Re: next/master bisection: baseline.login on rk3288-rock2-square - Russell King

From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: "kernelci-results@groups.io" <kernelci-results@groups.io>,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	Nicolas Pitre <nico@fluxnic.net>,
	Guillaume Tucker <guillaume.tucker@collabora.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: next/master bisection: baseline.login on rk3288-rock2-square
Date: Thu, 4 Feb 2021 10:47:15 +0000	[thread overview]
Message-ID: <20210204104714.GU1463@shell.armlinux.org.uk> (raw)
In-Reply-To: <CAMj1kXFog3=5zD7+P=cRfRLj1xfD1h1kU58iifASBSXkRe-E6g@mail.gmail.com>

On Thu, Feb 04, 2021 at 11:27:16AM +0100, Ard Biesheuvel wrote:
> Hi Russell,
> 
> If Guillaume is willing to do the experiment, and it fixes the issue,
> it proves that rk3288 is relying on the flush before the MMU is
> disabled, and so in that case, the fix is trivial, and we can just
> apply it.
> 
> If the experiment fails (which would mean rk3288 does not tolerate the
> cache maintenance being performed after cache off), it is going to be
> hairy, and so it will definitely take more time.
> 
> So in the latter case (or if Guillaume does not get back to us), I
> think reverting my queued fix is the only sane option. But in that
> case, may I suggest that we queue the revert of the original by-VA
> change for v5.12 so it gets lots of coverage in -next, and allows us
> an opportunity to come up with a proper fix in the same timeframe, and
> backport the revert and the subsequent fix as a pair? Otherwise, we'll
> end up in the situation where v5.10.x until today has by-va, v5.10.x-y
> has set/way, and v5.10y+ has by-va again. (I don't think we care about
> anything before that, given that v5.4 predates any of this)

I'm suggesting dropping your fix (9052/1) and reverting
"ARM: decompressor: switch to by-VA cache maintenance for v7 cores"
which gets us to a point where _both_ regressions are fixed.

I'm of the opinion that the by-VA patch was incorrect when it was
merged (it caused a regression), and it's only a performance
improvement. Our attempts so far to fix it are just causing other
regressions. So, I think it is reasonable to revert both back to a
known good point which has worked over a decade. If doing so causes
regressions (which I think is unlikely), then that would be unfortunate
but alas is a price that's worth paying to get back to a known good
point - since then we're not stacking regression fixes on top of other
regression fixes.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

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