[U-Boot] [PATCH] RFC: am35xx: Rearrange SPL on am35xx

[Simon Glass]

Hi Tom,

On 21 December 2014 at 05:30, Tom Rini <trini at ti.com> wrote:
> On Fri, Dec 19, 2014 at 08:32:35AM -0700, Simon Glass wrote:
>> Hi Tom, Albert,
>>
>> On 19 December 2014 at 07:40, Tom Rini <trini at ti.com> wrote:
>> > On Thu, Dec 18, 2014 at 05:21:21PM -0700, Simon Glass wrote:
>> >
>> >> This is an attempt to tidy up the early SPL code in an attempt to pave
>> >> the way for driver model in SPL:
>> >>
>> >> - Avoid setting up SDRAM before board_init_f()
>> >> - Avoid touching global_data before board_init_f()
>> >> - Allow board_init_f() to set up a new stack (seems that the SRAM stack
>> >> is not large enough on these boards)
>> >>
>> >> This needs more work but it does boot on Beaglebone Black.
>> >>
>> >> Signed-off-by: Simon Glass <sjg at chromium.org>
>> >> ---
>> >>
>> >>  arch/arm/cpu/armv7/am33xx/board.c  | 60 ++++++++++++++++++++++++++------------
>> >>  arch/arm/cpu/armv7/lowlevel_init.S |  4 ---
>> >>  arch/arm/include/asm/spl.h         |  3 ++
>> >>  arch/arm/lib/crt0.S                |  9 ++++++
>> >>  include/configs/ti_armv7_common.h  |  5 ++--
>> >>  5 files changed, 56 insertions(+), 25 deletions(-)
>> >
>> > This takes things in the wrong direction I think.  Since omap3/4/5 have
>> > the same problem we're going to have to duplicate a bunch of this code.
>> > But we can do omap_save_boot_params a bit later I'm pretty sure we can
>> > shove it into spl_board_init() in
>> > arch/arm/cpu/armv7/omap-common/boot-common.c and I'm going to do my best
>> > to do that today and test it on at least a few boards.
>>
>> I don't have a lot of background on SPL stuff as I only know one
>> implementation in detail. So these comments may be a bit off.
>>
>> There seem to be two drivers causing this oddness:
>>
>> 1. The need to save boot params before global_data is available. I
>> wonder if it is possible to avoid overwriting the boot params, and
>> save them later, in board_init_f()? If not, then I don't think the
>> global_data structure should be used. A static local variable in the
>> data section, with just a few fields in it, could be used instead.
>> That avoids the temptation to thing that we are creating a global_data
>> structure before crt0.S does it officially. If the data had just been
>> stored into the data section, without messing with global_data, then I
>> don't think we would have this problem.
>
> This is actually covered today, really.  We do "save_boot_params" first
> thing and that function must save a magic register value from ROM (or
> whatever) to a "stable" magic location in memory.

Ah OK. So long as it doesn't save to global_data we are fine. It could
just have a little structure somewhere in the data segment.

>
>> 2. Need for more stack that can be fitted into SRAM. I think the only
>> sensible option here is to change the stack after board_init_f(). As
>> Albert says this should be done in crt0.S (in fact that's where I put
>> my code). Forcing the dram init to before board_init_f() in SPL seems
>> broken to me.
>
> This is also covered.  The flow is cpu_init_crit -> lowlevel_init (set
> stack to CONFIG_SYS_INIT_SP_ADDR) -> s_init (init DDR) -> _main (set
> stack to CONFIG_SPL_STACK, can be in DDR).

Well s_init is a board-specific thing. What I mean is that crt0.S
should set up an initial stack - there should be no stack before that.
Then board_init_f() uses the initial stack, sets up DRAM, then we move
to a second larger stack before board_init_r() is called.

>
>> I think we should try to have the same flow as U-Boot proper:
>>
>> start.S
>> lowlevel_init (no stack, no global_data, no dram) - can only use
>> 'data' section to write stuff
>> crt0.S (sets up stack and global_data, no dram)
>> board_init_f (sets up dram)
>> relocate stack if required (but not code)
>> board_init_r (running with full stack in dram)
>
> I suppose since old school PowerPC needed i2c to read spd eeproms it's
> possible this ordering could work still today where there's a good bit
> of stuff neeed in order to bring DDR on some ARM platforms.

(Note I am talking just about SPL here, as above)

I suppose what I am saying is that there is no point in setting up
DRAM before board_init_f() in SPL. There is no global_data available,
nor is there any console, so any code that happens before
board_init_f() should be absolutely essential for getting to
board_init_f(). I can't think of much that fits in that category,
maybe some clock things?

Regards,
Simon