On Tue, 2012-07-24 at 14:07 -0500, Jon Hunter wrote:

Hi Vinod,

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Required property:
    dmas: list of one or more dma specifiers, each consisting of
     - phandle pointing to dma controller node
     - flags word, a bit map that can hold these flags
       * 0x00000001 channel can be used for transfer from device
       * 0x00000002 channel can be user for transfer to device

Is this for identifying which channel is for TX and RX? If not I am not
sure I understood it well

Yes, but we can potentially add more flags here.

The argument we had when coming up with this was roughly:

* we need to identify which specifiers are referring to the same
  conceptual channel and can be used as alternatives
* this could be done just using the dma-names property, but making
  dma-names mandatory adds complexity for everyone.
* Most devices have just one or two channels, and if they have two,
  there is usually one input and one output.

=> if the common dmaengine code can find out whether a channel is
  input or output without looking at the dmac driver specific configuration,
  we don't need to add dma-names in most cases, but just let the client
  driver ask for "give me a channel with these flags".

No we don't export the direction of the channel and usually channel can
be configured either way.

So yes I can see that a channel itself could be configured to support a
given direction, but when we ask for a channel via dma_request_channel()
we are going to get a channel that matches the criteria we pass using
the filter parameter. So here the thinking was that "flags" is a filter
parameter that the user could specify and one example being direction
but it could be something else too.

Yes that can be done, but I am leaning towards clients not have to do
anything :) DMAEngine needs to know mapping and when
dma_request_channel() is called it _always_ gives you the right channel.

Maybe for slave case we need to create dma_request_slave_channel() which
has additional arguments for dmaengine to do the filtering.

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But from a client POV it makes sense as with the given direction you
would need a specific request line for a channel. So this is right.
But direction is something I don't expect to be used for "give me a
channel" 

Ok. The thought was that the user would have the following means of
requesting a channel ...

1. By name

Bare name maynot be enough. In a dmac we have many channels which one to
choose?

2. By a filter parameter (flags)

Even with direction same problem can arise

3. By name and a filter parameter

Additionally we need to say which channel, or making dmaengine already
aware will help here

So we would have the following APIs ...

struct dma_chan
*of_dma_request_channel(struct device_node *node, unsigned int flags);
struct dma_chan
*of_dma_request_named channel(struct device_node *node, char *name,
unsigned int flags);

In both of these the filter parameter flags is optional.

Let me know your thoughts on this.

I would call them dma_request_slave_channel and try to add to it for
additional filtering



-- 
~Vinod