Re: RPi4 can't deal with 64 bit PCI accesses

[Florian Fainelli <f.fainelli@gmail.com>]

On 2/24/2021 12:25 PM, 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.

That would work. The use case described by Robin is highly unlikely to
exist on the Pi4 given that you cannot easily access the PCIe bus and
plug an arbitrary GPU, so maybe there is nothing to do for framebuffer
memory.
-- 
Florian