Message-ID: <CAGo_u6p7ErrKOMy4Ljg3nehVpd=DOg3XDn6gZ1nrFE_OuOjsWg@mail.gmail.com>
Date:	Tue, 14 Jan 2014 14:56:42 -0600
From:	Nishanth Menon <nm@...com>
To:	Victor Kamensky <victor.kamensky@...aro.org>
Cc:	Linaro Kernel <linaro-kernel@...ts.linaro.org>,
	Russell King <linux@....linux.org.uk>,
	Patch Tracking <patches@...aro.org>,
	Tony Lindgren <tony@...mide.com>,
	Taras Kondratiuk <taras.kondratiuk@...aro.org>,
	open list <linux-kernel@...r.kernel.org>,
	Tero Kristo <t-kristo@...com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Linaro Networking <linaro-networking@...aro.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: OMAP4: sleep: byteswap data for big-endian

On Tue, Jan 14, 2014 at 2:20 PM, Victor Kamensky
<victor.kamensky@...aro.org> wrote:
> On 14 January 2014 09:56, Nishanth Menon <nm@...com> wrote:
>> On Tue, Jan 14, 2014 at 11:35 AM, Victor Kamensky
>> <victor.kamensky@...aro.org> wrote:
>>>
>>> When BE kernel is built Makefile does take of compiling code in BE
>>> mode. I.e all proper flags like -mbig-endian and -Wl,--be8 will be set.
>>
>> Agreed, and I assume you cannot instead switch to LE mode when
>> entering assembly assuming LE?
>
> Yes. Note that this asm interacts with other data in kernel that would
> be in BE form. And after all linker will not allow to put together files
> that were compiled in different endianity.
>
>> The reason I ask this is - most of our development is NOT in BE mode.
>> we will continue to manipulate and add assembly - AM335x, DRA7/OMAP5
>> etc.. and obviously not every code change will indulge in ensuring
>> right markers will be in place.
>>
>> by ensuring readl_relaxed handles the variations, you have ensured
>> that I dont need to care about drivers other than to ensure they use
>> _relaxed variants. in the case of assembly, this does not seem long
>> term manageable.
>
> Yes, I agree if it is outside of main use case it will get broken if not
> attended to. Definitely universe entropy increases :) - if left without
> attention things will decay. Note we admit that even with ARM CPU
> core BE changes similar decay can happen eventually ...that is
> what LNG BE group trying to prevent. I think
> the deal here is that next user of it can/need to fix things if they
> decayed.
>

Personally, I have no idea what "LNG BE" stands for.. I see the
approach we are attempting here is to do any interaction external to
ARM boundary to go through the ARM_BE8 macro.

If we want to make this something we can live with, then the platforms
will have to ensure memory operations external to ARM must be operated
upon by these macros. Instead, an approach that may be used is to
introduce accessors macros that will provide right instruction based
on BE/LE build and BE/LE SoC peripheral behavior.

>>>
>>>> is the idea of BE build meant to deal with having a single BE kernel
>>>> build work for all platforms (including LE ones)?
>>>
>>> Sort of. The idea here to run BE image on OMAP4 chip, with
>>> kernel that would deals with LE periphery correctly, but ARM
>>> core run in BE with special kernel that compiled for BE case (i.e
>>> CONFIG_CPU_BIG_ENDIAN is set).
>>
>> I still dont get the usecase - other than "hey, we do this coz we can
>> do it!".. I mean, yep, it sounds great and all.. but 4 years down the
>> line, is this still going to work? is this going to be interesting
>> careabout? or we are just maintaining additional code for a passing
>> fancy or proof-of-concept?
>
> Valid concern. From LNG BE group point of view it is not "we can do
> it". It is more like we've done it. We have Pandaboard ES running BE
> kernel for a while. It is in LNG BE tree. We used it as development
> and testing vehicle for BE work for a while. We are very grateful to
> the platform for that - it is affordable and easily available! Given,
> beyond ongoing BE testing on Pandaboard in LNG there may not be valid
> use case for further things on OMAP4 BE. We had discussion
> with Santosh Shilimkar from TI during last Linaro connect what to
> do with LNG BE Pandaboard series. Santosh suggested and we
> agreed that we would try to contribute them back to community. And
> that is what Taras is doing. IMHO even though there may not be real

ok.. some sort of Linaro thing about which I have no background about
- but dont really care in this context.

> product use case for BE OMAP4, it could serve in kernel source as good
> example that shows how to work with LE periphery from BE kernel. After
> all, these changes are noop for LE case and they don't introduce
> a lot of clutter: all BE asm rev and rev16 instructions wrapped around
> with ARM_BE8 macro.
>

So, why are we not doing this for *all* OMAP platforms? we try very
hard not to regress on features introduced and wish to ensure all
platforms are equivalent in terms of usage. it makes code sharing a
little harder if we have to ensure that ARM_BE8 is supported in a mix
of reusable codebases.

Personally, here is what I might expect to see:
a) if we are going to force usage of BE and LE build options - then we
*must* be able to reuse code across platforms without having to worry
about breaking platform x etc..
b) this seems like - "take this code in" if something breaks, the guy
who needs it later will come and fix - Shrug, I personally dont buy
that. if something benefits many folks, lets all use it in upstream,
if there is just an exception usage for a short duration which may
impact code scalability and hard to maintain, keep it forked.
c) considering this is non standard usage, some sort of regular test
procedure and owner to ensure this feature is kept sane

With out the above, we are just adding dead code IMHO.

Regards,
Nishanth Menon
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