[U-Boot] [PATCH 1/3] dm: Add migration plan for CONFIG_BLK

[André Przywara]

Hi,

On 01/04/18 14:19, Tom Rini wrote:
> On Tue, Mar 27, 2018 at 11:34:19PM +0530, Jagan Teki wrote:
>> On Mon, Sep 4, 2017 at 9:57 PM,  <sjg at google.com> wrote:
>>> Hi Tom,
>>>
>>> On 7 August 2017 at 09:39, Tom Rini <trini at konsulko.com> wrote:
>>>> On Sat, Aug 05, 2017 at 03:45:53PM -0600, Simon Glass wrote:
>>>>
>>>>> The CONFIG_BLK conversion involves quite invasive changes in the U-Boot
>>>>> code, with #ifdefs and different code paths. We should try to move over to
>>>>> this soon so we can drop the old code.
>>
>> I hope this will applicable to SPL too?
>>
>> If so, we are having SPL size issues with few Allwinner families, if
>> enable SPL_DM any suggestions?
> 
> How close, and have you looked at the u-boot-spl.map to see what you can
> maybe trim?  Or areas to look at reducing in code complexity?

The Boot ROM limit for all Allwinner SoCs known so far is 32KB. The A64
SPL (AArch64) stands at ~31KB at the moment. Yes, we went over the map
and picked most low hanging fruits already.
So far we discussed several mitigations, but mostly to cover the
"natural" SPL code size grow over time:
1) The AArch64 exception vectors take 1KB, plus an unnecessary ~1.6KB of
padding (for a 2KB architectural alignment). Given that the vectors are
used only for debugging purposes, we could scrap them entirely or
construct them on the fly in some other SRAM. So would free about 2.5KB,
ideally. Lowest hanging fruit so far.
2) We can compile the SPL in AArch32 mode, which can use the Thumb2
encoding. This reduces the size significantly, to about 20KB. The
disadvantage is using a second cross-compiler or even a additional
cross-compiler for native builds, complicating the build process.
I maintain a branch for enabling FEL booting here [1], which provides
two _defconfigs (one 32-bit for SPL, one 64-bit for U-Boot proper).
There are no technical disadvantages in running the SPL in 32-bit, so
this is mostly a build issue.
3) Try to use ILP32 for the AArch64 SPL build. This reduces the pointer
size and sizeof(long) to be 32-bit and should help, though I haven't
been able to successfully compile it yet (relocation types problems).
Despite lacking mainline support for AArch64 ILP32 in Linux and
glibc(?), GCC supports it for quite a while already. Unknown saving effect.
4) Use runtime decompression. Most SoCs have larger or more SRAM than
the 32KB, so we could leverage this. Siarhei knows more about this.
5) Use a TPL. Haven't looked at this in detail yet.

So 1) would be the easiest to pursue, but 2.5KB are not enough to offset
the >10 KB toll the DM_SPL support actually takes.

Cheers,
Andre.