[V2,5/5] dmaengine: Documentation: Add documentation for multi chan testing

[Peter Ujfalusi <peter.ujfalusi@ti.com>]

Hi,

On 2018-08-31 16:41, Seraj Mohammed wrote:
>> Can I still use:
>> echo "" > /sys/module/dmatest/parameters/channel
>>
>> to stop dmatest to run on the specific channel(s) or do I need to reboot
>> the machine to do so?
> 
> is this behavior documented somewhere? or did you discover this from your
> experience with using the module?

If the channels is "" then it is not set. We can change the channels
currently and there is no difference between selecting a channel or
selecting none.

> The short answer to your question is no, with this change you cannot stop
> a test by echoing an empty string to the channel parameter, you will either
> have to run the test, or reboot the machine.
> 
> Setting the channel parameter to an empty string will request all
> channels up to "max_channels" parameter value.

Yes, and it is very useful. What would be even more useful is if via the
channel I could select the starting channel from where the max_channels
will start...

> This is because with this
> change, editing the channel parameter will invoke the process of allocating
> a thread for the specified channel and storing that thread in a thread
> list, after that, setting run to 1 will simply iterate through the thread
> list and start all pending threads.

We have automated regression tests via dmatest's memcpy, it does changes
parameters between runs to gather the data for different transfer sizes,
runs in selected channels and runs also with different max_channels.

It is a regression if we need to reboot between every test to change
parameters, channels, etc.

- Péter

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