[U-Boot] [PATCH 2/2] sunxi: Enable pre-console buffer

[Ian Campbell]

On Fri, 2015-01-09 at 13:13 +0200, Siarhei Siamashka wrote:
> > If yes then I think it is confusing to modify this comment, and the
> > comment before the pre-console #defines should mention that the buffer
> > is boottime only/short lived etc.
> 
> Just in case if something goes really wrong (in theory it shouldn't,
> but in practice you know...) it is still somewhat safer to keep the
> buffer in its own dedicated area and keep everything else out.

Nothing in those defines is protecting anything though, if the kernel is
more than 15M it will still overwrite that area.

> > Perhaps a better place for the pre-console buffer would be right before
> > the framebuffer (or at the top of RAM if no video on the board), with
> > modifications to bootm_size or not depending on the answer to the
> > original question above.
> 
> If this needs any kind of runtime address calculations instead of
> compile time constants, then IMHO it becomes unnecessarily complicated.

One for Hans I think, my understanding was that the framebuffer was at
the top of RAM, but having bootm_size set to 0xf0000000 unconditionally
doesn't match that. I suppose the idea is that it corresponds with the
smallest board because it's not worth making it dynamic (I think I
recall Hans saying something like that at the time).

I think you could safely put the early buffer at 0xf000000-DELTA (and
adjust bootm_size to match), rather than worrying about packing it up
below the real framebuffer.

> Anyway. The sunxi part of these changes just needs to assign some
> memory area to the pre-console buffer. In the end it does not really
> matter which one. The size also does not need to be too large.
> For example 1920 * 1080 / (8 * 16) = 16200. It means that only ~16K
> of the log buffer can fully cover the FullHD display using the 8x16
> font. And this is even assuming no line breaks. I picked 1M only
> because it was the smallest unit of the address space allocation in
> sunxi-common.h :-)

I don't think it needs to be allocated in 1M chunks, it just happens to
have been arbitrarily chosen that way so far.

If you want to keep the early buffer down in that region then I think
it'd be better to steal a few KB from the end of the fdt, script or pxe
(all of which will never be anywhere near 1MB) than stealing 1M off the
end of the kernel (it's not totally inconceivable that a kernel might be
approaching ~15M in size)

Ian.