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

[Neil Armstrong <narmstrong@baylibre.com>]

On 24/02/2021 21:35, Florian Fainelli wrote:
> 
> 
> 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.
> 

Erf, not really, with the compute module ATX/ITX boards are being designed with a full PCIe connector like:
https://www.indiegogo.com/projects/over-board-raspberry-pi-4-mini-itx-motherboard/#/

Neil