[PATCH 3/3] document dma_flags_set/get_*()

[PATCH 3/3] document dma_flags_set/get_*()

[akepner]

Document the dma_flags_set/get_*() interfaces.

Signed-off-by: Arthur Kepner <akepner@sgi.com>

--- 

 DMA-API.txt |   38 ++++++++++++++++++++++++++++++++++++++
 1 files changed, 38 insertions(+)

diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
index b939ebb..00919b0 100644
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -547,3 +547,41 @@ size is the size (and should be a page-sized multiple).
 The return value will be either a pointer to the processor virtual
 address of the memory, or an error (via PTR_ERR()) if any part of the
 region is occupied.
+
+int 
+dma_flags_set_attr(u32 attr, enum dma_data_direction dir)
+
+Amend dir with a platform-specific "dma attribute".
+
+The only attribute currently defined is DMA_BARRIER_ATTR, which causes 
+in-flight DMA to be flushed when the associated memory region is written 
+to (see example below).  Setting DMA_BARRIER_ATTR provides a mechanism 
+to enforce ordering of DMA on platforms that permit DMA to be reordered 
+between device and host memory (within a NUMA interconnect).  On other 
+platforms this is a nop.
+
+DMA_BARRIER_ATTR would be set when the memory region is mapped for DMA, 
+e.g.:
+
+	int count;
+	int flags = dma_flags_set_attr(DMA_BARRIER_ATTR, DMA_BIDIRECTIONAL);
+	....
+	count = dma_map_sg(dev, sglist, nents, flags);
+
+As an example of a situation where this would be useful, suppose that 
+the device does a DMA write to indicate that data is ready and 
+available in memory.  The DMA of the "completion indication" could 
+race with data DMA.  Using DMA_BARRIER_ATTR on the memory used for 
+completion indications would prevent the race.
+
+int
+dma_flags_get_dir(int flags)
+
+Retrieve the original DMA direction, where flags was returned from 
+dma_flags_set_attr().
+
+int
+dma_flags_get_attr(int flags)
+
+Retrieve the "dma attributes", where flags was returned from 
+dma_flags_set_attr().

Re: [PATCH 3/3] document dma_flags_set/get_*()

[Andrew Morton]

On Tue, 16 Oct 2007 18:44:17 -0700 akepner@sgi.com wrote:

> @@ -547,3 +547,41 @@ size is the size (and should be a page-sized multiple).
>  The return value will be either a pointer to the processor virtual
>  address of the memory, or an error (via PTR_ERR()) if any part of the
>  region is occupied.
> +
> +int 
> +dma_flags_set_attr(u32 attr, enum dma_data_direction dir)
> +
> +Amend dir with a platform-specific "dma attribute".
> +
> +The only attribute currently defined is DMA_BARRIER_ATTR, which causes 
> +in-flight DMA to be flushed when the associated memory region is written 
> +to (see example below).  Setting DMA_BARRIER_ATTR provides a mechanism 
> +to enforce ordering of DMA on platforms that permit DMA to be reordered 
> +between device and host memory (within a NUMA interconnect).  On other 
> +platforms this is a nop.
> +
> +DMA_BARRIER_ATTR would be set when the memory region is mapped for DMA, 
> +e.g.:
> +
> +	int count;
> +	int flags = dma_flags_set_attr(DMA_BARRIER_ATTR, DMA_BIDIRECTIONAL);
> +	....
> +	count = dma_map_sg(dev, sglist, nents, flags);
> +

Isn't this rather a kludge?

What would be the cost of doing this cleanly and either redefining
dma_data_direction to be a field-of-bits or just leave dma_data_direction
alone (it is quite unrelated to this work, isn't it?) and adding new
fields/arguments to manage this new functionality?