Re: [PATCH/RFC v7] ARM: boot: Obtain start of physical memory from DTB - Russell King

From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: "Kumar Gala" <kumar.gala@linaro.org>,
	"Geert Uytterhoeven" <geert+renesas@glider.be>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Nicolas Pitre" <nico@fluxnic.net>,
	"Stephen Boyd" <sboyd@kernel.org>,
	"Linus Walleij" <linus.walleij@linaro.org>,
	"Lukasz Stelmach" <l.stelmach@samsung.com>,
	"Bjorn Andersson" <bjorn.andersson@linaro.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	"Chris Brandt" <chris.brandt@renesas.com>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Eric Miao" <eric.miao@nvidia.com>,
	"Dmitry Osipenko" <digetx@gmail.com>,
	"Laura Abbott" <labbott@redhat.com>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	"Linux ARM" <linux-arm-kernel@lists.infradead.org>,
	"Marek Szyprowski" <m.szyprowski@samsung.com>
Subject: Re: [PATCH/RFC v7] ARM: boot: Obtain start of physical memory from DTB
Date: Sat, 15 Aug 2020 10:18:27 +0100	[thread overview]
Message-ID: <20200815091827.GW1551@shell.armlinux.org.uk> (raw)
In-Reply-To: <CAMj1kXFcS2+i7+Uf6mSkg+aO6j3FOaTJhN70vbWUDZe2CBm1Hg@mail.gmail.com>

On Fri, Aug 14, 2020 at 04:06:03PM +0200, Ard Biesheuvel wrote:
> On Fri, 14 Aug 2020 at 16:03, Linus Walleij <linus.walleij@linaro.org> wrote:
> >
> > On Thu, Jul 23, 2020 at 3:19 AM Stephen Boyd <sboyd@kernel.org> wrote:
> >
> > > > > textofs-$(CONFIG_ARCH_IPQ40XX) := 0x00208000
> > > > > textofs-$(CONFIG_ARCH_MSM8X60) := 0x00208000
> > > > > textofs-$(CONFIG_ARCH_MSM8960) := 0x00208000
> > > >
> > > > But what on earth is this? I just deleted this and the platform
> > > > boots just as well.
> > >
> > > We need to shift the kernel text to start 2MB beyond the start of memory
> > > because there is the shared memory region used to communicate with other
> > > processors in the SoC there. It took a while for us to convince other OS
> > > folks in the company to put shared memory somewhere else besides the
> > > start of RAM, but eventually we won that battle.
> > >
> > > Does your booted kernel have its text section at the start of RAM or is
> > > it offset by 2MB without this change? Check out /proc/iomem to see where
> > > the kernel text is in relation to the start of RAM.
> >
> > The memory on this machine starts at 0x40200000 since the effect
> > of the current code is to take pc &= 0xf8000000 and that results in
> > 0x40000000 and then this adds textofs 0x00208000 to that
> > resulting in 0x40208000 for the kernel physical RAM. Which
> > is what we want to achieve since the RAM starts at
> > 0x40200000.
> >
> > But TEXT_OFFSET is also used inside the kernel to offset the
> > virtual memory. This means that when we set up the virtual
> > memory split, the kernel virtual memory is also bumped by
> > these 2 MB so the virtual memory starts at 0xC0208000
> > instead of 0xC0008000 as is normal.
> >
> > It looks weird to me but maybe someone can explain how
> > logical that is?
> >
> 
> The ARM mm code assumes that the relative alignment between PA and VA
> is 16 MB, so if we skip 2 MB in the physical space, we must do the
> same in the virtual space.

That is a good thing; it makes the virtual to physical translations
easy - we only have facility for offsets with bits 23..0 clear to
make the code rewriting simple.

-- 
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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