linux-kernel - Re: DMA coherency in drivers/tty/serial/mpsc.c

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Date:   Wed, 26 Jun 2019 08:48:37 +0200
From:   Christoph Hellwig <hch@....de>
To:     Mark Greer <mgreer@...malcreek.com>
Cc:     Christoph Hellwig <hch@....de>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Dale Farnsworth <dale@...nsworth.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: DMA coherency in drivers/tty/serial/mpsc.c

On Tue, Jun 25, 2019 at 09:37:22AM -0700, Mark Greer wrote:
> Yeah, the mpsc driver had lots of ugly cache related hacks because of
> cache coherency bugs in the early version of the MV64x60 bridge chips
> that it was embedded in.  That chip is pretty much dead now and I've
> removed core support for it from the powerpc tree.  Removing the mpsc
> driver is on my todo list but I've been busy and lazy.  So, to sum it
> up, don't spend any more time worrying about it as it should be removed.
> 
> I'll post a patch to do that tonight and I'm sorry for any time you've
> spent looking at it so far.

No problem.  And if future such broken chips show up we now have
support for per-device DMA coherency settings and could actually
handle it in a reaѕonably clean way.