[U-Boot] [PATCH] arch: armv8: Remove the error when dcache is off

[Alexander Graf]

On 27.06.17 13:52, Michal Simek wrote:
> On 27.6.2017 13:46, Alexander Graf wrote:
>>
>>
>> On 27.06.17 13:20, Michal Simek wrote:
>>> Hi,
>>>
>>> On 27.6.2017 13:01, Alexander Graf wrote:
>>>> I don't think that's going to work - at least not without compiler flag
>>>> changes. By default, gcc will happily generate unaligned accesses. If
>>>> you disable dcache, these will trap.
>>>
>>> What's that compiler flags we should be using to avoid that?
>>
>> It's a combination of
>>
>>    -mstrict-align
>>
>> plus crossing fingers with lots of praying plus making sure that every
>> code you call also follows -mstrict-align plus double-checking that you
>> don't break the kernel booting ABI:
>>
>>
>> http://elixir.free-electrons.com/linux/v4.12-rc7/source/Documentation/arm64/booting.txt
>>
>>
>> In the booti case, disabling dcache seems to be legitimate. In the
>> bootefi case however, it's not.
> 
> Non wants to boot the kernel. It is really about programming stuff.
> 
>>
>> So you will also need to set CONFIG_EFI_LOADER to depend on
>> !CONFIG_SYS_DCACHE_OFF which means you will want to convert
>> CONFIG_SYS_DCACHE_OFF to Kconfig first :).
> 
> ok. I will let Siva to do it just wanted to refresh this topic.
> 
> 
>>> The reason for this change is to have really small u-boot which fits to
>>> OCM without DDR to be able to do initial programming.
>>
>> Yup, makes sense. I'm just slightly scared by the idea :).
> 
> The same stuff we did on Zynq in past.
> I have never had enough time to look at that MMU mapping why it is so
> huge. Maybe reduce size of that tables or using different page size is
> better way to go.

Actually thinking about this one again. I'm fairly sure you could 
provide page-aligned gigabyte page maps in .rodata manually for your 
board in your specific configuration. The mmu setup function is weak, so 
you can simply override it and have it point the mmu to your 
pregenerated page table.

With that, the overhead of having caches on shouldn't be too much. 
Especially if you can make the page table aligned, but not padded to 4k.


Alex