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

[Siarhei Siamashka]

On Sat, 10 Jan 2015 08:24:52 -0700
Simon Glass <sjg at chromium.org> wrote:

> Hi,
> 
> On 10 January 2015 at 03:50, Ian Campbell <ijc at hellion.org.uk> wrote:
> > 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)
> 
> I don't think it is a good idea to use the 'pre-console buffer' after
> the console exists. It is a misnomer.

If one is fixated on the idea that "console" == "UART serial console",
then yes, keeping the pre-console buffer after the UART console
is initialized looks like a misnomer. However there are other
consoles too. And in the case of tablets, the "LCD console" may
be the only console that is (reasonably easily) accessible. IMHO,
the "pre-vga console buffer" interpretation is just as valid as the
"pre-serial console buffer".

Again, if we look at the pre-console buffer original idea. It just
does initial logging in the memory buffer because the serial console
becomes available relatively late and needs somewhat complicated
initialization (complicated compared to trivial logging in the memory
buffer). The vga console is in principle exactly the same, except that
it brings "late" and "complicated" to the entirely new level.

I understand that the pre-console buffer had been introduced to solve
one particular problem. But giving it a fresh look, we can see that it
already works perfectly fine for solving other problems too.
 
> Also, the reason that the pre-console buffer has pre-allocated memory
> is to work around the lack of memory allocation before relocation. Now
> that we have initf_mallloc() called very early in boot we could
> consider allocating the space instead.

My understanding is that one of the important reasons to have this
buffer at a known predictable memory location is to simplify JTAG
debugging.

> I think this patch is a good feature to implement, but I agree with
> others that hard-coded memory locations for U-Boot features should not
> exist except in exceptional circumstances (e.g. very early boot).

The platform code is in a perfect control of its address space. It can
make decisions about how much RAM to use (for example, assume that
256MB is the minimum possible amount of RAM on sunxi hardware), chop
off some amount of RAM for simplefb, etc. We are just having a hair
splitting argument with Ian about the suitable location and size of
the pre-console buffer. But no problem exists there in principle.

Doing the fixed memory location reservation provides absolutely the
best console log buffer. No malloc, UART hardware or anything else
is required. You can start logging right away. This could possibly
be enough to justify the exception.

Also because of assigning a fixed location to the pre-console buffer,
it is trivially easy to additionally extract and prepend the log from
SPL with really minimal changes. I can submit an updated patch with SPL
log extraction support just to demonstrate how it works.

> Re passing the U-Boot console to the kernel, see as an example the
> cbmem_console.c driver. It only works on x86 at present and only with
> coreboot.

Cool. It is good to know that this is already at least partially
solved. What makes it x86 and coreboot specific? Is it difficult
to make this usable on all platforms?

> It works as a stdio driver, so skips the first part of U-Boot's output.

By applying my pre-console buffer tweak and enabling pre-console in
the platform header file, the first part of U-Boot's output should be
available for cbmem_console too.

> So I suggest:
> 
> 1. Remove the pre-console address and just have a size. Allocate the
> space after initf_malloc(). Store the details (buffer start, size and
> current pointer) in global_data
> 2. Add a general mechanism to record the console into a buffer by
> renaming and adjusting the existing code. It can then be set up
> pre-console, post-console but pre-stdio, and then post-stdio for
> recording what is passed to Linux.

If this means getting rid of the fixed address for the pre-console
buffer reservation entirely, then we lose the ability to log before
malloc, JTAG debugging becomes more complicated and extracting the SPL
log is also not easy anymore. Or am I missing something?

If we keep the fixed pre-console buffer reservation, but migrate the
pre-console buffer into a new malloc allocated entity as soon as
malloc becomes available, then what do we really gain by doing this?

-- 
Best regards,
Siarhei Siamashka