RPi4 can't deal with 64 bit PCI accesses

[David Woodhouse]

On Wed, 2021-02-24 at 20:25 +0000, Christoph Hellwig wrote:
> 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.

Is it completely impossible to do 64-bit cycles with this host bridge?

I'm now having nasty flashbacks to an SH platform with a host bridge
that screwed up byte access for direct MMIO — but *could* do those MMIO
cycles accurately if we did them through an indirect method similar to
config cycles. Is there any such mechanism on the offending hardware?
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