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

Lad, Prabhakar
2023-01-09T12:04:46Z

On Sun, Jan 8, 2023 at 12:08 AM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Sat, Jan 7, 2023, at 23:10, Lad, Prabhakar wrote:
>
> >> > +
> >> > + memset(&thead_cmo_ops, 0x0, sizeof(thead_cmo_ops));
> >> > + if (IS_ENABLED(CONFIG_ERRATA_THEAD_CMO)) {
> >> > + thead_cmo_ops.clean_range = &thead_cmo_clean_range;
> >> > + thead_cmo_ops.inv_range = &thead_cmo_inval_range;
> >> > + thead_cmo_ops.flush_range = &thead_cmo_flush_range;
> >> > + riscv_noncoherent_register_cache_ops(&thead_cmo_ops);
> >> > + }
> >>
> >> The implementation here looks reasonable, just wonder whether
> >> the classification as an 'errata' makes sense. I would probably
> >> consider this a 'driver' at this point, but that's just
> >> a question of personal preference.
> >>
> > zicbom is a CPU feature that doesn't have any DT node and hence no
> > driver and similarly for T-HEAD SoC.
>
> A driver does not have to be a 'struct platform_driver' that
> matches to a device node, my point was more about what to
> name it, regardless of how the code is entered.
>
> > Also the arch_setup_dma_ops()
> > happens quite early before driver probing due to which we get WARN()
> > messages during bootup hence I have implemented it as errata; as
> > errata patching happens quite early.
>
> But there is no more patching here, just setting the
> function pointers, right?
>
Yes that's right.

> >> > +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
>
> 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.

Cheers,
Prabhakar