[PATCH v2] ARM: realview: basic device tree implementation

On Thursday 22 May 2014 14:39:20 Nicolas Pitre wrote:

On Thu, 22 May 2014, Arnd Bergmann wrote:

quoted

On Thursday 22 May 2014 14:09:52 Nicolas Pitre wrote:

quoted

On Thu, 22 May 2014, Arnd Bergmann wrote:

quoted

On Thursday 22 May 2014 15:45:03 Catalin Marinas wrote:

quoted

By making the DMA memory Strongly Ordered, we avoid the
L220 write buffer. In addition to the above commit, we also had
2503a5ecd86c (ARM: 6201/1: RealView: Do not use outer_sync() on
ARM11MPCore boards with L220).

Ah, so this also stands in the way of multiplatform realview 

Isn't the real value of a multiplatform realview in the ability to use 
it with Qemu?  Did Qemu go to the extreme of emulating all those cache 
issues?

The qemu developers have in the past always argued strongly that they
want to run the same kernel that runs on real hardware. Having a kernel
that runs only on the emulated version but not on actual hardware is
clearly against that goal, and  we should use CONFIG_ARCH_VIRT for
that purpose.

But the kernel that actually runs on the real hardware also runs on Qemu 
already.  It is just that, by some coincidence, a multi-platform kernel 
that pretends to have the same peripherals as a Realview board could 
also be used on Qemu.

Otherwise I really don't see the point in making the code more complex 
just to make a multi-platform kernel run on actual Realview hardware 
that is quite obsolete now and that very few people have access to.

The main reason is that out of principle I want to get to the point where
every ARMv6 and ARMv7 machine is part of CONFIG_ARCH_MULTIPLATFORM.

Then again, once we introduce CONFIG_BROKEN_MULTIPLATFORM, we could
have an option that depends on that, and turns on both
ARM_DMA_MEM_BUFFERABLE as well as the hack to remove the outer_sync
from mb() and wmb() for CONFIG_SMP.

	Arnd