On Fri, Aug 01, 2014 at 03:30:20PM +0100, Russell King - ARM Linux wrote: > On Fri, Aug 01, 2014 at 10:51:26AM +0200, Laurent Pinchart wrote: > > I'll take this opportunity to question why we have a separation between > > tx_submit and issue_pending. What's the rationale for that, especially given > > that dma_issue_pending_all() might kick in at any point and issue pending > > transfers for all devices. A driver could thus see its submitted but not > > issued transactions being issued before it explicitly calls > > dma_async_issue_pending(). > > A prepared but not submitted transaction is not a pending transaction. > > The split is necessary so that a callback can be attached to the > transaction. This partially comes from the async-tx API, and also > gets a lot of use with the slave API. > > The prepare function allocates the descriptor and does the initial > setup, but does not mark the descriptor as a pending transaction. > It returns the descriptor, and the caller is then free to add a > callback function and data pointer to the descriptor before finally > submitting it. This sequence must occur in a timely manner as some > DMA engine implementations hold a lock between the prepare and submit > callbacks (Dan explicitly permits this as part of the API.) > > > The DMA_PRIVATE capability flag seems to play a role here, but it's far from > > being clear how that mechanism is supposed to work. This should be documented > > as well, and any light you could shed on this dark corner of the API would > > help. > > Why do you think that DMA_PRIVATE has a bearing in the callbacks? It > doesn't. DMA_PRIVATE is all about channel allocation as I explained > yesterday, and whether the channel is available for async_tx usage. > > A channel marked DMA_PRIVATE is not available for async_tx usage at > any moment. A channel without DMA_PRIVATE is available for async_tx > usage until it is allocated for the slave API - at which point the > generic DMA engine code will mark the channel with DMA_PRIVATE, > thereby taking it away from async_tx API usage. When the slave API > frees the channel, DMA_PRIVATE will be cleared, making the channel > available for async_tx usage. > > So, basically, DMA_PRIVATE set -> async_tx usage not allowed. > DMA_PRIVATE clear -> async_tx usage permitted. It really is that > simple. > > > Similarly, the DMA engine API is split in functions with different > > prefixes (mostly dmaengine_*, dma_async_*, dma_*, and various > > unprefixed niceties such as async_tx_ack or txd_lock. If there's a > > rationale for that (beyond just historical reasons) it should also > > be documented, otherwise a cleanup would help all the confused DMA > > engine users (myself included). > > dmaengine_* are generally the interface functions to the DMA engine code, > which have been recently introduced to avoid the multiple levels of > pointer indirection having to be typed in every driver. > > dma_async_* are the DMA engine interface functions for the async_tx API. > > dma_* predate the dmaengine_* naming, and are probably too generic, so > should probably end up being renamed to dmaengine_*. > > txd_* are all about the DMA engine descriptors. > > async_tx_* are the higher level async_tx API functions. Ah looks like I repeated the good answers from you. Should have read all replied first -- ~Vinod -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
