On 17 May 2012 05:29, Stephen Warren [off-list ref] wrote:

quoted

quoted

Generic binding to provide a way to provide the client-channel map and
other dmac specific parameters to the dma controller driver

DMA Model:-
  Only the most common characteristics of a dma setup are assumed
in this binding.
Client: Some h/w controller that could request a DMA controller in
the system to perform data transfer on its behalf. Example SPI, MMC,
I2S etc.
DMAC: A DMA Controller instance. Example, PL330, PL08X, SDMA etc.

 The assumed model of the DMAC, in this binding, has P peripheral
interfaces (P request signals) that could request a data transfer
and C physical channels that actually do the data transfers, hence,
at most C out of P peripherals could be served by the DMAC at any
point of time. Usually C := P, but not always. Usually, any of the
physical channels could be employed by the DMAC driver to serve any
client.
 The DMAC driver identifies a client by its i/f number, 'peri_id'
on the given DMAC. For example, TX for SPI has 7th while RX_TX
(half-duplex) for MMC has 10th peripheral interface (request-signal)
on a given DMAC. Usually, any of the physical channels could be
employed by the DMAC driver to serve a client.

I'm still anything but convinced by this model. Basically it's the
exact opposite of what we do for every other subsystem (irq, pinctrl,
regulator, gpio, ...), where the device using some infrastructure
contains the information about who provides it, whereas here you
move all the information into the device that provides the functionality,
and have no handle in the device using it by which the driver can
identify it.

Yes, I guess this is backwards. But, the HW is a little different too;
GPIOs (and probably interrupts) don't have multiple places they could
come from.

The problem is that if we did something like this in the DMA client:

dma-reqs = <&dmac1 DMAC1_DMA_REQ_6 &dmac1 DMAC1_DMA_REQ_8>;

how do we know if the client is emitting 2 DMA request signals that get
routed to two different inputs on the DMA controller, or whether this is
two alternatives for the same signal.

FWIW, I wouldn't lose sleep over the possibility of redundancy on same DMAC.
If a client's request signal is routed to 2 inputs, they are usually
on different DMACs.
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