PWM DMA interface

PWM DMA interface

[Gordon Hollingworth]

Hi,

I've been writing a driver that uses the PWM to talk to devices that
communicate through a pwm like signal.  To achieve this some devices have
a dma interface that you can stream data at to the pwm...

We use something similar to this on Raspberry Pi to do audio output, we
use the PWM interface to output a 'stream' of audio data which is
subsequently filtered (RC filter) to give an audio output.

I was hoping to create a driver that was suitably upstreamable and thought
about implementing within the world of the pwm class (and was coming here
to ask for guidance).  Basically I was thinking about implementing it in
as an extension of the pwm class:

Add parameter to config() to enable dma access, this will then create a
suitable /dev/pwm_dma0 etc. interface to write data to

Only issue is we have a number of parameters / modes we'd like to support
that are likely to be platform specific rather than general:

1) Ability to switch between three different modes:
	A) serial mode (where the output is actually just a serialisation of the
fifo data.)
	B) two formats of mark space ratio, giving a standard duty cycle or a
distributed mark space
2) Ability to set a 'silence value' i.e. what value you transmit when the
fifo runs dry

Whats your take on it?  In the end if not I'll implement a pwm_dma driver
as a platform device specific to the 2835 device

Thanks

Gordon


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Re: PWM DMA interface

[Thierry Reding]

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On Mon, Jun 09, 2014 at 10:49:46AM +0000, Gordon Hollingworth wrote:
> Hi,
> 
> I've been writing a driver that uses the PWM to talk to devices that
> communicate through a pwm like signal.  To achieve this some devices have
> a dma interface that you can stream data at to the pwm...
> 
> We use something similar to this on Raspberry Pi to do audio output, we
> use the PWM interface to output a 'stream' of audio data which is
> subsequently filtered (RC filter) to give an audio output.

What kind of audio data is this? Is it able to play back any arbitrary
audio signal?

Thierry

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Re: PWM DMA interface

[Gordon Hollingworth]

Yes within the limitations of the simple RC filter but basically it's working like a one bit DAC

Gordon


> On 17 Jun 2014, at 23:54, "Thierry Reding" <thierry.reding@gmail.com> wrote:
> 
>> On Mon, Jun 09, 2014 at 10:49:46AM +0000, Gordon Hollingworth wrote:
>> Hi,
>> 
>> I've been writing a driver that uses the PWM to talk to devices that
>> communicate through a pwm like signal.  To achieve this some devices have
>> a dma interface that you can stream data at to the pwm...
>> 
>> We use something similar to this on Raspberry Pi to do audio output, we
>> use the PWM interface to output a 'stream' of audio data which is
>> subsequently filtered (RC filter) to give an audio output.
> 
> What kind of audio data is this? Is it able to play back any arbitrary
> audio signal?
> 
> Thierry

Re: PWM DMA interface

[Thierry Reding]

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On Wed, Jun 18, 2014 at 08:20:58AM +0000, Gordon Hollingworth wrote:
> Yes within the limitations of the simple RC filter but basically it's
> working like a one bit DAC

Perhaps a better way to expose this, then, would be as a proper sound
device so that the standard ALSA tools can be used to play back audio
rather than needing a set of custom tools.

Thierry

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Re: PWM DMA interface

[Gordon Hollingworth]

On 18/06/2014 23:12, "Thierry Reding" <thierry.reding@gmail.com> wrote:


>On Wed, Jun 18, 2014 at 08:20:58AM +0000, Gordon Hollingworth wrote:
>> Yes within the limitations of the simple RC filter but basically it's
>> working like a one bit DAC
>
>Perhaps a better way to expose this, then, would be as a proper sound
>device so that the standard ALSA tools can be used to play back audio
>rather than needing a set of custom tools.
>
>Thierry

Yes, although it would be better to expose the PWM DMA interface through a
PWM class and then implement an ALSA driver on top of this rather than
just hacking in a Pi specific pwm driver...

Mostly because I want to write a different driver that uses the PWM DMA
interface on the Pi but it seems wrong to do this without having a common
PWM_DMA interface


Gordon