[PATCH V3 1/2] of: Add generic device tree DMA helpers

[Jassi Brar]

On 16 May 2012 21:31, Jon Hunter <jon-hunter at ti.com> wrote:
> On 05/16/2012 08:15 AM, Jon Hunter wrote:
>> Hi Jassi,
>>
>> On 05/16/2012 07:37 AM, Jassi Brar wrote:
>>> Hi Jon,
>>>
>>> On 16 May 2012 06:41, Jon Hunter <jon-hunter at ti.com> wrote:
>>>> On 05/04/2012 02:01 PM, Jassi Brar wrote:
>>>>>
>>>>> +       i2c1: i2c at 1 {
>>>>> +               ...
>>>>> +               dma = <&sdma 2 1 &sdma 3 2>;
>>>>> +               ...
>>>>> +       };
>>>>>>
>>>>> I see this requires a client driver to specify a particular req_line on a
>>>>> particular dma controller. I am not sure if this is most optimal.
>>>>
>>>> Actually, no. The phandle in the DT specifies the DMA controller to use.
>>>> Then the client simply asks for a channel with a particular property,
>>>> for example, DMA_MEM_TO_DEV (ie. TX) and the channel information is return.
>>>>
>>> See below.
>>>
>>>>> I think such client->req_line map should be provided to the dmac controller
>>>>> driver via its dt node in some format. The dmac driver could then populate
>>>>> a dma_chan, or similar, only for that req_line and not for the unused one
>>>>> which otherwise could also have served the same client.
>>>>>
>>>>> Ideally the I2C driver should simply ask, say, a channel for TX and another
>>>>> for RX, everything else should already be setup via dmac's dt nodes.
>>>>
>>>> Yes that is the intention here.
>>>>
>>> But the client is required to specify the dmac that would serve it.
>>> Which is more
>>> than simply asking for "some suitable channel".
>>
>> No this is not the case with what I propose. The client knows nothing
>> about the dmac.
>
> By the way, I do see your point. You wish to describe the all the
> mappings available to all dma controllers and then set a mapping in the
> device tree. Where as I am simply setting a mapping and do not list all
> other possibilities (assuming that there some).
>
> What is still unclear to me, is if you use this token approach how
> readable is the device-tree? For example, if you have a client that can
> use one of two dmac and for each dmac the request/channel number is
> different, then by using a global token how can I determine what the
> options available for this client are?
>
Simple - you/client need not know about any option at all :)

Client driver would simply request some channel and if it
doesn't get it, it bails out.

It would be the DMACs' DT node that would contain that info.

> Take your example ...
>
> mmc1: mmc at 13002000 {
>        ...
>        dma_tx = <891>   //some platform-wide unique value
>        dma_rx = <927>   //some platform-wide unique value
>        ...
> };
>
> DMAC's Node:-
>
> pdma2: pdma at 10800000 {
>         .......
>        dma_map = <891, 7>,       // Map mmc1_tx onto i/f 7
>                  <927, 8>,       // Map mmc1_rx onto i/f 8
>        .......
> };
>
> But now I have another dmac which has the following options ...
>
> pdma1: pdma at 10000000 {
>         .......
>        dma_map = <598, 2>,       // Map mmc1_tx onto i/f 2
>                  <230, 3>,       // Map mmc1_rx onto i/f 3
>        .......
> };
>
No, rather the pdma1 node should look like

 pdma1: pdma at 10000000 {
         .......
        dma_map = <891, 2>,       // Map mmc1_tx onto i/f 2
                   <927, 3>,       // Map mmc1_rx onto i/f 3
        .......
};

Because the tokens 891 and 927 are came from the client's node/driver.

After the DMAC driver has probed both pdma-0 & 1, it would
know that MMC1 could be served by 2 DMACs. And basically
its the dmac driver that should be making about routing decisions,
not the client.

Btw, everything remains same, only we have now decided to not
use tokens but phandle+req_sig_ids instead.