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

[Alistair Delva]

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

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).

> 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)

> 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.

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.