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

[Conor Dooley <conor.dooley@microchip.com>]

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On Mon, Jan 09, 2023 at 01:59:12PM +0100, Arnd Bergmann wrote:
> On Mon, Jan 9, 2023, at 13:03, Lad, Prabhakar wrote:
> > On Sun, Jan 8, 2023 at 12:08 AM Arnd Bergmann <arnd@arndb.de> wrote:
> >> >> > +struct riscv_cache_ops {
> >> >> > +     void (*clean_range)(unsigned long addr, unsigned long size);
> >> >> > +     void (*inv_range)(unsigned long addr, unsigned long size);
> >> >> > +     void (*flush_range)(unsigned long addr, unsigned long size);
> >> >> > +     void (*riscv_dma_noncoherent_cmo_ops)(void *vaddr, size_t size,
> >> >> > +                                           enum dma_data_direction dir,
> >> >> > +                                           enum dma_noncoherent_ops ops);
> >> >> > +};
> >> >>
> >> >> I don't quite see how the fourth operation is used here.
> >> >> Are there cache controllers that need something beyond
> >> >> clean/inv/flush?
> >> >>
> >> > This is for platforms that dont follow standard cache operations (like
> >> > done in patch 5/6) and there drivers decide on the operations
> >> > depending on the ops and dir.
> >>
> >> My feeling is that the set of operations that get called should
> >> not depend on the cache controller but at best the CPU. I tried to
> >> enumerate how zicbom and ax45 differ here, and how that compares
> >> to other architectures:
> >>
> >>                   zicbom      ax45,mips,arc      arm           arm64
> >> fromdevice      clean/flush   inval/inval   inval/inval   clean/inval
> >> todevice        clean/-       clean/-       clean/-       clean/-
> >> bidi            flush/flush   flush/inval   clean/inval   clean/inval

I did a bit of digging on lore for context on why the ops are what they
are..
In v3 of the Zicbom enablement patchset, things looked like:
fromdevice	inval/inval
todevice	clean/-
bidi		flush/inval

v3:
https://lore.kernel.org/linux-riscv/20220610004308.1903626-3-heiko@sntech.de/

Samuel had some comments about the invals:
https://lore.kernel.org/linux-riscv/342e3c12-ebb0-badf-7d4c-c444a2b842b2@sholland.org/

In v4 it was changed to:
fromdevice	inval/flush
todevice	clean/-
bidi		flush/flush

v4:
https://lore.kernel.org/linux-riscv/20220619203212.3604485-4-heiko@sntech.de/

Christoph replied to that one, linking the thread belonging to the
commit you pointed out earlier:
https://lore.kernel.org/linux-riscv/20220620061607.GB10485@lst.de/

v5 produced what you have in your table above:
https://lore.kernel.org/linux-riscv/20220629215944.397952-4-heiko@sntech.de/

> >>
> >> So everyone does the same operation for DMA_TO_DEVICE, but
> >> they differ in the DMA_FROM_DEVICE handling, for reasons I
> >> don't quite see:
> >>
> >> Your ax45 code does the same as arc and mips. arm and
> >> arm64 skip invalidating the cache before bidi mappings,
> >> but arm has a FIXME comment about that. arm64 does a
> >> 'clean' instead of 'inval' when mapping a fromdevice
> >> page, which seems valid but slower than necessary.
> >>
> >> Could the zicbom operations be changed to do the same
> >> things as the ax45/mips/arc ones, or are there specific
> >> details in the zicbom spec that require this?
> >>
> > I'll let the RISC-V experts respond here.
> 
> Adding Christoph Hellwig and Will Deacon to Cc as well.
> 
> 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
> 
> 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.

Hope the lore archaeology helps jog people's memories...

Conor


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