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

[Alexander Graf]

On 03.07.17 08:29, Siva Durga Prasad Paladugu wrote:
> Hi Alex,
> 
>> -----Original Message-----
>> From: Alexander Graf [mailto:agraf at suse.de]
>> Sent: Thursday, June 29, 2017 7:34 PM
>> To: Siva Durga Prasad Paladugu <sivadur at xilinx.com>; Michal Simek
>> <michal.simek at xilinx.com>
>> Cc: u-boot at lists.denx.de; Tom Rini <trini at konsulko.com>; Simon Glass
>> <sjg at chromium.org>
>> Subject: Re: [U-Boot] [PATCH] arch: armv8: Remove the error when dcache is
>> off
>>
>>
>>
>> On 29.06.17 15:44, Siva Durga Prasad Paladugu wrote:
>>> Hi,
>>>
>>>> -----Original Message-----
>>>> From: Alexander Graf [mailto:agraf at suse.de]
>>>> Sent: Tuesday, June 27, 2017 6:08 PM
>>>> To: Michal Simek <michal.simek at xilinx.com>
>>>> Cc: Siva Durga Prasad Paladugu <sivadur at xilinx.com>;
>>>> u-boot at lists.denx.de; Tom Rini <trini at konsulko.com>; Simon Glass
>>>> <sjg at chromium.org>; Siva Durga Prasad Paladugu <sivadur at xilinx.com>
>>>> Subject: Re: [U-Boot] [PATCH] arch: armv8: Remove the error when
>>>> dcache is off
>>>>
>>>>
>>>>
>>>>> Am 27.06.2017 um 13:52 schrieb Michal Simek
>> <michal.simek at xilinx.com>:
>>>>>
>>>>>> 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/Documentati
>>>>>> on
>>>>>> /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 :).
>>>
>>> I can see that, we are anyway avoiding that error message for SPL
>>> BUILD where dcache is off.
>>> Also, dcache will be ON by default, until and unless user disable it
>>> through CONFIG_SYS_DCACHE_OFF.
>>>
>>> -mstrict-align is already used for armv8 in
>>> arch/arm/cpu/armv8/config.mk
>>>
>>> Finally I feel, we can remove this check or atleast we should make it as
>> #warning instead of #error.
>>
>> I put it in because UEFI binaries can assume that unaligned instructions
>> work. So please at least convert CONFIG_SYS_DCACHE_OFF to Kconfig and
>> make it mutually exclusive to CONFIG_EFI_LOADER.
> 
> I got your point but, Iam bit concerned to do this change as it may break for someone
> if they are using both DCACHE_OFF and EFI_LOADER, Isn't it?

Today you can not use DCACHE_OFF at all, so I don't think it'd be a 
regression ;). I want to make sure that we prohibit the combination on 
the Kconfig level already to make sure that we do not break anyone using 
DCACHE_OFF if we start to allow it.

> I don’t have idea on UEFI side, all assume unaligned instructions work or it is configurable when building?

With UEFI you run 3rd party binaries in U-Boot context. The UEFI spec 
explicitly states that unaligned accesses are valid. So because we by 
definition don't control the payloads, we can not break unaligned 
accesses which DCACHE_OFF does.


Alex