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

[David Laight]

From: 'Christoph Hellwig'
> Sent: 08 June 2021 16:32
> 
> 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

I guess that each transmit has to be split into enough
fragments that they fill a cache line.
That won't work with some (probably old now) devices that
require the first fragment to be 64 bytes because it won't
back-up the descriptors after a collision.

It's all as horrid as a DSP we have that can't receive ethernet
frames onto a 4n+2 boundary and doesn't support misaligned accesses.

Mind you, Sun's original Sbus ethernet board had to be given
a 4n aligned rx buffer and then a misaligned copy done in kernel
in order to not drop packets!

	David

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