Re: [RFC v5.1 9/9] [DON'T APPLY] cache: sifive-ccache: add cache flushing capability

Conor Dooley
2023-01-04T15:11:36Z

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Hey Arnd,

On Wed, Jan 04, 2023 at 11:19:44AM +0100, Arnd Bergmann wrote:
> On Wed, Jan 4, 2023, at 10:23, Conor Dooley wrote:
> >>Right, no need to touch the existing file as part of this series,
> >>it probably just gets in the way of defining a good interface here.
> >
> > Sure. Can leave it where it was & I'll sort it out later when it's
> > errata etc get added.
> >
> > Btw, would you mind pointing out where you wanted to have that if/else
> > you mentioned on IRC?
>
> I meant replacing both of the runtime patching indirections in
> arch_sync_dma_for_device(). At the moment, this function calls
> ALT_CMO_OP(), which is patched to either call the ZICBOM or the
> THEAD variant, and if I read this right you add a third case
> there with another level of indirection using static_branch.

Yah, pretty much.

> I would try to replace both of these indirections and instead
> handle it all from C code in arch_sync_dma_for_device() directly,
> for the purpose of readability and maintainability.
> static inline void dma_cache_clean(void *vaddr, size_t size)
> {
> if (!cache_maint_ops.clean)
> zicbom_cache_clean(vaddr, size, riscv_cbom_block_size);

And I figure that this function is effectively a wrapper around ALT_CMO_OP()?

> else
> cache_maint_ops.clean(vaddr, size, riscv_cbom_block_size);

And this one gets registered by the driver using an interface like the
one I already proposed, just with the cache_maint_ops struct expanded?

Extrapolating, with these changes having an errata would not even be
needed in order to do cache maintenance.
Since the ALT_CMO_OP() version would only be used inside
zicbom_cache_clean(), assuming I understood correctly, a driver could
just register cache_maint_ops for a given platform without having to
muck around with errata.
If so, that seems like a distinct improvement over my suggestion & gets
around the thing I mentioned in 0/9 about a shared case in the
alternative application code.

Again, assuming I understood correctly, I like this a lot.

> }
>
> void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
> enum dma_data_direction dir)
> {
> void *vaddr = phys_to_virt(paddr);
>
> switch (dir) {
> case DMA_TO_DEVICE:
> case DMA_FROM_DEVICE:
> dma_cache_clean(vaddr, size);
> break;
> case DMA_BIDIRECTIONAL:
> dma_cache_flush(vaddr, size);
> break;
> default:
> break;
> }
> }
>
> which then makes it very clear what the actual code path
> is, while leaving the zicbom case free of indirect function
> calls. You can still use a static_branch() to optimize the
> conditional, but I would try to avoid any extra indirection
> levels or errata checks.

The other thing that I like about this is we can then remove the various
calls to ALT_CMO_OP() that are scattered around arch/riscv now & replace
them with functions that have more understandable names.

Thanks Arnd!
Conor.

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