Re: [PATCH 04/12] soc: sifive: ccache: Add non-coherent DMA handling

[Emil Renner Berthing]

On Thu, 16 Feb 2023 at 19:51, Conor Dooley <conor@xxxxxxxxxx> wrote:
>
> Emil,
>
> +CC Daire
>
> On Sat, Feb 11, 2023 at 05:18:13AM +0200, Cristian Ciocaltea wrote:
> > From: Emil Renner Berthing <kernel@xxxxxxxx>
> >
> > Add functions to flush the caches and handle non-coherent DMA.
> >
> > Signed-off-by: Emil Renner Berthing <kernel@xxxxxxxx>
> > [replace <asm/cacheflush.h> with <linux/cacheflush.h>]
> > Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@xxxxxxxxxxxxx>
> > ---
>
> > +void *sifive_ccache_set_uncached(void *addr, size_t size)
> > +{
> > +     phys_addr_t phys_addr = __pa(addr) + uncached_offset;
> > +     void *mem_base;
> > +
> > +     mem_base = memremap(phys_addr, size, MEMREMAP_WT);
> > +     if (!mem_base) {
> > +             pr_err("%s memremap failed for addr %p\n", __func__, addr);
> > +             return ERR_PTR(-EINVAL);
> > +     }
> > +
> > +     return mem_base;
> > +}
>
> The rest of this I either get b/c we did it, or will become moot so I
> amn't worried about it, but can you please explain this, in particular
> the memremap that you're doing here?

No, I can't really. As we talked about it's also based on a prototype
by Atish. I'm sure you know that the general idea is that we want to
return a pointer that accesses the same physical memory, but through
the uncached alias. I can't tell you exactly why it's done this way
though, sorry.

/Emil

> Cheers,
> Conor.
>
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