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From:   'Christoph Hellwig' <hch@lst.de>
To:     David Laight <David.Laight@ACULAB.COM>
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Subject: Re: [PATCH RFC 0/3] riscv: Add DMA_COHERENT support
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On Tue, Jun 08, 2021 at 03:00:17PM +0000, David Laight wrote:
> 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.

It is horrible, but it has been done.  Take a look at:

drivers/net/ethernet/i825xx/lasi_82596.c and
drivers/net/ethernet/seeq/sgiseeq.c