Re: [PATCH v2] ARM: realview: basic device tree implementation

[Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>]

On Thursday 22 May 2014 15:45:03 Catalin Marinas wrote:
> On Thu, May 22, 2014 at 09:31:37AM +0100, Arnd Bergmann wrote:
> > commit 42c4dafe803dcad82980fd8b0831a89032156f93
> > Author: Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>
> > Date:   Thu Jul 1 13:22:48 2010 +0100
> > 
> >     ARM: 6202/1: Do not ARM_DMA_MEM_BUFFERABLE on RealView boards with L210/L220
> >     
> >     RealView boards with certain revisions of the L210/L220 cache controller
> >     may have issues (hardware deadlock) with the mandatory barriers (DSB
> >     followed by an L2 cache sync) when ARM_DMA_MEM_BUFFERABLE is enabled.
> >     The patch disables ARM_DMA_MEM_BUFFERABLE for these boards.
> >     
> >     Tested-by: Linus Walleij <linus.walleij-0IS4wlFg1OjSUeElwK9/Pw@public.gmane.org>
> >     Signed-off-by: Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>
> >     Signed-off-by: Russell King <rmk+kernel-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>
> 
> Looking through the L220 errata document, I think this relates to 425331
> which says that a cache sync can deadlock the system if the write buffer
> is not empty.

Hmm, but isn't the write buffer empty here because we have just issued
a dsb()?

> By making the DMA memory Strongly Ordered, we avoid the
> L220 write buffer. In addition to the above commit, we also had
> 2503a5ecd86c (ARM: 6201/1: RealView: Do not use outer_sync() on
> ARM11MPCore boards with L220).

Ah, so this also stands in the way of multiplatform realview :(

> The above only solves the issue for coherent DMA allocations. Streaming
> DMA requires additional changes to cache-l2x0.c but they are all weird
> and depend on how the L2x0 is wired (for example, using SWP to access
> the L2x0 registers works on one RealView board but not on another).
> Anyway, I don't think we have any streaming DMA used on these RealView
> boards.

Yes, I think that is correct. MMCI, SSP and serial could all use
DMA but are not configured to do it.

> If we only use the DMA coherent buffers for CLCD on these boards, we
> don't really need the outer_sync() either so we could go back to
> bufferable DMA memory (strangely enough, this L2 cache workaround looks
> similar to Marvell's PCI+PL310 problem where we want to avoid
> outer_sync()).

Ah, so you think we can add another hook to NULL out outer_cache.sync
per platform?

	Arnd
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