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

[Jassi Brar <jaswinder.singh@linaro.org>]

On 10 May 2012 00:40, Stephen Warren <swarren@wwwdotorg.org> wrote:
> On 05/08/2012 01:09 PM, Jassi Brar wrote:
>>
>> There is important difference between providing data via clients
>> during runtime vs providing info about every client to the dmac driver
>> at one go during its probe.
>
> I certainly see that there is a difference.
>
> I don't understand why it's an important difference; I think everything
> needs to be handled at run-time no matter what:
>
> Just because there is information in the DT that "this client DT node
> uses this channel/request-ID/... of this DMA controller" doesn't mean
> that the driver for the client is going to be loaded, or that SW running
> on the system is going to cause that driver to actually be opened and do
> any DMA. As such, any resource allocation rules etc. must be evaluated
> only if/when a driver actually requests/uses a DMA channel, so having
> "all the information up front" doesn't seem like a theoretically
> possible thing anyway.
>
One point is about 'qos' here.... something like bandwidth allocation.
If the dmac driver knew up-front the max possible clients that could be
active simultaneously, it could "divide the bandwidth" accordingly.
Again, the example of PL330 employing 2physical channels for better
service - LLI (you got it right), where even 1 physical channel would
also have served only not as reliably. I believe there would be
more such scenarios.

Another minor benefit could be that the dmac driver populate only
as many "struct dma_chan" as there are actually clients on the
machine (and this population has to be done during probe).
It could mean ~8 instead of ~40 channels populated on a machine.
Note, a PL330 dmac can have 32 channels, OMAP's has 128....

Most importantly, I just think it's a cleaner approach.


> Very similar to this, in order to support your point that a given client
> could potentially use a channel from either of 2 different DMA
> controller instances, you don't know until run-time which controller
> will be used, so can't have up-front information in this scenario
> either, even if the DMA does actually take place.
>
Sorry, perhaps I wasn't clear enough.
A client sees _no_ difference between the two channels that could serve it.
And it can't start using two, if two are available.
Client just needs one suitable channel on whichever dmac that might be.
If the channel for a client is to be switched runtime, that decision
should lie with the dmac driver, not the client. And I am not really
after implementing that runtime decision support.


> Solving (b) seems to require something very roughly like:
>
> dma-controller@0 {
>    compatible = "foo,dmac-xxx";
>    dma-requests = <
>                &client0 0 // DMAC req 0 is client0's 0th req output
>                &client0 1
>                &client1 0
>                &client1 1
>                &client2 0
>                ...>;
> };
>
> dma-controller@1 {
>    compatible = "bar,dmac-yyy";
>    dma-requests = <
>                // Supports some client modules, but not the same ones
>                &client0 0
>                &client1 0
>                &client3 0
>                ...>;
> };
>
> client0: i2s {
>    /* has 2 DMA request output signals: 0, 1 */
> };
>
> client1: spdif {
>    /* has 2 DMA request signals: 0, 1 */
> };
>
> client2: foo {
>    /* has 1 DMA request signal: 0 */
> };
>
> client3: bar {
>    /* has 1 DMA request signal: 0 */
> };
>
> and then having the core DMA code have an API like:
>
> channel = dma_request(clients_dma_req_output_id)
>
> which looks in each DMA controller's dma-requests list for the client's
> phandle and matching dma_req_output_id.
>
Almost what I proposed in my third post in this thread !!

The minor difference being, you use the {request-signal, phandle} pair
to find the right channel, I used only 'token'.

Please note, with this method we specify info about every client at one
go and via dmac's DT node - hence those benefits.

Also note that, a client's dma specifier becomes perfectly general
and future-proof
   client1: spdif {
          dma_tx = <278>
          dma_rx = <723>
    };

otherwise the following representation
    client1: spdif {
               dma = <&sdma 2 1 &sdma 3 2>;
     };
could break with some weird dma setups (IIANW Russell already finds
it wouldn't work for him).

I am arguing for (b) for the above mentioned reasons, and not because
I want to have clients switching channels in runtime.

Thanks,
Jassi
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