Re: [RFC 1/5] RISC-V: Implement arch_sync_dma* functions

[Guo Ren]

On Tue, Jul 27, 2021 at 5:53 AM Atish Patra <atishp@xxxxxxxxxxxxxx> wrote:
>
> On Sun, Jul 25, 2021 at 11:57 PM Christoph Hellwig <hch@xxxxxx> wrote:
> >
> > > +#ifdef CONFIG_RISCV_DMA_NONCOHERENT
> > > +struct riscv_dma_cache_sync {
> > > +     void (*cache_invalidate)(phys_addr_t paddr, size_t size);
> > > +     void (*cache_clean)(phys_addr_t paddr, size_t size);
> > > +     void (*cache_flush)(phys_addr_t paddr, size_t size);
> > > +};
> > > +
> > > +void riscv_dma_cache_sync_set(struct riscv_dma_cache_sync *ops);
> > > +#endif
> >
> > As told a bunch of times before: doing indirect calls here is a
> > performance nightmare.  Use something that actually does perform
> > horribly like alternatives.  Or even delay implementing that until
> > we need it and do a plain direct call for now.
> >
>
> I was initially planning to replace this with alternatives in the
> future versions. But there is no point in doing it
> until we have CMO spec finalized. We also don't have any other
> platform using these apart from sifive l2
> cache controllers for now.
>
> I will change these to direct for now.
I think alternatives' would be helpful, I've rebased on your patchset, thx.
https://lore.kernel.org/linux-riscv/20210911092139.79607-6-guoren@xxxxxxxxxx/

>
> > static void __dma_sync(phys_addr_t paddr, size_t size, enum dma_data_direction dir)
> > > +{
> > > +     if ((dir == DMA_FROM_DEVICE) && (dma_cache_sync->cache_invalidate))
> > > +             dma_cache_sync->cache_invalidate(paddr, size);
> > > +     else if ((dir == DMA_TO_DEVICE) && (dma_cache_sync->cache_clean))
> > > +             dma_cache_sync->cache_clean(paddr, size);
> > > +     else if ((dir == DMA_BIDIRECTIONAL) && dma_cache_sync->cache_flush)
> > > +             dma_cache_sync->cache_flush(paddr, size);
> > > +}
> >
> > The seletion of flush types is completely broken.  Take a look at the
> > comment in arch/arc/mm/dma.c above arch_sync_dma_for_device for a good
> > explanation.
> >
>
> Thanks. I will fix it.
>
> > > +void arch_dma_prep_coherent(struct page *page, size_t size)
> > > +{
> > > +     void *flush_addr = page_address(page);
> > > +
> > > +     memset(flush_addr, 0, size);
> >
> > arch_dma_prep_coherent is not supposed to modify the content of
> > the data.
>
> Sorry. This was a leftover from some experimental code. It shouldn't
> have been included.
>
> > _______________________________________________
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>
>
>
> --
> Regards,
> Atish
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-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/