linux-kernel - Re: [PATCH 0/4] PCI SMC conduit, now with DT support

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Message-ID: <YvtOSedi3SqYngbV@arm.com>
Date:   Tue, 16 Aug 2022 08:59:05 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     Will Deacon <will@...nel.org>, linux-pci@...r.kernel.org,
        bhelgaas@...gle.com, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, lpieralisi@...nel.org,
        kw@...ux.com, mark.rutland@....com, sudeep.holla@....com,
        boqun.feng@...il.com, linux-arm-kernel@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        maz@...nel.org, jonmasters@...gle.com
Subject: Re: [PATCH 0/4] PCI SMC conduit, now with DT support

Hi Jeremy,

On Thu, Jul 28, 2022 at 12:20:55PM -0500, Jeremy Linton wrote:
> On 7/26/22 06:40, Will Deacon wrote:
> > On Mon, Jul 25, 2022 at 11:39:01AM -0500, Jeremy Linton wrote:
> > > This is a rebase of the later revisions of [1], but refactored
> > > slightly to add a DT method as well. It has all the same advantages of
> > > the ACPI method (putting HW quirks in the firmware rather than the
> > > kernel) but now applied to a 'pci-host-smc-generic' compatible
> > > property which extends the pci-host-generic logic to handle cases
> > > where the PCI Config region isn't ECAM compliant. With this in place,
> > > and firmware managed clock/phy/etc its possible to run the generic
> > > driver on hardware that isn't what one would consider standards
> > > compliant PCI root ports.
> > 
> > I still think that hiding the code in firmware because the hardware is
> > broken is absolutely the wrong way to tackle this problem and I thought
> > the general idea from last time was that we were going to teach Linux
> > about the broken hardware instead [1]. I'd rather have the junk where we
> > can see it, reason about it and modify it.
[...]
> Is it the official position of the Linux kernel maintainers that they will
> refuse to support future Arm standards in order to gate keep specific
> hardware platforms?

(just back from holiday; well, briefly, going away for a few days soon)

We shouldn't generalise what maintainers wwould accept or not. We decide
on a case by case basis. With speculative execution mitigations, for
example, we try to do as much as we can in the kernel but sometimes
that's just not possible, hence an EL3 call and we'd rather have this
standardised (e.g. custom branch loops to flush the branch predictor if
possible from the normal world, secure call if not).

You mention PSCI but that's not working around broken hardware, it was a
concious decision from the start to standardise the booting protocol and
CPU power management.

Now this PCI SMC protocol was simply created because hardware did not
comply with another PCI standard that has been around for a long time.
As with the speculative execution mitigations, we'd rather work around
broken hardware in the kernel first and, if it's not possible, we can
look at a firmware interface (and ideally standardised). Do you have an
example where we cannot work around the PCI hardware bugs in the kernel
and EL3 firmware involvement is necessary?

So, in summary, Arm Ltd proposing a new standard because hardware
companies can't be bothered with an existing one is not an argument for
accepting its support in the Linux kernel. This PCI SMC conduit is not
presented as a hardware bug workaround interface but rather as an
alternative to ECAM (and, yes, the kernel maintainers can choose not to
support specific "standards" in Linux).

-- 
Catalin