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

[Siva Durga Prasad Paladugu]

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/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 :).

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.

Thanks,
Siva.

> >
> > 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.
> 
> If you configure your section boundaries on natural alignments (1GB IIRC),
> you should get away with quite few tables.
> 
> Alex
>