[U-Boot] Making U-Boot smaller

[Tom Rini]

On Tue, May 21, 2019 at 09:32:29PM +0200, Marek Vasut wrote:
> On 5/21/19 6:56 PM, Jagan Teki wrote:
> > On Tue, May 21, 2019 at 10:14 PM Simon Glass <sjg at chromium.org> wrote:
> >>
> >> Hi,
> >>
> >> (moved from thread "U-Boot PXA support")
> >>
> >> We have of-platdata, which produces C data from the DT, for linking
> >> into U-Boot. It saves libfdt and DT space. But we still have the DM
> >> overhead.
> >>
> >> We have binman which can insert values into the binary after
> >> link-time. This is barely used at present, only for accessing the
> >> location of things in flash.
> >>
> >> Another thing is that every little tweak and feature adds a few bytes
> >> and there are dozens of them in each release. It would be interesting
> >> to build a board from 10 years ago (like PXA) and see where the growth
> >> is. My bet is that we could add Kconfig options to disable extra
> >> features (and enhancements of features) and make quite a difference.
> >>
> >> For DM, I think it would be interesting to revisit and compare against
> >> the initial release, and see if some features could be made optional
> >> in SPL.
> >>
> >> Finally I feel we could implement a single-device API for where
> >> CONFIG_SPL_DM is not set. We could use the debug UART for serial, a
> >> single instance of tiny MMC for MMC, etc.
> > 
> > This is what I'm looking for quite sometime, a tiny MMC which would
> > bypass the mmc stack and do the possible stuff in SPL, since we don't
> > have any option to use full DM in SPL (specifically for Allwinner 64
> > SoC's). API that would atleast compatible with DM with small
> > foot-print would help.
> 
> It's been in mainline for a long time, MMC_TINY. Creator CI20 uses it.

I want to start by saying I'm not criticizing MMC_TINY.  But I think
this highlights part of the problem of "lets do something that's not the
normal framework".  Developers come up with a one-off, do their best to
make others know about it, and then a year later when someone else has a
similar problem, they may or may not stumble into the alternate path
fix.

So, getting back to specifics, what would it take to do both:
- Make MMC_TINY abstract enough sunxi or TI or ... could plug-in and use
  this for the "we just need MMC read, ROM probably already did enough
  init for us for this"
- NOT blow up CI20 which I know is super tight on space.

-- 
Tom
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