linux-kernel - Re: DMA using data buffer vmapped in kernel space

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Message-ID: <20100306193127.GA13262@n2100.arm.linux.org.uk>
Date:	Sat, 6 Mar 2010 19:31:27 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Thomas Koeller <thomas@...ller.dyndns.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: DMA using data buffer vmapped in kernel space

On Sat, Mar 06, 2010 at 02:07:12PM +0100, Thomas Koeller wrote:
> Am Donnerstag, 4. März 2010 22:36:34 schrieb Russell King - ARM Linux:
> > Cache maintainence is done using virtual addresses for L1, and
> > physical addresses for L2.  There's the need for virtual addresses
> > to be translatable to physical addresses, which is only true for
> > the kernel direct mapped region (pages between PAGE_OFFSET and
> > high_memory).
> 
> Isn't the mapping created by vmap() sufficient for the virt/phys
> translation? In which way is this case different from a buffer
> passed in from user space, where the constituent pages are not
> in the directly mapped kernel region either?

No different.

The requirement is that dma_map_single() is passed a virtual address
in the kernel direct-mapped memory region, which is translatable using
virt_to_phys() and friends.

Anything which requires a page table lookup to obtain the physical
address is just not acceptable - that requires taking locks and other
messy things, plus is grossly inefficient.
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