linux-kernel - Re: [PATCH] ARM: avoid mis-detecting some V7 cores in the decompressor

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Date:	Mon, 03 Jun 2013 15:59:53 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Brian Swetland <swetland@...gle.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] ARM: avoid mis-detecting some V7 cores in the decompressor

On 06/03/13 15:45, Russell King - ARM Linux wrote:
> On Mon, Jun 03, 2013 at 03:37:39PM -0700, Stephen Boyd wrote:
>> In my case I'm booting a kernel with textoffset = 0x208000 but RAM
>> starts at 0x0. Does "minimum of RAM start" mean 0x0 or 0x200000?
> The basic requirement for zImage's is no less than the start of RAM
> plus 32K.  Or let me put it another way - start of writable memory
> plus 32K.
>
> Whether you need an offset of 0x200000 or not is not for the
> decompressor to know.  If you're having to avoid the first 0x200000
> bytes of memory for some reason (eg, secure firmware or DSP needs
> it left free) then there's no way for the decompressor to know that,
> so it's irrelevant.
>
> So, lets say that your platform has a DSP which needs the first 0x200000
> bytes left free.  So the boot loader _already_ needs to know to load
> the image not at zero, but above 0x200000.  The additional 32K
> requirement is really nothing new and so should be treated in just the
> same way.
>
> Leave at least 32K of usable memory below the zImage at all times.

Understood. On my device writeable RAM actually starts at 0x0 but I have
compiled in support for devices which don't have writeable memory at
0x0, instead they have writeable memory starting at 0x200000. Because I
have a kernel supporting more than one device with differing memory
layouts I run into this problem. The same problem will occur to any
devices in the multi-platform kernel when a device with unwriteable
memory near the bottom (such as MSM8960) joins the multi-platform defconfig.

Let me try to word it in your example. I have compiled in support for a
platform that has a DSP which needs the first 0x200000 bytes left free.
I have also compiled in support for a platform that doesn't have this
requirement. I plan to run the zImage on the second platform (the one
without the DSP requirement). The bootloader I'm running this zImage on
has no idea that I've compiled in support for the other platform with
the DSP requirement so it assumes it can load the zImage at the start of
RAM (0x0) plus 32K. This is bad because then the page tables get written
into my compressed data and it fails to decompress.

I believe you think the bootloader should know about this, but I can't
tell how it could know.

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation

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