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

[David Laight]

From: Christoph Hellwig
> Sent: 07 June 2021 07:27
> 
> 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
> or somewhat dynamic.

It is almost impossible to interface to many ethernet chips without
either coherent or uncached memory for the descriptor rings.
The status bits on the transmit ring are particularly problematic.

The receive ring can be done with writeback+invalidate provided you
fill a cache line at a time.

	David

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