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

From: Sascha Hauer <s.hauer@pengutronix.de>
To: Andrey Smirnov <andrew.smirnov@gmail.com>
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: Thu, 1 Oct 2015 08:22:32 +0200	[thread overview]
Message-ID: <20151001062232.GH7858@pengutronix.de> (raw)
In-Reply-To: <CAHQ1cqG311Dxq9A1wOOzu0WGJ8YcWnFJZ67Nf_3R6HPTGN7S9A@mail.gmail.com>

On Wed, Sep 30, 2015 at 10:56:20AM -0700, Andrey Smirnov wrote:
> >> 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.

Ok, I see. I think your original patch is almost fine, only the test if
you need to copy or not needs adjustment. You have to test if the fdt is
membase < dtb < membase + memsize. If it is you have to copy it,
otherwise it should be fine to use it in place. Would that work?

Sascha

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