Messages in this thread | | | Date | Fri, 3 Aug 2018 20:58:18 -0400 (EDT) | From | Mikulas Patocka <> | Subject | Re: framebuffer corruption due to overlapping stp instructions on arm64 |
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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
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