On Mon, Jun 06, 2011 at 05:06:03PM +0900, FUJITA Tomonori wrote: > max_segs isn't unrelated with the dma mapping API. I explained above, > IOMMUs doesn't increase the number of segments (could decrease the > number of segments by merging). > > The limitation about the number of segment already lives elsewhere > (e.g. queue's limits.max_segments). I think you're missing the point entirely. Lets take the problem at hand: you have two devices. One of them is handled by the DMA engine code. One of them is a block device. The block layer needs to know the various parameters of what is allowable for DMA, including such things as the maximum size of a segment, and the _number_ of segments that can be placed into any one request. As the DMA provider is _entirely_ separate and unknown to the block device driver, the block device driver has no way to sanely provide these parameters to the block layer - they are not a property of the block device driver, but of the DMA provider. However, the DMA provider can't know that it'll be interacting with the block layer, so there's no way for the DMA provider to tell the block layer about its parameters. The only way this can happen is for there to be some way for the DMA provider to export this information to the block device driver, so the block device driver can then inform its upper layers what the DMA capabilities are. So to say that "it lives elsewhere" is completely missing the problem trying to be addressed by these patches. -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
