Re: [PATCH 4/6] dma-mapping: merge direct and noncoherent ops
From: Maciej W. Rozycki
Date: Wed Oct 31 2018 - 12:31:46 EST
On Wed, 31 Oct 2018, Maciej W. Rozycki wrote:
> > All the cache maintainance is already stubbed out when not enabled,
> > but merging the two allows us to nicely handle the case where
> > cache maintainance is required for some devices, but not others.
> FYI, you commit bc3ec75de545 ("dma-mapping: merge direct and noncoherent
> ops") has caused:
> fddi0: DMA command request failed!
> fddi0: Adapter open failed!
> with the `defxx' driver on my R4400SC TURBOchannel DECstation (but not the
> R3400 one) and consequently the interface does not work anymore. Both are
> non-coherent cache systems, however the R3400 implements the write-through
> policy while the policy of the R4400SC is write-back (it also has 1MiB of
> secondary cache), which I suspect is the reason of the difference.
Doh! It would have helped if I actually had the right adapter installed
in the R3400 box. I've got a spare one, which I have now plugged in there
and the R3400 actually shows the same issue as the R4400SC does. So this
has nothing to do WRT write-through vs write-back.
Hmm, in `dfx_hw_dma_cmd_req' the driver polls the consumer block (which
is write-only by the adapter and read-only by the host, except for the
initialisation time before adapter's DMA engines have been started, and
write-through vs write-back indeed does not matter for this kind of use)
and there's no DMA synchronisation whatsoever in that.
However the block has been allocated with `dma_zalloc_coherent', which
means no synchronisation is supposed to be required. For non-coherent
cache systems that essentially means an uncached memory allocation.