[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: Monday, July 03, 2017 12:19 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 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/Documenta
> >>>>>> ti
> >>>>>> 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.

Thanks for clarification, then I will go ahead and make the change as you 
suggested.(convert CONFIG_SYS_DCACHE_OFF to Kconfig and make it mutually
exclusive to CONFIG_EFI_LOADER).

Thanks,
Siva
> 
> 
> Alex