Re: framebuffer corruption due to overlapping stp instructions on arm64

Ard Biesheuvel
2018-08-06T15:47:41Z

On 6 August 2018 at 14:42, Robin Murphy <robin.murphy@arm.com> wrote:
> On 06/08/18 11:25, Mikulas Patocka wrote:
> [...]
>>>
>>> None of this explains why some transactions fail to make it across
>>> entirely. The overlapping writes in question write the same data to
>>> the memory locations that are covered by both, and so the ordering in
>>> which the transactions are received should not affect the outcome.
>>
>>
>> You're right that the corruption couldn't be explained just by reordering
>> writes. My hypothesis is that the PCIe controller tries to disambiguate
>> the overlapping writes, but the disambiguation logic was not tested and it
>> is buggy. If there's a barrier between the overlapping writes, the PCIe
>> controller won't see any overlapping writes, so it won't trigger the
>> faulty disambiguation logic and it works.
>>
>> Could the ARM engineers look if there's some chicken bit in Cortex-A72
>> that could insert barriers between non-cached writes automatically?
>
>
> I don't think there is, and even if there was I imagine it would have a
> pretty hideous effect on non-coherent DMA buffers and the various other
> places in which we have Normal-NC mappings of actual system RAM.
>

Looking at the A72 manual, there is one chicken bit that looks like it
may be related:

CPUACTLR_EL1 bit #50:

0 Enables store streaming on NC/GRE memory type. This is the reset value.
1 Disables store streaming on NC/GRE memory type.

so putting something like

mrs x0, S3_1_C15_C2_0
orr x0, x0, #(1 << 50)
msr S3_1_C15_C2_0, x0

in __cpu_setup() would be worth a try.