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

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Message-ID: <20130604194705.GK599@codeaurora.org>
Date:	Tue, 4 Jun 2013 12:47:05 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	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 06/04, Nicolas Pitre wrote:
> On Mon, 3 Jun 2013, Stephen Boyd wrote:
> 
> > 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've looked at the code and I think that #1 in your initial options is 
> probably best here.  I agree with Russell about #2 being way too complex 
> for only this case.
> 
> So, right before calling into cache_on, you could test if r4 - 16K >= pc 
> and r4 < pc + (_end - .) then skip cache_on.
> 
> Something like this untested patch:

So this would cause the decompression to run without the cache on
if we have to relocate the decompression code to avoid
overwriting ourselves? It seems that the memcpy is fairly quick
on my hardware in comparison to the decompression so moving the
cache_on() call to right before we run decompression keeps things
pretty fast. It's very possible different hardware will have
different results. This is what I meant by option #1. I suppose
we can make it smarter and conditionalize it on if we relocated
or not?

----8<---
diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index fe4d9c3..fcf3ff3 100644
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -182,8 +182,6 @@ not_angel:
                ldr     r4, =zreladdr
 #endif
 
-               bl      cache_on
-
 restart:       adr     r0, LC0
                ldmia   r0, {r1, r2, r3, r6, r10, r11, r12}
                ldr     sp, [r0, #28]
@@ -464,6 +462,7 @@ not_relocated:      mov     r0, #0
                cmp     r2, r3
                blo     1b
 
+               bl      cache_on
 /*
  * The C runtime environment should now be setup sufficiently.
  * Set up some pointers, and start decompressing.

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