Linux-PCI Archive on lore.kernel.org
 help / color / Atom feed
From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
To: Thierry Reding <thierry.reding@gmail.com>
Cc: Vidya Sagar <vidyas@nvidia.com>,
	bjorn@helgaas.com, Bjorn Helgaas <bhelgaas@google.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Len Brown <lenb@kernel.org>,
	Andrew Murray <andrew.murray@arm.com>,
	treding@nvidia.com, jonathanh@nvidia.com,
	linux-tegra@vger.kernel.org, linux-pci@vger.kernel.org,
	linux-acpi@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	kthota@nvidia.com, mmaddireddy@nvidia.com, sagar.tv@gmail.com
Subject: Re: [PATCH] PCI: Add MCFG quirks for Tegra194 host controllers
Date: Mon, 20 Jan 2020 15:18:49 +0000
Message-ID: <20200120151849.GA24402@e121166-lin.cambridge.arm.com> (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
> > ARM SBSA:
> > 
> > http://infocenter.arm.com/help/topic/com.arm.doc.den0029c/Server_Base_System_Architecture_v6_0_ARM_DEN_0029C_SBSA_6_0.pdf
> 
> 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.

Thanks,
Lorenzo

  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-03 17:49 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
2020-01-10 19:15     ` [PATCH V3 2/2] PCI: Add MCFG quirks for Tegra194 host controllers Vidya Sagar
2020-01-17 11:42       ` Thierry Reding
2020-01-16 17:18     ` [PATCH V3 0/2] " Vidya Sagar

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200120151849.GA24402@e121166-lin.cambridge.arm.com \
    --to=lorenzo.pieralisi@arm.com \
    --cc=andrew.murray@arm.com \
    --cc=bhelgaas@google.com \
    --cc=bjorn@helgaas.com \
    --cc=jonathanh@nvidia.com \
    --cc=kthota@nvidia.com \
    --cc=lenb@kernel.org \
    --cc=linux-acpi@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-pci@vger.kernel.org \
    --cc=linux-tegra@vger.kernel.org \
    --cc=mmaddireddy@nvidia.com \
    --cc=rjw@rjwysocki.net \
    --cc=sagar.tv@gmail.com \
    --cc=thierry.reding@gmail.com \
    --cc=treding@nvidia.com \
    --cc=vidyas@nvidia.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-PCI Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-pci/0 linux-pci/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-pci linux-pci/ https://lore.kernel.org/linux-pci \
		linux-pci@vger.kernel.org
	public-inbox-index linux-pci

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-pci


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git