[PATCH] arm: update advice on kernel and FDT load address.

[Nicolas Pitre]

On Mon, 29 Jul 2013, Ian Campbell wrote:

> On Mon, 2013-07-29 at 14:50 +0100, Will Deacon wrote:
> > Hi Ian,
> > 
> > /me plays English teacher :)
> > 
> > On Mon, Jul 29, 2013 at 01:56:18PM +0100, Ian Campbell wrote:
> > > Based on the discussion in [0]:
> > >   - Recommend that the kernel be placed under 128MiB ("for automatic RAM
> > >     location to work", says Nico)
> > >   - Recommend that DTB larger than ~16KiB be placed after the 128MiB boundary
> > >     (in order to avoid conflicting with the kernel decompressor, kernel is at
> > >     most 32MiB) and within the lowmem region. Somewhat arbitrarily recommend
> > >     placement below the 256MiB boundary, the real lowmem limit is likely up
> > >     around 600-800MiB but varies.
> > > 
> > > [0] http://article.gmane.org/gmane.linux.ports.arm.kernel/254538
> > > 
> > > Signed-off-by: Ian Campbell <ian.campbell at citrix.com>
> > > Cc: Russell King <linux at arm.linux.org.uk>
> > > Cc: Nicolas Pitre <nicolas.pitre at linaro.org>
> > > Cc: Will Deacon <will.deacon at arm.com>
> > > Cc: Dave Martin <dave.martin at linaro.org>
> > > Cc: Grant Likely <grant.likely at secretlab.ca>
> > > Cc: linux-arm-kernel at lists.infradead.org
> > > ---
> > >  Documentation/arm/Booting |   14 +++++++++++---
> > >  1 files changed, 11 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/Documentation/arm/Booting b/Documentation/arm/Booting
> > > index 0c1f475..72c035e 100644
> > > --- a/Documentation/arm/Booting
> > > +++ b/Documentation/arm/Booting
> > > @@ -125,6 +125,13 @@ with the caveat that it may not be located at physical address 0 since
> > >  the kernel interprets a value of 0 in r2 to mean neither a tagged list
> > >  nor a dtb were passed.
> > >  
> > > +If the DTB is larger than ~16KiB, such that placement in the first
> > > +16KiB would lead to a conflict with the kernel loaded at 32KiB (as
> > > +recommended below) which uses the memory between 16KiB-32KiB itself,
> > > +then the DTB should be placed after the first 128MiB of RAM in order
> > > +to avoid conflicting with the kernel decompressor. The recommnded
> > 
> > recommended
> 
> Should've pressed F5, sorry...
> 
> > It also looks like you're repeating part of what is said later regarding the
> > 16k of page tables below the image.
> 
> yes, this was tricky since I wanted to justify the larger than 16KiB but
> that was a forward reference. I went with "as recommended below".
> Perhaps I should just omit the justification? Or perhaps "If the DTB is
> larger than ~16KiB (see below for why this particular value
> matters)...". Clunky :-(

Why not simply:

	Make sure the loaded DTB doesn't conflict with the zImage 
	decompressor which may relocate itself.  A safe location is 
	just above the 128MB boundary from start of RAM.

> > Also, we use 20k with LPAE.
> 
> I'll fold in an update along those lines too.
> 
> However given that this then makes everything more complicated (e.g.
> it's now DTB > ~12KiB in that case) perhaps we should just ditch the
> original recommendation to use the first 16KiB and switch to a
> recommendation of "as high as possible below 256MiB" across the board?
> That would remove the issue with repeating stuff from the next section
> too.

Sure.

> 
> > > +placement in this case just below the 256MiB boundary.
> > > +
> > >  5. Calling the kernel image
> > >  ---------------------------
> > >  
> > > @@ -136,9 +143,10 @@ is stored in flash, and is linked correctly to be run from flash,
> > >  then it is legal for the boot loader to call the zImage in flash
> > >  directly.
> > >  
> > > -The zImage may also be placed in system RAM (at any location) and
> > > -called there.  Note that the kernel uses 16K of RAM below the image
> > > -to store page tables.  The recommended placement is 32KiB into RAM.
> > > +The zImage may also be placed in system RAM and called there.  The
> > > +kernel should be placed in the first 128MiB of RAM. Note that the
> > > +kernel uses 16KiB of RAM below the image to store page tables.  The
> > > +recommended placement is 32KiB into RAM.
> > 
> > This is a bit clunky since you say that the zImage may be placed in system
> > RAM, then restrict that in the next sentence.
> 
> How about:
> 
>         The kernel may also be placed in the first 128MiB of system RAM
>         and called there.

Not "may" but "must" if you have CONFIG_AUTO_ZRELADDR=y.

>	Note that the kernel uses 16KiB of RAM below
>       the image to store page tables.  The recommended placement is
>         32KiB into RAM.

Well... not completely right.

You may load zImage anywhere in the first 128MB of RAM if you have 
CONFIG_AUTO_ZRELADDR=y.  If CONFIG_AUTO_ZRELADDR=n then zImage may be 
loaded anywhere. In the former case, the zImage code will automatically 
determine the physical start of RAM.  In the later case the start of RAM 
is hardcoded in the zImage binary.

Then, the zImage code will decompress the actual kernel image at an 
offset of (in most cases) 32KB from start of RAM.  However, if the 
decompressed kernel would overwrite the loaded zImage where the 
decompressor code and data is located, then zImage will relocate itself 
just after the end of the decompressed kernel area before proceeding to 
the decompression.  This is where you need to be careful about 
ramdisk/initrd and DTB load addresses.

The initial page table is therefore (in most cases again) just below the 
decompressed kernel location i.e. either Ffrom 16KB from start of RAM, 
or from 12KB if using LPAE.

You can therefore load everything (zImage, initrd, DTB) sequentially 
from the 32MB mark in RAM and be safe.  By loading zImage that high, it 
most certainly won't need to relocate itself (unless the decompressed 
kernel is larger than 32MB which is very unlikely as I mentioned) and 
that will also make the boot process slightly faster.



Nicolas