linux-kernel - Re: [PATCH] mtd: only use __xipram annotation when XIP

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Message-ID: <20160305000225.GB55664@google.com>
Date:	Fri, 4 Mar 2016 16:02:25 -0800
From:	Brian Norris <computersforpeace@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	David Woodhouse <dwmw2@...radead.org>,
	linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org
Subject: Re: [PATCH] mtd: only use __xipram annotation when XIP_KERNEL is set

Hi Arnd,

I know you're travelling, but...

On Mon, Jan 25, 2016 at 04:41:50PM +0100, Arnd Bergmann wrote:
> When XIP_KERNEL is enabled, some functions are defined in the .data
> ELF section because we require them to be in RAM whenever we communicate
> with the flash chip. However this causes problems when FTRACE is
> enabled and gcc emits calls to __gnu_mcount_nc in the function
> prolog:
> 
> drivers/built-in.o: In function `cfi_chip_setup':
> :(.data+0x272fc): relocation truncated to fit: R_ARM_CALL against symbol `__gnu_mcount_nc' defined in .text section in arch/arm/kernel/built-in.o
> drivers/built-in.o: In function `cfi_probe_chip':
> :(.data+0x27de8): relocation truncated to fit: R_ARM_CALL against symbol `__gnu_mcount_nc' defined in .text section in arch/arm/kernel/built-in.o
> /tmp/ccY172rP.s: Assembler messages:
> /tmp/ccY172rP.s:70: Warning: ignoring changed section attributes for .data
> /tmp/ccY172rP.s: Error: 1 warning, treating warnings as errors
> make[5]: *** [drivers/mtd/chips/cfi_probe.o] Error 1
> /tmp/ccK4rjeO.s: Assembler messages:
> /tmp/ccK4rjeO.s:421: Warning: ignoring changed section attributes for .data
> /tmp/ccK4rjeO.s: Error: 1 warning, treating warnings as errors
> make[5]: *** [drivers/mtd/chips/cfi_util.o] Error 1
> /tmp/ccUvhCYR.s: Assembler messages:
> /tmp/ccUvhCYR.s:1895: Warning: ignoring changed section attributes for .data
> /tmp/ccUvhCYR.s: Error: 1 warning, treating warnings as errors

Can you provide a sample .config that DOES build correctly with
XIP_KERNEL enabled + this patch? My first attempt yields some other
failures I don't care to fixup right now...

Anyway, I don't doubt you have a good fix here, so I can probably take
it. Any review from others would be welcome though.

> Specifically, this does not work because the .data section is not
> marked executable, which leads LD to not generate trampolines for
> long calls.
> 
> This moves the __xipram functions into their own .xiptext section instead.
> The section is still placed next to .data and located in RAM but is marked
> executable, which avoids the build errors.
> 
> Also, we only need to place the XIP functions into a separate section
> if both CONFIG_XIP_KERNEL and CONFIG_MTD_XIP are set: When only MTD_XIP
> is used, the whole kernel is still in RAM and we do not need to worry
> about pulling out the rug under it. When only XIP_KERNEL but not MTD_XIP
> is set, the kernel is in some form of ROM, but we never write to it.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Brian