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+==============================
+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>