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
