linux-kernel - Re: [PATCH] ARM: Keystone: Introduce Kconfig option to compile in typical Keystone features

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Date:	Thu, 02 Jun 2016 00:31:37 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Nishanth Menon <nm@...com>, Russell King <linux@...linux.org.uk>,
	Santosh Shilimkar <ssantosh@...nel.org>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Lokesh Vutla <lokeshvutla@...com>,
	linux-kernel@...r.kernel.org, Tero Kristo <t-kristo@...com>,
	Murali Karicheri <m-karicheri2@...com>,
	Bill Mills <wmills@...com>
Subject: Re: [PATCH] ARM: Keystone: Introduce Kconfig option to compile in typical Keystone features

On Wednesday, June 1, 2016 4:31:54 PM CEST Nishanth Menon wrote:
> Introduce ARCH_KEYSTONE_TYPICAL which is common for all Keystone
> platforms. This is particularly useful when custom optimized defconfig
> builds are created for Keystone architecture platforms.
> 
> An example of the same would be a sample fragment ks_only.cfg:
> http://pastebin.ubuntu.com/16904991/ - This prunes all arch other than
> keystone and any options the other architectures may enable.
> 
> git clean -fdx && git reset --hard && \
>  ./scripts/kconfig/merge_config.sh -m \
>  	./arch/arm/configs/multi_v7_defconfig ~/tmp/ks_only.cfg &&\
>  make olddefconfig
> 
> The above unfortunately will disable options necessary for KS2 boards
> to boot to the bare minimum initramfs.
> 
> Hence the "KEYSTONE_TYPICAL" option is designed similar to commit 8d9166b519fd
> ("omap2/3/4: Add Kconfig option to compile in typical omap features")
> that can be enabled for most boards keystone platforms
> without needing to rediscover these in defconfig all over again -
> examples include multi_v7_defconfig base and optimizations done on top
> of them for keystone platform.

I'd rather remove the option for OMAP as well, it doesn't really fit in with
how we do things for other platforms, and selecting a lot of other Kconfig
symbols tends to cause circular dependencies.

> 
> NOTE: the alternative is to select the configurations under
> ARCH_KEYSTONE. However, that would fail multi_v7 builds on ARM
> variants that dont work with LPAE.

Please no arbitrary selects from the platform.

> Cc: Bill Mills <wmills@...com>
> Cc: Murali Karicheri <m-karicheri2@...com>
> Cc: Grygorii Strashko <grygorii.strashko@...com>
> Cc: Tero Kristo <t-kristo@...com>
> Cc: Lokesh Vutla <lokeshvutla@...com>
> Signed-off-by: Nishanth Menon <nm@...com>
> ---
> 
> Based on: next-20160601
> 
> Tested for basic initramfs boot for K2HK/K2G platforms with the
> http://pastebin.ubuntu.com/16904991/ fragment + multi_v7_defconfig
> 
> Side note on LPAE:
> For our current device tree and u-boot, LPAE is mandatory to bootup
> for current Keystone boards - but this is not a SoC requirement,
> booting without LPAE/HIGHMEM results in non-coherent DDR accesses.

This sounds like a regression, I thought we had this working when
keystone was initially merged and we got both the coherent and
non-coherent mode working with the same DT.

> Currently:
> - U-Boot assumes that lpae is always enabled in kennel and updates the 
> DT memory node with higher addresses. Because of which you are not 
> detecting any memory without lpae and kernel crashed very early, hence 
> no prints. So, make mem_lpae env setting as 0 in U-boot.

We could work around this in the kernel by detecting the faulty u-boot
behavior and fixing up the addresses in an early platform callback.

> - DT also assumes that lpae is always enabled, and always asks for 
> dma-address translation for higher addresses to lower addresses.
> Just delete the "dma-ranges" property or create a one-on-one mapping 
> like dma-ranges = <0x80000000 0x0 0x80000000 0x80000000>

This may be a bit trickier, I think originally keystone ignored the
dma-ranges property and hacked up its own offset by adding a magic
constant to the dma address using a bus notifier. We probably don't
want to bring that hack back, but maybe we can come up with another
solution.

	Arnd