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Re: framebuffer corruption due to overlapping stp instructions on arm64

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 

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.


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