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

[Seraj Mohammed <seraj.mohammed@sondrel.com>]

Hi,

On 31/08/18 15:22, Peter Ujfalusi wrote:
> 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...

Since this patch already allows you to specifically select which
channels to run on, i don't see the benefit in allocating channels using
ranges.

>> 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.

Once you have successfully completed a test, you don't need to reboot to
start a new one with different parameters.

> - Péter
> 
> Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
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>