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| -rw-r--r-- | Documentation/driver-api/dmaengine/pxa_dma.rst | 190 |
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diff --git a/Documentation/driver-api/dmaengine/pxa_dma.rst b/Documentation/driver-api/dmaengine/pxa_dma.rst new file mode 100644 index 000000000..442ee691a --- /dev/null +++ b/Documentation/driver-api/dmaengine/pxa_dma.rst @@ -0,0 +1,190 @@ +============================== +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 priorities 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@free.fr> |