linux-kernel - Re: [PATCH V2 22/22] MIPS: Add multiplatform BMIPS target

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Date:	Tue, 18 Nov 2014 11:19:51 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Kevin Cernekee <cernekee@...il.com>
Cc:	Ralf Baechle <ralf@...ux-mips.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Jon Fraser <jfraser@...adcom.com>,
	Dmitry Torokhov <dtor@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jonas Gorski <jonas.gorski@...il.com>
Subject: Re: [PATCH V2 22/22] MIPS: Add multiplatform BMIPS target

On Monday 17 November 2014 13:21:45 Kevin Cernekee wrote:
> On Mon, Nov 17, 2014 at 12:40 PM, Arnd Bergmann <arnd@...db.de> wrote:
> >> One possible complication: for BCM63xx/BCM7xxx (MIPS) there is no
> >> decompressor in the kernel.  The firmware loads an ELF image into
> >> memory and jumps directly to kernel_entry.
> >>
> >
> > Right, that complicates it a bit, but is there a reason why a decompressor
> > would be hard to do, or would be considered a bad thing?
> > There is already generic decompressor code in arch/mips/boot/compressed/
> > that I would assume you could use without firmware changes. Are you
> > worried about boot time overhead?
> 
> Currently the bootloader is responsible for decompressing the image.
> On STB the bootloader typically loads a gzipped ELF file; on DSL/CM
> the bootloader unpacks a custom image format containing an
> LZMA-compressed kernel in some form.  So we would be
> double-compressing the same kernel in this scheme.
> 
> STB/DSL should be able to boot the arch/mips/boot/compressed "vmlinuz"
> ELF file; I tested STB.  CM might be questionable, but doesn't need
> decompressor mods because the bootloader is DT-aware.
> 
> Also, the decompressor may need to be modified so that it recognizes /
> passes / doesn't overwrite DTB blobs coming from the bootloader.  And
> to make sure it doesn't stomp on any of the code or data that our
> bootloaders use for their callback mechanisms.
> 
> So, one possibility is to submit a V3 patch which allows 0 or 1 DTB
> files to be compiled in statically (similar to
> CONFIG_ARM_APPENDED_DTB) and requires a DT-aware bootloader or
> decompressor for anything else.  Any opinions?

That sounds like a good approach, in particular with the patch that
Jonas linked to. With that, most or all of your arch/mips/bmips/setup.c
should become completely generic, and I would think the rest can
be handled through a function that is called after looking up the
root "compatible" property in DT.

	Arnd
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