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From: Lorenzo Pieralisi <>
To: Thierry Reding <>
Cc: Vidya Sagar <>,, Bjorn Helgaas <>,
	"Rafael J. Wysocki" <>,
	Len Brown <>,
	Andrew Murray <>,,,,,, LKML <>,,,
Subject: Re: [PATCH] PCI: Add MCFG quirks for Tegra194 host controllers
Date: Mon, 20 Jan 2020 15:18:49 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <20200120111042.GA203160@ulmo>

On Mon, Jan 20, 2020 at 12:10:42PM +0100, Thierry Reding wrote:


> > > Currently the BSP has the kernel booting through Device Tree mechanism
> > > and there is a plan to support UEFI based boot as well in the future software
> > > releases for which we need this quirky way of handling ECAM.
> > > Tegra194 is going to be the only and last chip with this issue and next chip
> > > in line in Tegra SoC series will be fully compliant with ECAM.
> > 
> > ACPI on ARM64 works on a standard subset of systems, defined by the
> > 
> >
> I don't understand what you're saying here. Are you saying that you want
> to prevent vendors from upstreaming code that they need to support their
> ACPI based platforms? I understand that the lack of support for proper
> ECAM means that a platform will not be SBSA compatible, but I wasn't
> aware that lack of SBSA compatibility meant that a platform would be
> prohibited from implementing ACPI support in an upstream kernel.

ACPI on ARM64 requires a set of HW components described in the SBSA.

If those HW requirements are not fulfilled you can't bootstrap an ARM64
system with ACPI - it is as simple as that. It is not even appropriate
to discuss this on a Linux mailing list anymore since it is HW
requirements and it has been public information since ACPI on ARM64 was
first enabled.

> > These patches will have to be carried out of tree, the MCFG quirk
> > mechanism (merged as Bjorn said more than three years ago) was supposed
> > to be a temporary plaster to bootstrap server platforms with teething
> > issues, the aim is to remove it eventually not to add more code to it
> > indefinitely.
> Now, I fully agree that quirks are suboptimal and we'd all prefer if we
> didn't have to deal with them. Unfortunately the reality is that
> mistakes happen and hardware doesn't always work the way we want it to.
> There's plenty of other quirk mechanisms in the kernel, and frankly this
> one isn't really that bad in comparison.

Because you don't have to maintain it ;) - I think I said what I had to
say about the MCFG mechanism in the past - it has been three years
and counting - it is time to remove it rather that adding to it.

> > So I am afraid but this quirk (and any other coming our way) will not be
> > merged in an upstream kernel anymore - for any queries please put Nvidia
> > in touch.
> Again, I don't understand what you're trying to achieve here. You seem
> to be saying that we categorically can't support this hardware because
> it isn't fully SBSA compatible.

I am not trying to achieve anything - I am just stating public
information - let me repeat it again for interested readers: to
bootstrap an ARM64 system with ACPI the platform HW design must follow
the SBSA guidelines.

> Do you have any alternative suggestions on how we can support this in an
> upstream kernel?

Booting with a device tree ?

> We realized a while ago that we cannot achieve proper ECAM on Tegra194
> because of some issues with the hardware and we've provided this as
> feedback to the hardware engineers. As a result, the next generation of
> Tegra should no longer suffer from these issues.

We will bootstrap next generation Tegra with ACPI then, there are
SBSA tests available for compliancy - again, that's a matter for
Nvidia and Arm to settle, not a mailing list discussion.

> As for Tegra194, that chip taped out two years ago and it isn't possible
> to make it fully ECAM compliant other than by revising the chip, which,
> frankly, isn't going to happen.
> So I see two options here: either we find a way of dealing with this, by
> either merging this quirk or finding an alternative solution, or we make
> the decision that some hardware just can't be supported.
> The former is fairly common, whereas I've never heard of the latter.

What does this mean ? Should I wreck the upstream kernel to make it boot
with ACPI on *any* ARM64 platform out there then ?

My stance is clear above and the ACPI PCI programming model - inclusive
of firmware - has been there since ACPI was deployed, if ACPI support
is required HW must comply, either that or it is out of tree patches
and I can't be blamed for that.


  reply	other threads:[~2020-01-20 15:18 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-03 17:49 [PATCH] PCI: Add MCFG quirks for Tegra194 host controllers Vidya Sagar
2020-01-03 18:04 ` Bjorn Helgaas
2020-01-04  3:44   ` Vidya Sagar
2020-01-17 12:17     ` Lorenzo Pieralisi
2020-01-20 11:10       ` Thierry Reding
2020-01-20 15:18         ` Lorenzo Pieralisi [this message]
2020-01-21 13:44           ` Thierry Reding
2020-01-23 10:49             ` Lorenzo Pieralisi
2020-02-06 16:46               ` Thierry Reding
2020-02-07 14:50                 ` Bjorn Helgaas
2020-02-07 16:51                   ` Thierry Reding
2020-02-07 18:34                     ` Bjorn Helgaas
2020-01-04 21:53 ` kbuild test robot
2020-01-06  8:27 ` [PATCH V2] " Vidya Sagar
2020-01-10 19:14   ` [PATCH V3 0/2] " Vidya Sagar
2020-01-10 19:14     ` [PATCH V3 1/2] arm64: tegra: Re-order PCIe aperture mappings to support ACPI boot Vidya Sagar
2020-06-29 13:31       ` Jon Hunter
2020-06-30 10:52         ` Vidya Sagar
2020-01-10 19:15     ` [PATCH V3 2/2] PCI: Add MCFG quirks for Tegra194 host controllers Vidya Sagar
2020-01-17 11:42       ` Thierry Reding
2021-03-05 21:57       ` Bjorn Helgaas
2021-03-05 23:04         ` Krzysztof Wilczyński
2021-04-16 13:45         ` Vidya Sagar
2021-04-16 13:45       ` [PATCH V4] " Vidya Sagar
2021-04-16 19:06         ` Bjorn Helgaas
2021-05-13  9:40       ` [PATCH V3 2/2] " Qu Wenruo
2021-05-13 13:05         ` Vidya Sagar
2020-01-16 17:18     ` [PATCH V3 0/2] " Vidya Sagar

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