Re: Interrupt handler responsibility

[Hans Verkuil <hverkuil@xs4all.nl>]

On 11/21/2015 11:20 PM, Ran Shalit wrote:
> Hello,
> 
> I am trying to understand the interrupt handler responsibility in
> v4l2, also with respect to dma usage. I see that it is not defined as
> part of the videobuf2 API.
> 
> This is what I understand this far:
> 1. start_streaming is responsible for getting into "streaming" state.
> dma start should be trigggered at this point.

Right.

> 2. interrupt handler: is responsible for passing back the buffer to
> user using vb2_buffer_done() call.

Right.

> 
> But what is the exact reponsibility of interrupt handler with respect
> to dma usage  ?

Typically when the DMA has finished DMAing a frame it will generate an interrupt.
The interrupt handler will then call vb2_buffer_done() handing the buffer back
to the vb2 framework. Think of it as who owns the buffer: when buf_queue is called
the buffer is handed from vb2 to the driver, and the driver calls vb2_buffer_done()
when it is finished with the buffer (i.e. the data is DMAed into the buffer) and
it hands it back to vb2.

> In some of the drivers I see that the interrupt start/stop dma, but in
>  v4l2-pci-skeleton.c I don't see any usage of dma in the interrupt
> handler, so I'm not sure.

The interrupt handler is called in response of a DMA interrupt. How that interrupt
is generated is hardware specific, so that's why you don't see it in the skeleton
driver.

Regards,

	Hans