Document the new design of the pxa dma driver. Signed-off-by: Robert Jarzmik <robert.jarzmik@xxxxxxx> --- Since v2: Vinod's review --- Documentation/dmaengine/pxa_dma.txt | 153 ++++++++++++++++++++++++++++++++++++ 1 file changed, 153 insertions(+) create mode 100644 Documentation/dmaengine/pxa_dma.txt diff --git a/Documentation/dmaengine/pxa_dma.txt b/Documentation/dmaengine/pxa_dma.txt new file mode 100644 index 0000000..413ef9c --- /dev/null +++ b/Documentation/dmaengine/pxa_dma.txt @@ -0,0 +1,153 @@ +PXA/MMP - DMA Slave controller +============================== + +Constraints +----------- + a) Transfers hot queuing + A driver submitting a transfer and issuing it should be granted the transfer + is queued even on a running DMA channel. + This implies that the queuing doesn't wait for the previous transfer end, + and that the descriptor chaining is not only done in the irq/tasklet code + triggered by the end of the transfer. + A transfer which is submitted and issued on a phy doesn't wait for a phy to + stop and restart, but is submitted on a "running channel". The other + drivers, especially mmp_pdma waited for the phy to stop before relaunching + a new transfer. + + b) All transfers having asked for confirmation should be signaled + Any issued transfer with DMA_PREP_INTERRUPT should trigger a callback call. + This implies that even if an irq/tasklet is triggered by end of tx1, but + at the time of irq/dma tx2 is already finished, tx1->complete() and + tx2->complete() should be called. + + c) Channel running state + A driver should be able to query if a channel is running or not. For the + multimedia case, such as video capture, if a transfer is submitted and then + a check of the DMA channel reports a "stopped channel", the transfer should + not be issued until the next "start of frame interrupt", hence the need to + know if a channel is in running or stopped state. + + d) Bandwidth guarantee + The PXA architecture has 4 levels of DMAs priorities : high, normal, low. + The high prorities get twice as much bandwidth as the normal, which get twice + as much as the low priorities. + A driver should be able to request a priority, especially the real-time + ones such as pxa_camera with (big) throughputs. + +Design +------ + a) Virtual channels + Same concept as in sa11x0 driver, ie. a driver was assigned a "virtual + channel" linked to the requestor line, and the physical DMA channel is + assigned on the fly when the transfer is issued. + + b) Transfer anatomy for a scatter-gather transfer + +------------+-----+---------------+----------------+-----------------+ + | desc-sg[0] | ... | desc-sg[last] | status updater | finisher/linker | + +------------+-----+---------------+----------------+-----------------+ + + This structure is pointed by dma->sg_cpu. + The descriptors are used as follows : + - desc-sg[i]: i-th descriptor, transferring the i-th sg + element to the video buffer scatter gather + - status updater + Transfers a single u32 to a well known dma coherent memory to leave + a trace that this transfer is done. The "well known" is unique per + physical channel, meaning that a read of this value will tell which + is the last finished transfer at that point in time. + - finisher: has ddadr=DADDR_STOP, dcmd=ENDIRQEN + - linker: has ddadr= desc-sg[0] of next transfer, dcmd=0 + + c) Transfers hot-chaining + Suppose the running chain is : + Buffer 1 Buffer 2 + +---------+----+---+ +----+----+----+---+ + | d0 | .. | dN | l | | d0 | .. | dN | f | + +---------+----+-|-+ ^----+----+----+---+ + | | + +----+ + + After a call to dmaengine_submit(b3), the chain will look like : + Buffer 1 Buffer 2 Buffer 3 + +---------+----+---+ +----+----+----+---+ +----+----+----+---+ + | d0 | .. | dN | l | | d0 | .. | dN | l | | d0 | .. | dN | f | + +---------+----+-|-+ ^----+----+----+-|-+ ^----+----+----+---+ + | | | | + +----+ +----+ + new_link + + If while new_link was created the DMA channel stopped, it is _not_ + restarted. Hot-chaining doesn't break the assumption that + dma_async_issue_pending() is to be used to ensure the transfer is actually started. + + One exception to this rule : + - if Buffer1 and Buffer2 had all their addresses 8 bytes aligned + - and if Buffer3 has at least one address not 4 bytes aligned + - then hot-chaining cannot happen, as the channel must be stopped, the + "align bit" must be set, and the channel restarted As a consequence, + such a transfer tx_submit() will be queued on the submitted queue, and + this specific case if the DMA is already running in aligned mode. + + d) Transfers completion updater + Each time a transfer is completed on a channel, an interrupt might be + generated or not, up to the client's request. But in each case, the last + descriptor of a transfer, the "status updater", will write the latest + transfer being completed into the physical channel's completion mark. + + This will speed up residue calculation, for large transfers such as video + buffers which hold around 6k descriptors or more. This also allows without + any lock to find out what is the latest completed transfer in a running + DMA chain. + + e) Transfers completion, irq and tasklet + When a transfer flagged as "DMA_PREP_INTERRUPT" is finished, the dma irq + is raised. Upon this interrupt, a tasklet is scheduled for the physical + channel. + The tasklet is responsible for : + - reading the physical channel last updater mark + - calling all the transfer callbacks of finished transfers, based on + that mark, and each transfer flags. + If a transfer is completed while this handling is done, a dma irq will + be raised, and the tasklet will be scheduled once again, having a new + updater mark. + + f) Residue + Residue granularity will be descriptor based. The issued but not completed + transfers will be scanned for all of their descriptors against the + currently running descriptor. + + g) Most complicated case of driver's tx queues + The most tricky situation is when : + - there are not "acked" transfers (tx0) + - a driver submitted an aligned tx1, not chained + - a driver submitted an aligned tx2 => tx2 is cold chained to tx1 + - a driver issued tx1+tx2 => channel is running in aligned mode + - a driver submitted an aligned tx3 => tx3 is hot-chained + - a driver submitted an unaligned tx4 => tx4 is put in submitted queue, + not chained + - a driver issued tx4 => tx4 is put in issued queue, not chained + - a driver submitted an aligned tx5 => tx5 is put in submitted queue, not + chained + - a driver submitted an aligned tx6 => tx6 is put in submitted queue, + cold chained to tx5 + + This translates into (after tx4 is issued) : + - issued queue + +-----+ +-----+ +-----+ +-----+ + | tx1 | | tx2 | | tx3 | | tx4 | + +---|-+ ^---|-+ ^-----+ +-----+ + | | | | + +---+ +---+ + - submitted queue + +-----+ +-----+ + | tx5 | | tx6 | + +---|-+ ^-----+ + | | + +---+ + - completed queue : empty + - allocated queue : tx0 + + It should be noted that after tx3 is completed, the channel is stopped, and + restarted in "unaligned mode" to handle tx4. + +Author: Robert Jarzmik <robert.jarzmik@xxxxxxx> -- 2.1.4 -- 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
