Re: [PATCH 3/3] dmaengine: tegra-adma: Add support for Tegra210 ADMA

[Jon Hunter <jonathanh-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>]

On 29/09/15 16:34, Arnd Bergmann wrote:
> On Tuesday 29 September 2015 15:18:07 Jon Hunter wrote:
>> On 29/09/15 13:39, Arnd Bergmann wrote:
>>> Ok, that makes more sense then. A few questions still:
>>>
>>> * Would the admaif in turn provide a #dma-cells and have the real slaves
>>>   hooked up to it?
>>
>> I don't think that that would be necessary. If you look at the existing
>> tegra i2s binding [0], there is a ahub-cif-ids property which maps the
>> actual slave to the apbif (equivalent of the adma-if). This ID is used
>> to get the appropriate dma channel for this client interface (cif).
>>
>>> * How do you model the xbar in this scenario?
>>
>> The xbar is common to existing tegra devices and today is configured via
>> the ahub-cif-ids property.
> 
> I see, so instead of modeling the xbar as part of the DMA engine in DT, you
> model it as part of the slave. This probably adds a bit of complexity
> and a somewhat nonstandard binding, but it's too late to change that anyway.
> 
>>> * How does the adma driver know whether you are using a rx or tx channel
>>>   in the above example, should that perhaps be another cell in dma
>>>   specifier? It seems that the numbers are all overlapping between rx and
>>>   tx (each has 1 through 10).
>>
>> The channels can be configured to be either rx or tx and yes the IDs do
>> overlap so you have 1-10 for both rx and tx. However, if you are asking
>> how do I ensure that only one channel uses a specific hardware request,
>> then yes I probably need to add the direction to the specifier to ensure
>> only one channel uses a request at any given time.
> 
> If each channel is either rx or tx, but the two cannot be used simultaneously,
> you can just list each of them only once, like this:
> 
> 	dmas = <&adma 0>, <&adma 1>, <&adma 2>, ...
> 	dma-names = "if0", "if1", "if2", ...
> 
> Otherwise you end up allocating twice the number of channels that you
> really need.

Each dma channel can be configured for either tx or rx. There are 10 tx
ports on the adma-if and 10 rx. So you could have 20 dma channels
configured for each one of the tx and rx ports. It is a bit confusing
because rx port 1 has a request signal of 1 and tx port 1 has a request
signal of 1 as well. So if you are wondering how the dma distinguishes
between rx port 1 and tx port 1, it does so by having a separate field
for the tx and rx request in the dma channel control register.

Cheers
Jon