On Wed, Jun 29, 2022 at 9:36 AM Michael Schmitz <schmitzmic@xxxxxxxxx> wrote:
Am 29.06.2022 um 18:19 schrieb Arnd Bergmann:
On Wed, Jun 29, 2022 at 3:16 AM Michael Schmitz <schmitzmic@xxxxxxxxx> wrote:
V1 of a patch series to convert m68k Amiga WD33C93 drivers to the
DMA API.
This series was precipitated by Arnd removing CONFIG_VIRT_TO_BUS. The
m68k WD33C93 still used virt_to_bus to convert virtual addresses to
physical addresses suitable for the DMA engines (note m68k does not
have an IOMMU and uses a direct mapping for DMA addresses).
Arnd suggested to use dma_map_single() to set up dma mappings instead
of open-coding much the same in every driver dma_setup() function.
It appears that m68k (MMU, except for coldfire) will set up pages for
DMA transfers as non-cacheable, thus obviating the need for explicit
cache management.
To clarify, the dma-mapping API has two ways of dealing with this:
- the streaming API (dma_map/unmap_...) uses explicit cache flushes
- the coherent API (dma_alloc_coherent etc) uses page protections
to prevent caching.
These three drivers use the streaming API because they operate on
data passed in from the outside, so the non-cacheable protection bits
are not used here.
I had feared you'd say something along these lines ...
Now that throws up a possible problem for the gvp11 driver: due to the
need to first map an allocated chunk, then possibly discard that and try
another allocation strategy, copying of data to the bounce buffer is
deferred until after the final mapping has been successful. This means
for writes, we have done the cache flushing before we have actually
written any data to the buffer.
I don't think it is safe to omit the explicit cache flush for writes in
this case.
I think it's fine as long as you do things in the correct order: the
copy into the bounce buffer has to be done before the
dma_map_single() here, and conversely, the copy out of the
bounce buffer must happen after the dma_unmap_single().
Regarding the amiga_chip_alloc(), I don't know what this means
for caching. If chip memory is cache-coherent (either uncached
or by snooping), then there should not be any
dma_map()/dma_unmap() for that case, but instead the
amiga_chip_alloc() function should return both the pointer
and the dma_addr_t token.
Arnd
