[PATCH v2 1/5] Documentation: dmaengine: pxa-dma design

Robert Jarzmik
2015-04-11T19:42:00Z

Document the new design of the pxa dma driver.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
---
Documentation/dmaengine/pxa_dma.txt | 157 ++++++++++++++++++++++++++++++++++++
1 file changed, 157 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..63db9fe
--- /dev/null
+++ b/Documentation/dmaengine/pxa_dma.txt
@@ -0,0 +1,157 @@
+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.
+
+ 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 residue calculation
+ A channel should be able to report how much advanced is a transfer. The
+ granularity is still descriptor based.
+
+ d) 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.
+
+ e) 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.
+
+ f) Transfer reusability
+ An issued and finished transfer should be "reusable". The choice of
+ "DMA_CTRL_ACK" should be left to the client, not the dma driver.
+
+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
+
+ b) 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.
+
+ c) 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.
+
+ d) 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.
+
+ e) 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.
+
+ f) 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>
--
2.1.4