Re: [PATCH 2/3] dma: override "dma_flags_set_dmaflush" for sn-ia64

akepner
2007-08-21T19:35:22Z

> I'm a little concerned about changing the API for the dma_ foo
> functions, which are defined cross platform. If you want to change
> that, I think it will require updating the documentation explaining
> it.....

What do you think of the following? (And is there anyone else
I should be cc-ing for review?)

Document semantics of dma_flags_set_dmaflush()

Signed-off-by: Arthur Kepner <akepner@sgi.com>
--
DMA-API.txt | 22 ++++++++++++++++++++++
1 files changed, 22 insertions(+)

diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
index cc7a8c3..e117b72 100644
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -392,6 +392,28 @@ Notes: You must do this:

See also dma_map_single().

+int
+dma_flags_set_dmaflush(int dir)
+
+Amend dir (one of the enum dma_data_direction values), with a platform-
+specific "dmaflush" attribute. Unless the platform supports "posted DMA"
+this is a no-op.
+
+On platforms that support posted DMA, dma_flags_set_dmaflush() causes
+device writes to the associated memory region to flush in-flight DMA.
+This can be important, for example, when (DMA) writes to the memory
+region indicate that DMA of data is complete. If DMA of data and DMA of
+the completion indication race, as they can do when the platform supports
+posted DMA, then the completion indication may arrive in host memory
+ahead of some data.
+
+To prevent this, you might map the memory region used for completion
+indications as follows:
+
+ int count, flags = dma_flags_set_dmaflush(DMA_BIDIRECTIONAL);
+ .....
+ count = dma_map_sg(dev, sglist, nents, flags);
+

Part II - Advanced dma_ usage
-----------------------------
--
Arthur