linux-kernel - Re: [PATCH] Fix DMA on Dreamcast

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Date:	Sat, 28 Jul 2007 03:50:02 +0200
From:	"Peter Bortas" <bortas@...il.com>
To:	"Paul Mundt" <lethal@...ux-sh.org>,
	"Peter Bortas" <bortas@...il.com>,
	"Marcus Comstedt" <marcus@...pp.se>,
	"Adrian McMenamin" <adrianmcmenamin@...il.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix DMA on Dreamcast

On 7/26/07, Paul Mundt <lethal@...ux-sh.org> wrote:
> On Thu, Jul 26, 2007 at 02:59:51PM +0200, Peter Bortas wrote:
> > On 7/26/07, Marcus Comstedt <marcus@...pp.se> wrote:
> > > "Peter Bortas" <bortas@...il.com> writes:
> > > > On 7/21/07, Adrian McMenamin <adrianmcmenamin@...il.com> wrote:
> > > >> On 21/07/07, Peter Bortas <bortas@...il.com> wrote:
> > > >> > Sidenote: Does Linux handle the Dreamcast DMA errata?
> > > >>
> > > >> You need to explain what you mean (at least to me!).
> > > >>
> > > >> If you mean will it degrade gracefully - not without this patch if set
> > > >> to the (correct) defconfig. With iffy settings it will.
> > > >
> > > > If I remember correctly (and that's a big if since I last looked at it
> > > > in 2001) some revisions of the Dreamcast hardware would sporadically
> > > > lock up if you scheduled a new DMA request to quickly after a previous
> > > > one, even if you checked the ready bit. It's worked around by a delay
> > > > of X microseconds as recommended by Sega engineers. I don't remember
> > > > the value of X, nor where exactly in the flow this workaround should
> > > > be applied.
> > > >
> > > > Adding Marcus in case he has a better memory than me.
> > >
> > > I don't remeber any such delay.  Are you sure you're not thinking
> > > about the G2 bus problem (where accesses need to be programatically
> > > serialized, whether they are PIO or DMA)?
> >
> > In that case my memory is worse than I thought. I'll see if I can dig
> > up my old notes.
> >
> We've never hit any problems with the SH DMAC, so it would be interesting
> if you had some more information on this. The G2 problems are well known
> and documented, and the driver takes care of those issues already.

After grep-ing through some 8GiB of archived mails and notes it seems
marcus is absolutely correct, I'm thinking of the known G2 problem.

-- 
Peter
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