Hi Arnd,
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.
DMA setup on a3000 host adapters can be simplified to skip bounce
buffer use (assuming SCSI buffers passed to the driver are cache> Thanks, and Cheers,
Michael
line aligned; a safe bet except for maybe sg.c input).
On gvp11 and a2091 host adapters, only the lowest 16 MB of physical
memory can be directy addressed by DMA, and bounce buffers from that
space must still be used (possibly allocated from chip RAM using the
custom allocator) if buffers are located in the higher memory regions.
The m68k VME mvme147 driver has no DMA addressing or alignment
restrictions and can be converted in the same way as the Amiga a3000
one, but will require conversion to a platform device driver first.
Right, it seems that the old hack of passing a NULL device pointer
no longer works, and that is probably for the better.
If you want an easy way out, the driver can just call
platform_device_register_full() to create its own device with a
dma_mask set up, and use that device for the DMA API, but
not actually match the device to a driver.
I'll leave it to Geert to decide whether he prefers setting up a
platform device in arch/m68k/mvme147/config.c or use the shortcut. I've
used platform_device_register_simple() in a few other drivers so don't
mind that much.
Cheers,
Michael
Only compile tested so far, and hardware testing might be hard to do.
I'd appreciate someone giving this a thorough review.
Looks all good to me.
Arnd
