[U-Boot] [PATCH 0/5] sunxi: env: Load environment from boot media

[Maxime Ripard]

Hi!

On Tue, Jun 11, 2019 at 10:28:19AM -0400, Tom Rini wrote:
> On Tue, Jun 11, 2019 at 11:37:28AM +0200, Maxime Ripard wrote:
> > On Mon, Jun 10, 2019 at 10:11:39AM +0100, Andre Przywara wrote:
> > > On Mon, 10 Jun 2019 10:30:37 +0200
> > > Maxime Ripard <maxime.ripard at bootlin.com> wrote:
> > >
> > > Hi Maxime,
> > >
> > > thanks for having a look!
> > >
> > > > On Sat, Jun 08, 2019 at 02:26:53AM +0100, Andre Przywara wrote:
> > > > > At the moment we need to configure the place where U-Boot tries to load
> > > > > its environment from at compile time. This is not only inflexible, but
> > > > > also unnecessary, as we have easy access to the boot source.
> > > > >
> > > > > This series prepares U-Boot on Allwinner boards to load the environment
> > > > > from the same media where the SPL and U-Boot proper were loaded from.
> > > > > This allows to keep one firmware binary, and copy it to an SD card,
> > > > > eMMC or even SPI flash, without needing to configure it differently.
> > > >
> > > > This does change a couple of things though. The environment used to be
> > > > loaded always from the same source, no matter the boot device. This
> > > > means that if you would set an SD card, you would get the environment
> > > > from the eMMC. Same thing for FEL. This is no longer the case.
> > > >
> > > > I don't know whether it's a good or a bad thing, but it should be
> > > > mentionned.
> > >
> > > This is true, I failed to mention that.
> > >
> > > To start a discussion on this:
> > > I consider the current (fixed location) behaviour somewhat surprising and
> > > limiting, and couldn't find a real use case where this would be required.
> > > Happy to hear of one!
> > > Instead I thought about those cases:
> > > - There is some botched U-Boot plus environment on the eMMC. You want to
> > > boot from SD card to have a clean start, possibly to fix it. But it will
> > > load the possibly outdated, broken or even unrelated environment from eMMC.
> >
> > This one might be a feature though. Being able to restore / fix an
> > environment in the eMMC running from an SD card has save me a couple
> > of times. Or booting from the SD card because the U-Boot on the eMMC
> > is broken, while the environment is working.
> >
> > > - You want to boot from SD card without touching the eMMC at all. Saving
> > > the environment will spoil that.
> >
> > But it goes against that one, which might be more important / sensible.
> >
> > > - You want to have one image for all possible boot media.
> >
> > That won't happen, only because NAND is a thing.
>
> Some of this is perhaps an argument for adding a sub-command to specify
> where the environment is to be read from.  Heuristics are still only a
> best guess and won't get it right every time.

I was mostly talking about the distribution of the image itself. While
eMMC, SD and SPI flash can be made to take the same image, NAND will
require a particular ECC and randomizer setup that requires that it's
bundled separately.

> > And even then, I'm not really sure that it's a good thing. A U-Boot
> > build these days is roughly in the same sizes than a stripped down
> > Linux image. For an inferior solution in pretty much every aspect.
>
> Hey now.  We aren't _quite_ that large.  And we are (really!) trying to
> find a happy medium between "distros want X/Y/Z for everyone" and "can
> we commonly get back to UNDER 512kB maybe?  Please?".

I'm exagerating a little, but barely. A current build for the SoC
Andre was mentionning takes 600kB. And since the trend has been for
the binaries to grow for quite some time now, I'm pretty sure everyone
will reserve 1MB just to be sure they have some room to spare.

Now, getting a kernel image to fit in a MB takes a bit of time, but
it's not really impossible to achieve. And if you can reclaim that MB
dedicated to U-Boot, it's actually fairly easy to do.

I've tried to have the size reduced at some time because we were
corrupting our U-Boot binary as soon as we where using saveenv (which
is probably one of the worst issue we could have), and it ended up
with everyone agreeing that we needed to reduce the size, just not the
one they were using.

It's kind of weird that people haven't yet embraced defconfig as they
are in Linux, where every distro has its own configuration, and it's
perfectly fine.

Maxime

--
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
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