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

[Siva Durga Prasad Paladugu]

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

Thanks,
Siva

> 
> 
> Alex