RPi4 can't deal with 64 bit PCI accesses

[Christoph Hellwig]

On Wed, Feb 24, 2021 at 08:55:10AM -0800, Florian Fainelli wrote:
> > Working around kernel I/O accessors is all very well, but another
> > concern for PCI in particular is when things like framebuffer memory can
> > get mmap'ed into userspace (or even memremap'ed within the kernel). Even
> > in AArch32, compiled code may result in 64-bit accesses being generated
> > depending on how the CPU and interconnect handle LDRD/STRD/LDM/STM/etc.,
> > so it's basically not safe to ever let that happen at all.
> 
> Agreed, this makes finding a generic solution a tiny bit harder. Do you
> have something in mind Nicolas?

The only workable solution is a new

bool 64bit_mmio_supported(void)

check that is used like:

	if (64bit_mmio_supported())
		readq(foodev->regs, REG_OFFSET);
	else
		lo_hi_readq(foodev->regs, REG_OFFSET);

where 64bit_mmio_supported() return false for all 32-bit kernels,
true for all non-broken 64-bit kernels and is an actual function
for arm64 multiplatforms builds that include te RPi quirk.

The above would then replace the existing magic from the
<linux/io-64-nonatomic-lo-hi.h> and <linux/io-64-nonatomic-hi-lo.h>
headers.