Re: [PATCH] arm64: PCI: Enable SMC conduit

[Will Deacon <will@kernel.org>]

Hi Lorenzo,

On Fri, Jan 08, 2021 at 10:32:16AM +0000, Lorenzo Pieralisi wrote:
> On Thu, Jan 07, 2021 at 04:05:48PM -0500, Jon Masters wrote:
> > On 1/7/21 1:14 PM, Will Deacon wrote:
> > 
> > > On Mon, Jan 04, 2021 at 10:57:35PM -0600, Jeremy Linton wrote:
> > > > Given that most arm64 platform's PCI implementations needs quirks
> > > > to deal with problematic config accesses, this is a good place to
> > > > apply a firmware abstraction. The ARM PCI SMMCCC spec details a
> > > > standard SMC conduit designed to provide a simple PCI config
> > > > accessor. This specification enhances the existing ACPI/PCI
> > > > abstraction and expects power, config, etc functionality is handled
> > > > by the platform. It also is very explicit that the resulting config
> > > > space registers must behave as is specified by the pci specification.
> > > > 
> > > > Lets hook the normal ACPI/PCI config path, and when we detect
> > > > missing MADT data, attempt to probe the SMC conduit. If the conduit
> > > > exists and responds for the requested segment number (provided by the
> > > > ACPI namespace) attach a custom pci_ecam_ops which redirects
> > > > all config read/write requests to the firmware.
> > > > 
> > > > This patch is based on the Arm PCI Config space access document @
> > > > https://developer.arm.com/documentation/den0115/latest
> > > 
> > > Why does firmware need to be involved with this at all? Can't we just
> > > quirk Linux when these broken designs show up in production? We'll need
> > > to modify Linux _anyway_ when the firmware interface isn't implemented
> > > correctly...
> > 
> > I agree with Will on this. I think we want to find a way to address some
> > of the non-compliance concerns through quirks in Linux. However...
> 
> I understand the concern and if you are asking me if this can be fixed
> in Linux it obviously can. The point is, at what cost for SW and
> maintenance - in Linux and other OSes, I think Jeremy summed it up
> pretty well:
> 
> https://lore.kernel.org/linux-pci/61558f73-9ac8-69fe-34c1-2074dec5f18a@arm.com
> 
> The issue here is that what we are asked to support on ARM64 ACPI is a
> moving target and the target carries PCI with it.
> 
> This potentially means that all drivers in:
> 
> drivers/pci/controller
> 
> may require an MCFG quirk and to implement it we may have to:
> 
> - Define new ACPI bindings (that may need AML and that's already a
>   showstopper for some OSes)
> - Require to manage clocks in the kernel (see link-up checks)
> - Handle PCI config space faults in the kernel
> 
> Do we really want to do that ? I don't think so. Therefore we need
> to have a policy to define what constitutes a "reasonable" quirk and
> that's not objective I am afraid, however we slice it (there is no
> such a thing as eg 90% ECAM).

Without a doubt, I would much prefer to see these quirks and workarounds
in Linux than hidden behind a firmware interface. Every single time.

This isn't like the usual fragmentation problems, where firmware swoops in
to save the day; CPU onlining, spectre mitigations, early entropy etc. All
of these problems exist because there isn't a standard method to implement
them outside of firmware, and so adding a layer of abstraction there makes
sense.

But PCIe is already a standard!

We shouldn't paper over hardware designers' inability to follow a ~20 year
old standard by hiding it behind another standard that is hot off the press.
Seriously.

There is not a scrap of evidence to suggest that the firmware
implementations will be any better, but they will certainly be harder to
debug and maintain.  I have significant reservations about Arm's interest in
maintaining the spec as both more errata appear and the PCIe spec evolves
(after all, this is outside of SBSA, no?). The whole thing stinks of "if all
you have is a hammer, then everything looks like a nail". But this isn't the
sort of problem that is solved with yet another spec -- instead, how about
encouraging vendors to read the specs that already exist?

> The SMC is an olive branch and just to make sure it is crystal clear
> there won't be room for adding quirks if the implementation turns out
> to be broken, if a line in the sand is what we want here it is.

I appreciate the sentiment, but you're not solving the problem here. You're
moving it somewhere else. Somewhere where you don't have to deal with it
(and I honestly can't blame you for that), but also somewhere where you
_can't_ necessarily deal with it. The inevitable outcome is an endless
succession of crappy, non-compliant machines which only appear to operate
correctly with particularly kernel/firmware combinations. Imagine trying to
use something like that?

The approach championed here actively discourages vendors from building
spec-compliant hardware and reduces our ability to work around problems
on such hardware at the same time.

So I won't be applying these patches, sorry.

Will