Re: [PATCH RFC 0/3] riscv: Add DMA_COHERENT support

[Guo Ren]

On Mon, Jun 7, 2021 at 2:27 PM Christoph Hellwig <hch@xxxxxx> wrote:
>
> On Mon, Jun 07, 2021 at 11:19:03AM +0800, Guo Ren wrote:
> > >From Linux non-coherency view, we need:
> >  - Non-cache + Strong Order PTE attributes to deal with drivers' DMA descriptors
> >  - Non-cache + weak order to deal with framebuffer drivers
> >  - CMO dma_sync to sync cache with DMA devices
>
> This is not strictly true.  At the very minimum you only need cache
> invalidation and writeback instructions.  For example early parisc
> CPUs and some m68knommu SOCs have no support for uncached areas at all,
> and Linux works.  But to be fair this is very painful and supports only
> very limited periphals.  So for modern full Linux support some uncahed
> memory is advisable.  But that doesn't have to be using PTE attributes.
> It could also be physical memory regions that are either totally fixed
Double/Triple the size of physical memory regions can't be accepted by
SOC vendors, because it wastes HW resources.
Some cost-down soc interconnects only have 32bit~34bit width of
physical address, are you sure you could force them to expand it? (I
can't)

> or somewhat dynamic.
How can HW implement with dynamic modifying PMA? What's the granularity?



-- 
Best Regards
 Guo Ren

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