Re: [PATCH v4] ARM: boot: Obtain start of physical memory from DTB

From: Ard Biesheuvel <ardb@kernel.org>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "Arnd Bergmann" <arnd@arndb.de>,
	"Nicolas Pitre" <nico@fluxnic.net>,
	"Russell King" <linux@armlinux.org.uk>,
	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>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	"Marek Szyprowski" <m.szyprowski@samsung.com>
Subject: Re: [PATCH v4] ARM: boot: Obtain start of physical memory from DTB
Date: Wed, 15 Apr 2020 14:57:30 +0200	[thread overview]
Message-ID: <CAMj1kXEgXuizeQzBrt6aC-QODRGinoU5sjFrx0a1LRMg7zD85w@mail.gmail.com> (raw)
In-Reply-To: <CAMuHMdUkrF9qBaZre0EJ-cuzPcL7A1j2ANmQNYV7FAngybb1bA@mail.gmail.com>

On Wed, 15 Apr 2020 at 14:45, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Ard,
>
> On Tue, Apr 14, 2020 at 10:07 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> > On Wed, 25 Mar 2020 at 17:40, Ard Biesheuvel <ardb@kernel.org> wrote:
> > > On Fri, 20 Mar 2020 at 15:43, Geert Uytterhoeven
> > > <geert+renesas@glider.be> wrote:
> > > > Currently, the start address of physical memory is obtained by masking
> > > > the program counter with a fixed mask of 0xf8000000.  This mask value
> > > > was chosen as a balance between the requirements of different platforms.
> > > > However, this does require that the start address of physical memory is
> > > > a multiple of 128 MiB, precluding booting Linux on platforms where this
> > > > requirement is not fulfilled.
> > > >
> > > > Fix this limitation by obtaining the start address from the DTB instead,
> > > > if available (either explicitly passed, or appended to the kernel).
> > > > Fall back to the traditional method when needed.
> > > >
> > > > This allows to boot Linux on r7s9210/rza2mevb using the 64 MiB of SDRAM
> > > > on the RZA2MEVB sub board, which is located at 0x0C000000 (CS3 space),
> > > > i.e. not at a multiple of 128 MiB.
> > > >
> > > > Suggested-by: Nicolas Pitre <nico@fluxnic.net>
> > > > Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> > > > Reviewed-by: Nicolas Pitre <nico@fluxnic.net>
> > > > Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
> > > > ---
> > > > v4:
> > > >   - Fix stack location after commit 184bf653a7a452c1 ("ARM:
> > > >     decompressor: factor out routine to obtain the inflated image
> > > >     size"),
> > > >
> > >
> > > Apologies for the breakage. I was aware of the existence of this
> > > patch, but I didn't realize it was accessing LC0 early on to find the
> > > stack pointer value.
> > >
> > > Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
> >
> > OK, so one thing I did notice when playing with this code is that the
> > phys/virt patching code requires that 'PHYS_OFFSET - PAGE_OFFSET' is a
> > multiple of 16 MB, and so this needs to be taken into account by this
> > change as well, given that PHYS_OFFSET is based on the placement of
> > the uncompressed kernel in the physical address space.
>
> You mean fdt_get_mem_start() should round up the address to make sure
> it is a multiple of 16 MiB (assumed PAGE_OFFSET is a multiple of 16 MiB,
> too)?

Yes.

> Can PAGE_OFFSET actually be not a multiple of 16 MiB?

ARM's Kconfig has

config PAGE_OFFSET
        hex
        default PHYS_OFFSET if !MMU
        default 0x40000000 if VMSPLIT_1G
        default 0x80000000 if VMSPLIT_2G
        default 0xB0000000 if VMSPLIT_3G_OPT
        default 0xC0000000

which means that PHYS_OFFSET - PAGE_OFFSET is guaranteed to be 16 MB
aligned if PHYS_OFFSET is 16 MB aligned.

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