Re: [RFC v2 2/5] dmaengine: Add slave DMA interface

On Jan 30, 2008 3:52 AM, David Brownell <david-b@xxxxxxxxxxx> wrote:
On Wednesday 30 January 2008, Haavard Skinnemoen wrote:
Descriptor-based vs. register-based transfers sounds like something the
DMA engine driver is free to decide on its own.

Not entirely. The current interface has "dma_async_tx_descriptor"
wired pretty thoroughly into the call structure -- hard to avoid.
(And where's the "dma_async_rx_descriptor", since that's only TX??
Asymmetry like that is usually not a healthy sign.) The engine is
not free to avoid those descriptors ...


For better or worse I picked async_tx to represent "asynchronous
transfers/transforms", not "transmit". So there is no asymmetry as it
is used for operations in any direction, or multiple directions as is
the case with xor. It is simply a gathering point for the common
functionality of descriptor-based offload-engines plus some extra
stuff to deal with creating arbitrary dependency chains.

And consider that many DMA transfers can often be started (after
cache synch operations) by writing less than half a dozen registers:
source address, destination address, params, length, enable. Being
wildly generous, let's call that a couple dozen instructions, including
saving "what to do when it's done". The current framework requires
several calls just to fill descriptors ... burning lots more than that
many instructions even before getting around to the Real Work! (So I
was getting at low DMA overheads there, more than any particular way
to talk to the controller.)


Well, it has gone from 4 calls to 2 recently for the memcpy case. The
only reason it is not 1 call is to support switching dependency chains
between channels i.e. performing some copies on one channel followed
by an xor an another.

--
Dan
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