From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Timur Tabi <timur@freescale.com>
Cc: Dan Malek <ppc6dev@digitaldans.com>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>
Subject: Re: minimum guaranteed alignment of dma_alloc_coherent?
Date: Tue, 29 Mar 2011 11:32:39 +1100 [thread overview]
Message-ID: <1301358759.2402.665.camel@pasglop> (raw)
In-Reply-To: <4D500C28.7000704@freescale.com>
On Mon, 2011-02-07 at 09:13 -0600, Timur Tabi wrote:
> > This behavior is modified if you specify a restricted DMA
> > mask for the device. In this case, dma_alloc_coherent
> > will force GFP_DMA on your behalf (on PowerPC).
>
> Isn't it required for all callers of dma_alloc_coherent to specify a
> mask (via
> dma_set_mask) first?
GFP_DMA historically means ISA DMA memory on x86 :-) Other platforms had
different "use" for it but basically it's old style drivers.
The problem with dma_set_mask() is that there's no way without an iommu
that the implementation of dma_alloc_coherent() can satisfy arbitrary
masks since you don't control where the page allocator gets you the
pages from ... unless you have zones. That's what ZONE_DMA and
ZONE_DMA32 are for.
Cheers,
Ben.
prev parent reply other threads:[~2011-03-29 0:32 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-02-05 0:14 minimum guaranteed alignment of dma_alloc_coherent? Timur Tabi
2011-02-05 1:09 ` Dan Malek
2011-02-05 2:04 ` Tabi Timur-B04825
2011-02-05 2:56 ` Dan Malek
2011-02-07 15:13 ` Timur Tabi
2011-03-29 0:32 ` Benjamin Herrenschmidt [this message]
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