[U-Boot] [PATCH v2 23/23] sunxi: A64: add 32-bit SPL support

[Maxime Ripard]

On Mon, Dec 12, 2016 at 04:04:23PM +0000, Andre Przywara wrote:
> Hi,
> 
> On 12/12/16 15:13, Maxime Ripard wrote:
> > On Tue, Dec 06, 2016 at 12:22:59PM +0000, Andre Przywara wrote:
> >> Hi,
> >>
> >> On 06/12/16 11:28, Maxime Ripard wrote:
> >>> On Mon, Dec 05, 2016 at 01:52:30AM +0000, Andre Przywara wrote:
> >>>> When compiling the SPL for the Allwinner A64 in AArch64 mode, we can't
> >>>> use the more compact Thumb2 encoding, which only exists for AArch32
> >>>> code. This makes the SPL rather big, up to a point where any code
> >>>> additions or even a different compiler may easily exceed the 32KB limit
> >>>> that the Allwinner BROM imposes.
> >>>> Introduce a separate, mostly generic sun50i-a64 configuration, which
> >>>> defines the CPU_V7 symbol and thus will create a 32-bit binary using
> >>>> the memory-saving Thumb2 encoding.
> >>>
> >>> "mostly generic". Where do you draw the line? How do you deal with a
> >>> board that would use a different UART? a different MMC? different
> >>> memory configuration.?
> >>
> >> My impression was that it's rather pointless to provide another set of
> >> 32-bit SPL defconfigs for each board again, especially given that for
> >> the SPL's needs the boards so far seem to be very similar.
> >> For the loading part we will probably go with what the BROM already
> >> started: load more data from one of the BROM boot sources, which is
> >> fixed in the SoC and can't be really changed by a board vendor anyway.
> >> Which really leaves the DRAM setup and the UART.
> > 
> > So you plan on enabling all BROM boot sources as well (NAND, SPI) ?
> 
> In fact SPI works already (with little to no changes).

It's the little changes that I'm interested in to be honest :)

> And I don't care about NAND, really ;-)
> Is anyone aware of an A64 board using this?

Well, it's one of the possible boot source, so we have to consider it
and not ignore it entirely hoping that no one will use it, ever.

SPI booting was not used for 5-6 years, until someone started that
trend and now we have a significant number of boards implementing it.

> >> I can't predict the future, but so far those A64 boards look fairly
> >> similar in this respect. So I just avoid having another SPL defconfig
> >> for the BananaPi M64, for instance. I just added MMC_SUNXI_SLOT_EXTRA
> >> because this doesn't hurt on the Pine64, so less churn here.
> >>
> >> So if you know of any board which breaks this assumption, I am happy to
> >> hear about it and see if it can be integrated.
> > 
> > I know at least of one board that uses the UART3 on A33, instead of
> > UART0. The trend is very clear on the A64 and the previous SoCs, but
> > we also had some variations, so we need to take that into
> > account. Which brings me back to my original question, where do you
> > draw the line ? :)
> 
> I don't know, and to make this clear: I see the point in having separate
> configs for the SPL, but due to the 32-bit/64-bit split we probably need
> _two_ sets of defconfigs, which gets pretty messy very quickly.
> Especially given that they are very similar.
> 
> So how do we avoid this? Can we somehow share a defconfig between armv8
> and armv7? In the moment "CONFIG_CPU_V7" and "CONFIG_ARM64" conflict in
> the same file.

I guess the easiest and most robust solution to do this would be to
just generate it from the defconfig. It really feels from your patches
that it's just a matter of sed -i 's/CONFIG_MACH_SUN50I/CONFIG_MACH_SUN50I_32'
on the "real" defconfig.

And we don't have to matter about keeping it in sync, or the board
specific init that might need to be done.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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