the definition of it. It just
> happens to be the device that was chosen for emulation by crosvm.

I'm still sceptical that there is any other user... I checked all U-Boot
drivers and there was no such HW, no such driver. And because
PCI Mechanism #2 has some kind of memory mapped config space access I'm
in impression that it was #2 what was used on real HW. As I was
confirmed that there was real HW with #2 support. And this could explain
possible mix of #1 and #2 which I described in previous emails.

> > > > > This access looks like a mix of "PCI Configuration Mechanism #1" and
> > > > > "PCI Configuration Mechanism #2" from PCI Local Bus Specification
> > > > > (rev 2.1, sections 3.7.4.1 and 3.7.4.2) and incompatible with both of
> > > > > them. It has layout similar to Mechanism #1 and access similar to #2.
> > > > >
> > > > > PCI Configuration Mechanism #1 uses two registers, one which select
> > > > > config address and second for accessing config space (selected address).
> > > > > But that U-Boot "PCI CAM" is implemented as memory mapped address space,
> > > > > something similar to PCI Configuration Mechanism #2 but with different
> > > > > layout. Also that "PCI CAM" does not set "enable" bit which is per PCI
> > > > > Configuration Mechanism #1 required to access PCI config space.
> > > > >
> > > > > Recently I converted all PCI drivers in U-Boot which uses PCI
> > > > > Configuration Mechanism #1 to use PCI_CONF1_ADDRESS() macro for
> > > > > accessing PCI config space. Basically every HW which uses PCI
> > > > > Configuration Mechanism #1 requires to set "enable" bit like it is
> > > > > described in PCI local bus spec. There is only one exception pci_msc01.c
> > > > > which requires to have "enable" bit unset. And I'm not sure if this is
> > > > > not rather bug in U-Boot driver (but it is in U-Boot in this state for a
> > > > > long time).
> > > > >
> > > > > Do you have some references to this "PCI CAM" specification? Because for
> > > > > me it looks like some vendor/proprietary undocumented API and
> > > > > incompatible with everything which I saw.
> > > > >
> > > > > Therefore I would suggest to not call it "pci-host-cam-generic" or
> > > > > TYPE_PCI as it is not generic for sure (like PCIe ECAM which is
> > > > > documented in PCIe base spec) and also because it is not PCI type (does
> > > > > not match neither PCI Mechanism #1 nor Mechanism #2).
> > > > >
> > > > > Anyway, I would like to know, which hardware uses this unusual PCI
> > > > > config space access?
> > > >
> > > > I don't know what real hardware uses it, but it is used by crosvm
> > > > (https://chromium.googlesource.com/chromiumos/platform/crosvm)
> > >
> > > I did not know about crosvm project. But seems that this is really used,
> > > but just only in emulated hardware?
> 
> Yes, and we're mostly using the QEMU backend in U-Boot. We're using it
> as a virtual device reference for Android, and U-Boot's ability to
> boot Android boot.imgs has been very useful.
> 
> > > > > Btw, that commit probably does not work. It uses construction:
> > > > >
> > > > >   (PCI_FUNC(bdf) << 8) | offset
> > > > >
> > > > > offset passed by U-Boot is number between 0..4095 and therefore it
> > > > > overlaps with PCI function number. Either shift by 8 is wrong and it
> > > > > should be shift by 12 or offset needs to be limited just to 0..255. But
> > > > > then there would be no access to PCIe extended space (256..4095), only
> > > > > PCI and I doubt that somebody in 2022 is still doing new development for
> > > > > Conventional PCI local bus hardware.
> > > >
> > > > I think that's the case for this device, unfortunately. Perhaps we
> > > > should cap offset between 0..255.
> > > >
> > > > Our change does work; without it, U-Boot can't see any PCI devices.
> > > > With it, they are all shown.
> > >
> > > Well, in that commit is overlapping offset and function. So accessing
> > > offsets above 255 would overwrite also function number and so, register
> > > access goes into different function/device. U-Boot would see PCI device
> > > but content in registers above 255 would be just garbage.
> > >
> > > If your (emulated) hw has really function number starting at bit 8, and
> > > not 12 then offset has to be limited just to 0..255. Meaning that for
> > > offsets above 255, u-boot driver has to return fabricated value zeros
> > > for any read attempts (and ignores write attempts).
> 
> Understood. We'll send a patch to fix it (soon hopefully).
> 
> > > > The other shifts in the change look the same as the Linux driver which
> > > > adjusts the shift from 20 to 16 here:
> > > > https://elixir.bootlin.com/linux/latest/source/drivers/pci/controller/pci-host-generic.c#L18
> > > >
> > > > I admit, the added logic looks different though:
> > > > https://elixir.bootlin.com/linux/latest/source/drivers/pci/ecam.c#L187
> > > >
> > > > > Also in my opinion as this "PCI CAM" does not set "enable" bit, there is
> > > > > needed some other code which sets it via vendor-specific API.
> > > >
> > > > What should we do for now? Do you need any help getting set up with
> > > > this environment? I think we could look at adding the pcie ecam device
> > > > to crosvm in parallel.
> > >
> > > As this access mechanism is already used by HW (it is only emulator, but
> > > that does not matter), it makes sense to have u-boot driver. But as this
> > > access mechanism is custom / non-standard, I would suggest to call it
> > > e.g. "google,pci-crosvm" or something like that. For sure not
> > > "pci-host-cam-generic" as this is not generic (or unless somebody points
> > > to the PCI specification where it is documented that is is really
> > > generic). Term "generic" is not really correct here.
> 
> I'm afraid I don't agree. The DT binding for U-Boot should be the same
> as the DT binding for Linux, and as Linux calls this
> "pci-host-cam-generic", and this device has really nothing to do with
> Google, I don't think it should be changed.

Well, if bindings is incorrect then it should be fixed. And argument
that somebody use something incorrect is not the reason to expand its
usage.

I was really trying to find some other user of such thing, but seems
that this google crosvm is the only current user.

Lets start doing it properly. You are targeting support for google
crossvm (virtual hardware), so name it according to scheme like it is
used for other vendors.

There are many PCIe hardware parts based on Synopsys Designware IPs, and
probably this is the most common usage nowadays. But even Designware
drivers do not use "generic" bindings and instead are named
specifically.

So if something seems to be marked as generic, it is Designware used by
many HW, and not something which is used currently only by google
crossvm.

I saw in past that Google wanted and tried people to force their
technology as a standard even nobody except google used it. And this
stuff looks very similar to those attempts.

In my opinion, "generic" should be used for something which is really
generic, something which is standardized and defined. And this google
crossvm stuff is not in any PCIe spec.

> > > And also please use new U-Boot PCI_CONF1_ADDRESS macro for constructing
> > > "PCI Configuration Mechanism #1"-like values, what I did recently for
> > > all U-Boot pci drivers.
> 
> We'll take a look at this too.
> 
> > > And for future, I would really suggest to implement standard PCIe ECAM
> > > mechanism (as defined in PCIe standards) into crosvm project. For PCIe
> > > devices it is really required to be able to access also config registers
> > > above offset 255. Limitation for 0..255 is there just for compatibility
> > > with old Conventional PCI local bus hardware (and possibility to connect
> > > PCI-to-PCIe switches to these old hardware).
> 
> I agree. Once we have migrated to PCIe ECAM emulation, we can move our
> U-Boot configuration to that as well, and potentially remove the code
> added to support the PCI CAM version. However, this will take some
> time, and the impact of this code in the eCAM file is very limited. I
> think with the fixups you have requested, we can live with it for a
> little longer.