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Subject: Re: [PATCH 7/8] powerpc: use the newly added get_required_mask
	dma_map_ops hook
From: Milton Miller <miltonm@bga.com>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Thu, 12 May 2011 02:32:37 -0500
Cc: cbe-oss-dev@lists.ozlabs.org,
	FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>,
	Greg Kroah-Hartman <gregkh@suse.de>, Arnd Bergmann <arnd@arndb.de>,
	Geoff Levand <geoff@infradead.org>,
	Sean MacLennan <smaclennan@pikatech.com>,
	linux-kernel@vger.kernel.org, Paul Mackerras <paulus@samba.org>,
	Will Schmidt <will_schmidt@vnet.ibm.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Nishanth Aravamudan <nacc@us.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>, linuxppc-dev@lists.ozlabs.org,
	"David S. Miller" <davem@davemloft.net>
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> On Thu, May 12, 2011 at 00:25, Nishanth Aravamudan <nacc@us.ibm.com> wrote:
> > diff --git a/arch/powerpc/platforms/ps3/system-bus.c b/arch/powerpc/platforms/ps3/system-bus.c
> > index 23083c3..688141c 100644
> > --- a/arch/powerpc/platforms/ps3/system-bus.c
> > +++ b/arch/powerpc/platforms/ps3/system-bus.c
> > @@ -695,12 +695,18 @@ static int ps3_dma_supported(struct device *_dev, u64 mask)
> > 				return mask >= DMA_BIT_MASK(32);
> > 	}
> >
> > +static u64 ps3_dma_get_required_mask(struct device *_dev)
> > +{
> > + 			return DMA_BIT_MASK(32);
> 
> Why 32 and not 64?

I based it on the return of ps3_dma_supported, which you can see just
above says anything at or above a 32 bit mask is ok.

I don't really know the platform, but digging a bit deeper, it looks
like this goes to ps3_map_dma in ps3/mm.c.  It looks like that translates
the virt to phys to lpar (similar to absolute in iseries), and the
maps it to a bus address by a linear mapping.  But no where do I see
mention of a device dma mask (neither in mm.c or system-dev.c (except
for the ps3_dma_supported local), so I assume that 32 bits is sufficient
for any device.  It appears to me the code establishs a 1:1 mapping
of all possible memory with no provision for allocating blocks or
checking that a bus address belongs to another memory segment.

Feel free to point out any errors in the above analysis, otherwise
I assume the required mask matches the dma_supported op.

Does the lv1 hypervisor offer more than 4G of memory to the lpar?

milton