Hi will, everyone,
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...
Several folks here (particularly Lorenzo) have diligently worked hard
over the past few years - and pushed their patience - to accommodate
hardware vendors with early "not quite compliant" systems. They've taken
lots of quirks that frankly shouldn't continue to be necessary were it
even remotely a priority in the vendor ecosystem to get a handle on
addressing PCIe compliance once and for all. But, again frankly, it
hasn't been enough of a priority to get this fixed. The third party IP
vendors *need* to address this, and their customers *need* to push back.
We can't keep having a situation in which kinda-sorta compliant stuff
comes to market that would work out of the box but for whatever the
quirk is this time around. There have been multiple OS releases for the
past quite a few years on which this stuff could be tested prior to ever
taping out a chip, and so it ought not to be possible to come to market
now with an excuse that it wasn't tested. And yet here we still are. All
these years and still the message isn't quite being received properly. I
do know it takes time to make hardware, and some of it was designed
years before and is still trickling down into these threads. But I also
think there are cases where much more could have been done earlier.
None of these vendors can possibly want this deep down. Their engineers
almost certainly realize that just having compliant ECAM would mean that
the hardware was infinitely more valuable being able to run out of the
box software that much more easily. And it's not just ECAM. Inevitably,
that is just the observable syndrome for worse issues, often with the
ITS and forcing quirked systems to have lousy legacy interrupts, etc.
Alas, this level of nuance is likely lost by the time it reaches upper
management, where "Linux" is all the same to them. I would hope that can
change. I would also remind them that if they want to run non-Linux
OSes, they will also want to be actually compliant. The willingness of
kind folks like Lorenzo and others here to entertain quirks is not
necessarily something you will find in every part of the industry.
But that all said, we have a situation in which there are still
platforms out there that aren't fully compliant and something has to be
done to support them because otherwise it's going to be even more ugly
with vendor trees, distro hacks, and other stuff.
Some of you in recent weeks have asked what I and others can do to help
from the distro and standardization side of things. To do my part, I'm
going to commit to reach out to assorted vendors and have a heart to
heart with them about really, truly fully addressing their compliance
issues. That includes Cadence, Synopsys, and others who need to stop
shipping IP that requires quirks, as well as SoC vendors who need to do
more to test their silicon with stock kernels prior to taping out. And I
would like to involve the good folks here who are trying to navigate.
I would also politely suggest that we collectively consider how much
wiggle room there can be to use quirks for what we are stuck with rather
than an SMC-based solution. We all know that quirks can't be a free ride
forever. Those who need them should offer something strong in return. A
firm commitment that they will never come asking for the same stuff in
the future. Is there a way we can do something like that?
Jon.
--
Computer Architect
