[PATCH 3/3] doc: environment: Further expand on Image locations and provide example

Sun Jul 10 18:17:33 CEST 2022

On Sun, Jul 10, 2022 at 02:26:04PM +0200, Heinrich Schuchardt wrote:
> On 6/30/22 12:06, Simon Glass wrote:
> > On Mon, 20 Jun 2022 at 08:32, Tom Rini <trini at konsulko.com> wrote:
> > > 
> > > Start by elaborating on what some of our constraints tend to be with
> > > image location values, and document where these external constraints
> > > come from.  Provide a new subsection, an example based on the TI ARMv7
> > > OMAP2PLUS families of chips, that gives sample values and explains why
> > > we use these particular values.  This is based on what is in
> > > include/configs/ti_armv7_common.h as of fb3ad9bd923d ("TI: Add, use a
> > > DEFAULT_LINUX_BOOT_ENV environment string") as this contains just the
> > > values referenced in this document now and not some of the further
> > > additions that are less generic.
> > > 
> > > Signed-off-by: Tom Rini <trini at konsulko.com>
> > > ---
> > >   doc/usage/environment.rst | 39 +++++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 39 insertions(+)
> > 
> > Reviewed-by: Simon Glass <sjg at chromium.org>
> 
> Below you want a change?

Yes, often Simon does that (and it's fine) to both offer a tag but if
another iteration is needed to make a minor adjustment to some wording
or another, or to make when applying.  Which is fine with me.

> > > diff --git a/doc/usage/environment.rst b/doc/usage/environment.rst
> > > index a9a4702632d2..f70ccd6a58ee 100644
> > > --- a/doc/usage/environment.rst
> > > +++ b/doc/usage/environment.rst
> > > @@ -404,6 +404,42 @@ device tree blob  fdtfile        fdt_addr_r       fdt_addr
> > >   ramdisk           ramdiskfile    ramdisk_addr_r   ramdisk_addr
> > >   ================= ============== ================ ==============
> > > 
> > > +When setting the RAM addresses for `kernel_addr_r`, `fdt_addr_r` and
> > > +`ramdisk_addr_r` there are several constraints to keep in mind. When booting
> > > +Linux, the `Booting ARM Linux`_ and `Booting AArch64 Linux`_ documents lay out
> > > +the requirements for booting all ARM platforms, including both alignment and
> > > +where within memory various things must be.  These guidelines tend to also be
> > > +correct for other OSes and unless specifically contradicted by documentation
> 
> What makes you think that BSD or Haiku have the same constraints as Linux?

Because of what I said, and experience?  Now, one may be a subset of
another, but that still means it will work for both.  This is intended
to be general best practices.  If you follow this then it's likely
anything else will work too.  The danger comes from trying to optimize
the sizes to be as small as possible, rather than as large/flexible as
will likely work anywhere.  I will try and expand on that idea in the
next iteration.

> > > +specific to another architecture, are good rules to follow for other
> > > +architectures as well.
> 
> No. RISC-V does not have the same requirements as ARM. E.g. the initrd
> can be located anywhere in memory.

Please point to documentation that confirms that, and some otherwise bad
examples that actually work.

> > > +
> > > +Example Image locations
> > > +^^^^^^^^^^^^^^^^^^^^^^^
> 
> You seem not to refer to a file 'Image'.
> 
> %s/Image/image/

OK.

> > > +
> > > +If we take the Texas Instruments OMAP2PLUS family of ARMv7 processors as an
> > > +example for the above listed variables, we would do::
> 
> %s/we would do/we chose/ ?

Either?  I don't see it mattering either way.

> > > +
> > > +    loadaddr=0x82000000
> > > +    kernel_addr_r=${loadaddr}
> > > +    fdt_addr_r=0x88000000
> > > +    ramdisk_addr_r=0x88080000
> > > +    bootm_size=0x10000000
> > > +
> > > +To explain this, we start by noting that DRAM starts at 0x80000000.  A 32MiB
> > 
> > Should it say 'We use a 32MiB' ?
> 
> Please, mention that MAX_UNCOMP_KERNEL_SIZE = 32 MiB is ARMv7 specific.

Sorry?  As I understood it last, the maximum size was something like
96MiB before you have to employ some funky tricks.

> > > +buffer from the start of memory as our default load address, and so where the
> > > +kernel would also be loaded to.  This will hopefully allow for us to have the
> 
> %s/allow for us/allow/
> 
> > > +whole of the compressed kernel image exist in memory above where the whole of
> > > +the decompressed kernel image will be, and allow for a quicker boot.  Next, we

We use 32MiB for the reason I said here.  Which is only a slight
rewording of the arm32 Linux booting document, and the section starts
out by saying this is an example for ARMv7 platforms.

> Please, mention that decompressor code otherwise will have to relocate
> the compressed kernel.

I'm not sure.  Perhaps it would be good to also link to some of the
articles expanding upon how Linux on ARM32 boots, as part of more
general documentation, rather than a specific example here.

> > > +say that the device tree will be placed at 128MiB offset from the start of
> 
> Please, mention that initrd must be
> 
> * within 512 MiB (0x20000000) of the memory start on arm
>   (which restricts initrd_high)
> * in a a 1 GB aligned region of size '1UL << (VA_BITS_MIN - 1)' that
>   includes the kernel on arm64

No, because this is not intended to list every constraint on ARM32 (nor
arm64, which would benefit from an example that's not TI, as TI arm64
platforms share the same base address for memory).

> On RISC-V such a limitation does not exist.

I do wish that RISC-V had a similar document to ARM/ARM64, in Linux.

> > > +memory.  This is suggested by the kernel documment as it is exceedingly
> 
> %s/documment/document
> 
> > > +unlikely to be overwritten by the kernel itself given other architectural
> > > +constraints.  We then allow for the device tree to be up to 512KiB in size
> > > +before placing the ramdisk in memory.  We then say that everything should be
> > > +within the first 256MiB of memory so that U-Boot can relocate things as needed
> > > +to ensure proper alignment.  We pick 256MiB as our value here because we know
> 
> If the load address ranges occupy the first 256 MiB, where will be the
> space for relocation on a 256 MiB board? Without mentioning initrd_high
> this paragraph is incomplete.

You don't need initrd_high / fdt_high when bootm_size is used.  Maybe
that needs to be clearer in this paragraph and also the rest of the
documentation.

> > > +there are very few platforms on in this family with less memory.  It could be
> > > +as high as 768MiB and still ensure that everything would be visible to the
> > > +kernel, but again we go with what we assume is the safest assumption.
> 
> A section per architecture clearly naming the architecture specific
> restrictions of Linux would be much more helpful.

I did ask for further explained examples as it would indeed be good to
have at least one per architecture.

-- 
Tom
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