Documentation: dmaengine: pxa-dma design

Document the new design of the pxa dma driver.
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

Documentation/dmaengine/pxa_dma.txt

+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@free.fr>