Re: framebuffer corruption due to overlapping stp instructions on arm64

[Mikulas Patocka]

On Fri, 3 Aug 2018, Richard Earnshaw (lists) wrote:

> Whoa, hold on.
> 
> Memcpy should never be used on device memory.  Period.  Memcpy doesn't
> know anything about what size of access is needed for accessing a device.
> 
> But why is the buffer in device memory rather than some other form of
> uncached memory?
> 
> If you change memcpy to deal with an aspect of the system hardware,
> you'll end up hosing performance EVERYWHERE.  DON'T DO IT!

memcpy in glibc uses ifunc selection and it already has optimized variants 
for Falkor and Thunder-X. You can add just another variant for Armada-8040 
that works around this bug and you won't be harming anyone but users of 
Armada-8040.

Furthermore, you can detect in the kernel that the PCI bus has some device 
with prefetchable BAR and activate the workaround only if there is 
videocard plugged in the PCIe slot.

> If you must, create a new API with tighter semantics, but don't change
> memcpy to accommodate this.
> 
> Anyway, back to the original report.  What memory mapping is being used?
>  In detail?

It is PCI prefetchable BAR. It is mapped using pgprot_writecombine, which 
results in MT_NORMAL_NC page attributes. (the MT_DEVICE_nGnRE can't be 
used because it results in crashes due to unaligned accesses to videoram).

> R.

Mikulas