Re: [RFC PATCH v6 1/6] riscv: mm: dma-noncoherent: Switch using function pointers for cache management

[Arnd Bergmann]

On Tue, Jan 10, 2023, at 08:01, Christoph Hellwig wrote:
> On Mon, Jan 09, 2023 at 01:59:12PM +0100, Arnd Bergmann wrote:
>> I had another look at the arm64 side, which (like the zicbom
>> variant) uses 'clean' on dma_sync_single_for_device(DMA_FROM_DEVICE),
>> as that has changed not that long ago, see
>> 
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c50f11c6196f45c92ca48b16a5071615d4ae0572
>
> which IIRC has been reverted recently.

To clarify: I was looking at arch_sync_dma_for_device(), which
changed from 'invalidate' to 'clean' last June in commit
c50f11c6196f ("arm64: mm: Don't invalidate FROM_DEVICE buffers
at start of DMA transfer"). I don't see  a revert for that.

The one that was reverted recently is arch_dma_prep_coherent, which
was changed and reverted in

c44094eee32 Aug 23 2022 flush->clean
b7d9aae4048 Dec 6  2022 clean->flush

I'm primarily interested in the streaming mappings (arch_sync_*)
at the moment.

>> I'm still not sure what the correct set of operations has
>> to be, but nothing in that patch description sounds ISA
>> or even microarchitecture specific.
>
> Nothing is ISA specific, and the only micro architecture related thing
> is if the specific core can speculate memory accesses.  See the table
> in arch/arc/mm/dma.c for details.
>
> And as commented on the arm64 patch I really hate architectures getting
> creative here, as I'd much prefer to move the choice of primitives to
> the core DMA code and just provide helpers to invalidate/writeback/
> writeback+invalidate from the architectures eventually.

Agreed, that's why I particularly don't like the idea of giving SoC
specific L2$ drivers the option of making custom choices here.

The arch_dma_prep_coherent() change is arguably arm64 specific
because it is only needed for machines sharing memory with EL3
firmware that enforces buffer ownership, but even for that it would
be nice to have a central definition and some architecture specific
flag that picks one behavior or the other. Same thing for the
speculative access difference.

I looked at all the implementations now and put them in a table[1]
to see what the differences are. The only bit that I think needs
discussion is the dma_sync_single_for_device(DMA_FROM_DEVICE) op
that I mentioned above. I see that arm64, csky, powerpc, riscv
and parisc all write out at least partical cache lines first to
avoid losing dirty data in the part that is not written by the
device[2][3], while the other ones don't[4]. 

The other differences I found are:

- arm and arm64 use clean instead of flush for
  dma_sync_single_for_device(DMA_BIDIRECTIONAL).
  This seems harmless, since there is another
  invalidation at the _for_cpu() step.

- hexagon, m68k, openrisc and sh don't deal with
  speculative loads

- m68k, nios2, openrisc, parisc and xtensa use
  flush instead of clean for DMA_TO_DEVICE, presumably
  because they don't have a flush without invalidate.

- microblaze, parisc and powerpc use the same function
  for _for_device and _for_cpu, which is slightly wasteful
  but harmless.

- openrisc lacks support for bidirectional mappings,
  which is a bug

- sparc32 and xtensa only supports writethrough caches
  and consequently skips the clean before DMA but
  instead invalidate the buffer in the _for_cpu
  operation.

I also see that at least arc, arm, mips and riscv all want
CPU specific cache management operations to be registered
at boot time. While Russell had some concerns about your
suggestion to generalize the arm version, we could start
by moving the abstracted riscv version into
kernel/dma/direct.c and make sure it can be shared with
at least mips and arc, provided that we can agree on the
DMA_FROM_DEVICE behavior.

     Arnd

[1] https://docs.google.com/spreadsheets/d/1qDuMqB6TnRTj_CgUwgIIm_RJ6EZO76qohpTJUMQjUEo/edit#gid=0
[2] https://git.kernel.org/torvalds/c/c50f11c6196f
[3] https://git.kernel.org/torvalds/c/03d70617b8a7
[4] https://lore.kernel.org/all/20220606152150.GA31568@willie-the-truck/