linux-kernel - Re: [PATCH] arm64: PCI: Enable SMC conduit

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Message-ID: <20210225093000.GA22843@e121166-lin.cambridge.arm.com>
Date:   Thu, 25 Feb 2021 09:30:00 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Jon Masters <jcm@...masters.org>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Jeremy Linton <jeremy.linton@....com>,
        Will Deacon <will@...nel.org>, mark.rutland@....com,
        linux-pci@...r.kernel.org, sudeep.holla@....com,
        lkml <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: PCI: Enable SMC conduit

On Thu, Feb 18, 2021 at 12:43:30PM -0500, Jon Masters wrote:
> Hi Bjorn, all,
> 
> On Thu, Jan 28, 2021 at 6:31 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
> 
>     On Tue, Jan 26, 2021 at 10:46:04AM -0600, Jeremy Linton wrote:
> 
>  
> 
>     > Does that mean its open season for ECAM quirks, and we can expect
>     > them to start being merged now?
> 
>     "Open season" makes me cringe because it suggests we have a license to
>     use quirks indiscriminately forever, and I hope that's not the case.
> 
>     Lorenzo is closer to this issue than I am and has much better insight
>     into the mess this could turn into.  From my point of view, it's
>     shocking how much of a hassle this is compared to x86.  There just
>     aren't ECAM quirks, in-kernel clock management, or any of that crap.
>     I don't know how they do it on x86 and I don't have to care.  Whatever
>     they need to do, they apparently do in AML.  Eventually ARM64 has to
>     get there as well if vendors want distro support.
> 
>     I don't want to be in the position of enforcing a draconian "no more
>     quirks ever" policy.  The intent -- to encourage/force vendors to
>     develop spec-compliant machines -- is good, but it seems like the
>     reward of having compliant machines "just work" vs the penalty of
>     having to write quirks and shepherd them upstream and into distros
>     will probably be more effective and not much slower.
> 
> 
> The problem is that the third party IP vendors (still) make too much junk. For
> years, there wasn't a compliance program (e.g. SystemReady with some of the
> meat behind PCI-SIG compliance) and even when there was the third party IP
> vendors building "root ports" (not even RCs) would make some junk with a hacked
> up Linux kernel booting on a model and demo that as "PCI". There wasn't the
> kind of adult supervision that was required. It is (slowly) happening now, but
> it's years and years late. It's just embarrassing to see the lack of ECAM that
> works. In many cases, it's because the IP being used was baked years ago or
> made for some "non server" (as if there is such a thing) use case, etc. But in
> others, there was a chance to do it right, and it still happens. Some of us
> have lost what hair we had over the years getting third party IP vendors to
> wake up and start caring about this.
> 
> So there's no excuse. None at all. However, this is where we are. And it /is/
> improving. But it's still too slow, and we have platforms still coming to
> market that need to boot and run. Based on this, and the need to have something
> more flexible than just solving for ECAM deficiencies (which are really just a
> symptom), I can see the allure of an SMC. I don't like it, but if that's where
> folks want to go, and if we can find a way to constrain the enthusiasm for it,
> then perhaps it is a path forward. But if we are to go down that path it needs
> to come with a giant warning from the kernel that a system was booted at is
> relying on that. Something that will cause an OS certification program to fail
> without a waiver, or will cause customers to phone up for support wondering why
> the hw is broken. It *must* not be a silent thing. It needs to be "this
> hardware is broken and non-standard, get the next version fixed".

It is a stance I agree with in many respects, it should be shared (it
was in HTML format - the lists unfortunately dropped the message) so I
am replying to it to make it public.

Thanks,
Lorenzo