Re: [patch] Introduce CONFIG_HAS_DMA.

["Dan Williams" <dan.j.williams@intel.com>]

[ apologies for reviving an old conversation ]

On 2/26/07, Heiko Carstens <heiko.carstens@de.ibm.com> wrote:
> From: Heiko Carstens <heiko.carstens@de.ibm.com>
>
> Architectures that don't support DMA can say so by adding a
> config NO_DMA to their Kconfig file. This will prevent compilation
> of some dma specific driver code. Also dma-mapping-broken.h isn't
> needed anymore on at least s390.
> This avoids compilation and linking of otherwise dead/broken code.

I went back and read the thread leading up to this patch and I am of
the opinion that John's approach (adding more stubs to
dma-mapping-broken.h:
http://marc.info/?l=linux-kernel&m=117219377712232&w=2)  is needed in
_addition_ to this Kconfig option.  In my particular case I have an
API with a DMA and a non-DMA path written in such a way that the DMA
path can be compiled away.  Without dma-mapping-broken.h support the
API is unnecessarily forced to violate point 2 of
Documentation/SubmittingPatches (#ifdefs are ugly).

In other words let CONFIG_HAS_DMA prevent pure DMA code from being
built, but do not preclude "clever" implementations from calling
broken code.

If there are no objections I plan to resubmit John's changes with the
addition below, which allows CONFIG_HAS_DMA=n architectures to delete
their respective dma-mapping.h file.

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 9a663c6..2dc21cb 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -31,7 +31,11 @@ static inline int valid_dma_direction(int dma_direction)
               (dma_direction == DMA_FROM_DEVICE));
}

+#ifdef CONFIG_HAS_DMA
#include <asm/dma-mapping.h>
+#else
+#include <asm-generic/dma-mapping-broken.h>
+#endif

/* Backwards compat, remove in 2.7.x */
#define dma_sync_single                dma_sync_single_for_cpu
-
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