[coreboot] [RFC]What to do with TINY_BOOTBLOCK?
kyosti.malkki at gmail.com
Mon Oct 24 20:37:18 CEST 2011
On Mon, 2011-10-24 at 10:22 -0600, Marc Jones wrote:
> On Mon, Oct 24, 2011 at 4:15 AM, Patrick Georgi <patrick at georgi-clan.de> wrote:
> > Hi all,
> > as you may be aware, coreboot has two different ROM layouts so far.
> > The older one is derived from what we did before CBFS, and has all code that
> > does RAM init (our "romstage") in the bootblock (up to 64k at the top end of
> > the image). This worked for a long time, but required some hackery for
> > supporting dual-image scenarios (like fallback/normal, where we normal
> > passed control to fallback by jumping to start-8 bytes), and it also broke
> > when AMD's RAM setup became so complicated that it doesn't fit in 64k
> > anymore.
> > Those 64k are mandated by ROM mappings of various chipsets which, by
> > default, only provide access to the upper 64k.
> > The newer one, created after the CBFS switch and exploiting its features,
> > has a tiny bootblock (hence the name), often less than 1k, which implements
> > some policy: By default, it simply looks up "fallback/romstage" in CBFS and
> > executes it. Our other policy does the old fallback/normal routine (using a
> > counter in nvram), but executing files in CBFS as well, instead of jumping
> > into the void and hoping that there's code there.
> > The problem with the new approach is that it requires full ROM mapping
> > rather early. Boards whose romstage fit in the 64k were free to defer
> > setting up mapping to whereever it is convenient inside the romstage, so
> > it's not all that easy to identify without means to test it. Unfortunately,
> > this is a runtime problem, not a build problem, so it's hard to test all our
> > 160 boards. For this reason, we kept both mechanisms in the tree, under the
> > monikers BIG_BOOTBLOCK and TINY_BOOTBLOCK.
> > Some chipsets that are in common use were converted rather early, so by now,
> > 100 boards use tiny bootblock, while 60 use the old method.
> > Since then - not much happened.
> > Kyösti Mälkki recently brought this issue up again (thanks!), and proposes
> > to invert the flags, making tiny bootblock the default, so "big" bootblock
> > has to be requested explicitely and also adding some "maybe" flag for boards
> > that might just work. This is quite a large change, but I fear it'll bring
> > relatively little progress - people will just copy the TINY_NO_BOOTBLOCK (or
> > what it's called in the latest patch iteration) flag and move on.
> > Therefore, I propose (http://review.coreboot.org/#change,320) to get rid of
> > the "big bootblock" variant altogether. This might break some boards
> > (silently: they still build, but they fail on boot), but at least it forces
> > action to fix them.
> > Advantages:
> > - one flag less to care about
> > - more uniform feature set (big bootblock didn't support any fallback
> > mechanism)
> > - more opportunities to clean out and simplify the build system and code -
> > there are some crude workarounds to make both mechanisms work
> > Disadvantages:
> > - Boards might be broken for a long time until someone tries them again. The
> > visible result is that the boot fails early (ie. no error signalling at all,
> > the system simply hangs, nothing visible).
> > It's possible to determine all boards that _might_ be affected (those that
> > use a big bootblock now), so I could add that list to the commit message,
> > hopefully helping whoever stumbles over this issue.
> > Comments?
> Hi Patrick,
> I think that this makes sense. It seems like the change would improve
> the build and standardize early coreboot. I think that we can support
> developers in the porting for those platforms when they come up. The
> ROM decode is a typically a southbridge setting, so do you know what
> southbridges would be untested?
I am glad this topic brings up discussion.
List of mainboards that currently do not select TINY_BOOTBLOCK
and do not select ROMCC in Kconfig:
List of southbridges on those mainboards, as taken from the
It appears all but the two below already have an implementation with
tiny bootblock on some other mainboard:
It may be that the only reason BIG_BOOTBLOCK gets dragged along, is that
there has been no clear migration path away from it. I think there now
exists a better choice, pushing the choice in menuconfig.
As a side note. I did not realise it was possible to have TINY_BOOTBLOCK
without Cache-As-Ram. There was no such implementation (besides QEMU).
I thought that tiny bootblock would also move memory controller
initialisation away from romstage (so it could be compiled with gcc) and
then utilise cache for its stack.
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