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

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Date:   Tue, 26 Jul 2022 12:40:00 +0100
From:   Will Deacon <will@...nel.org>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     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, catalin.marinas@....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

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.

What's changed?

In my mind, the main thing that's happened since we last discussed this
is that Apple shipped arm64 client hardware with working ECAM. *Apple*
for goodness sake: a company with basically no incentive to follow
standards for their vertically integrated devices! Perhaps others need
to raise their game instead of wasting everybody's time on firmware
hacks; getting the hardware right obviously isn't as difficult as folks
would lead us to believe.

Will

[1] https://lore.kernel.org/r/20210325131231.GA18590@e121166-lin.cambridge.arm.com