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

[Maxime Ripard]

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) ?

> 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 ? :)

And that would prevent us from doing any kind of DRAM settings
enhancements in the future, every one using the best common
denominator, which seems inefficient.

> Actually I had the idea of eventually going the other way around:
> Providing one U-Boot proper A64 defconfig and let the DT (provided by
> the SPL) sort out the differences. Then we might be able to live with
> separate SPL defconfigs. But that's another patchset and probably quite
> some work.

That would work for MMC and UART (in u-boot, not in the SPL), but not
for the RAM setup.

Maxime

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