Re: [PATCH] Add warning above get_ram_size

[Sascha Hauer <s.hauer@pengutronix.de>]

On Thu, Feb 14, 2013 at 02:59:48PM +0100, Maxime Ripard wrote:
> Hi Sascha,
> 
> Le 14/02/2013 12:35, Sascha Hauer a écrit :
> > On Thu, Feb 14, 2013 at 10:40:38AM +0100, Maxime Ripard wrote:
> >> Hi Sascha,
> >>
> >> Le 13/02/2013 18:16, Sascha Hauer a écrit :
> >>> Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
> >>> ---
> >>>  common/memsize.c |    3 +++
> >>>  1 file changed, 3 insertions(+)
> >>>
> >>> diff --git a/common/memsize.c b/common/memsize.c
> >>> index d149e41..ef6381b 100644
> >>> --- a/common/memsize.c
> >>> +++ b/common/memsize.c
> >>> @@ -33,6 +33,9 @@
> >>>   * Check memory range for valid RAM. A simple memory test determines
> >>>   * the actually available RAM size between addresses `base' and
> >>>   * `base + maxsize'.
> >>> + *
> >>> + * This function modifies the RAM. Do not use it if you're running from
> >>> + * the RAM you are going to detect!
> >>>   */
> >>
> >> Actually, I don't see how it modifies the RAM, at least permanently. The
> >> values it erase are backed up, and there's no concurrency at barebox
> >> level, so we are sure that the value saved will still be the one that
> >> would need to be backed up at the end of the function, right?
> > 
> > Yes, it restores the values, but how do you make sure the function does
> > not modify the instructions you are currently executing? You need bad
> > luck to hit this, but sooner or later this will happen.
> 
> Ah, yes, this would be nasty indeed. Is there a way to know the end
> address of barebox into RAM ? or the address it has been loaded to and
> the size of its binary, so that we can just check the part that doesn't
> hold barebox?

See include/asm-generic/sections.h. You have to avoid modifying
everything between _text and __bss_stop. I haven't looked how exactly
get_dram_size works. Normally this function would have to test every
location at a power of 2, that would be:

1 2 4 ... 64MiB 128MiB

It seems you have to make sure that your binary does not cross a power
of 2 boundary, then you should be safe.

Sascha

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