Re: [PATCH 2/2] arm/cpu/start.c: Avoid copying device-tree when possible

From: Andrey Smirnov <andrew.smirnov@gmail.com>
To: Sascha Hauer <s.hauer@pengutronix.de>
Cc: "barebox@lists.infradead.org" <barebox@lists.infradead.org>
Subject: Re: [PATCH 2/2] arm/cpu/start.c: Avoid copying device-tree when possible
Date: Wed, 30 Sep 2015 10:56:20 -0700	[thread overview]
Message-ID: <CAHQ1cqG311Dxq9A1wOOzu0WGJ8YcWnFJZ67Nf_3R6HPTGN7S9A@mail.gmail.com> (raw)
In-Reply-To: <20150930070016.GY7858@pengutronix.de>

>> I'd still like to discuss the possibility of introducing a feature
>> like that to the codebase. Right now I have a use-case where I use
>> Barebox as a DDR memory tuning/testing tool on i.MX6Q where I upload
>> the image to IRAM via JTAG and execute Barebox straight out of SRAM.
>
> I understand your usecase and I think it's worth supporting it.
>
> So what are our options? You could run the tuning/testing completely
> from the PBL. We now have console support in the PBL, so you can output
> the results. You cannot do any interactive things though. We could add
> simple getc() support to the PBL, but something like a shell is out of
> reach. Do you need interactive input anyway?
> Another possibility would be to make device tree support optional for
> i.MX6. It is optional for the other i.MXes for historic reasons, so we
> could make it optional for i.MX6 aswell. This would give you another
> ~30K which is now used by the dtb.
> I'm a bit afraid that the regular-barebox-in-SRAM usecase will break
> quite frequently upstream because the image gets too big or simply
> because some other changes have side effects. For this reason I would
> really prefer the PBL way if that's possible for you.
>

Oh, I don't think I mentioned in my previous e-mail, but I do have a
working Barebox image for that case. The way I have it implemented
right now is a vanilla, single board, no-PBL, no-relocation, i.MX6Q
SabreSD Barebox image with a minimal configuration. The only things I
had to change was device tree file -- default required to much RAM to
instantiate, so I had to trim it down -- and this patch to avoid
copying FTD that is already built-in. Oh, and I also had to disable
MMU, because page table takes about 1MB or RAM(I haven't had a chance
to spend any time trying to modify MMU code to support coarser
1MB-page page table).

The image is intended to be used by EEs to do DRAM related
experiments, so I do need a shell and that was the reason I went with
full Barebox instead of trying to cram it in PBL.

We certainly can make PBL work for that use-case -- and if you recall
I already submitted getc() implementation for it once -- but the
functionality available there is very limited(and as you mentioned
full shell would be out of reach) and everything we do there would
most likely be a code duplication, so I'd rather not go that route.

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