Re: [RFC PATCH V4 4/6] RISC-V: Implement arch_sync_dma* functions

[Anup Patel <anup@brainfault.org>]

On Thu, Sep 16, 2021 at 7:03 AM Guo Ren <guoren@kernel.org> wrote:
>
> On Wed, Sep 15, 2021 at 3:50 PM Christoph Hellwig <hch@lst.de> wrote:
> >
> > On Sat, Sep 11, 2021 at 05:21:37PM +0800, guoren@kernel.org wrote:
> > > +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);
> > > +}
> >
> > Despite various snipplets this is a still pretty much the broken previous
> > versions.  These need to use the CMO instructions directly which are
> > about to go into review, and then your SBI can trap on those can call
> > whatever non-standard mess you're using.
> I think you mean put an ALTERNATIVE slot in the prologue of __dma_sync?
>
> #define ALT_DMA_SYNC()                                           \
> asm(ALTERNATIVE(".rept 64\n nop\n .endr\n", "<vendor code>",
> XXX_VENDOR_ID,        \
>                 ERRATA_XXX, CONFIG_ERRATA_XXX)  \
>                 : : : "memory")
>
> static void __dma_sync(phys_addr_t paddr, size_t size, enum
> dma_data_direction dir)
> {
>         ALT_DMA_SYNC();
>
>         /* future cmo codes */
> }

I think Christoph is suggesting to always use CMO instructions for
implementing arch specific DMA sync functions. The SBI implementation
will trap-n-emulate CMO instructions when underlying HW does not
have it. This means custom cache instructions on D1 can reside in
the platform support code of OpenSBI.

I also agree with the above suggestion. At least, this will ensure that we
have only one way of doing cache operations from S-mode perspective
which is CMO instructions.

Regards,
Anup

>
>
>
>
> --
> Best Regards
>  Guo Ren
>
> ML: https://lore.kernel.org/linux-csky/