Re: [PATCH V3 10/15] [media] marvell-ccic: split mcam-core into 2 parts for soc_camera support

[Jonathan Corbet <corbet@lwn.net>]

My apologies for the slow response...I'm running far behind.

On Thu, 31 Jan 2013 00:29:13 -0800
Albert Wang <twang13@marvell.com> wrote:

> As you know, we are working on adding B_DMA_SG support on soc_camera mode.
> 
> We found there is some code we can't understand in irq handler:
> >>>>>>  
> if (handled == IRQ_HANDLED) {
> 	set_bit(CF_DMA_ACTIVE, &cam->flags);
> 	if (cam->buffer_mode == B_DMA_sg)
> 		mcam_ctlr_stop(cam);
> }
> <<<<<<
> 
> The question is why we need stop ccic in irq handler when buffer mode is B_DMA_sg?

That's actually intended to be addressed by this comment in the DMA setup
code:

/*
 * Frame completion with S/G is trickier.  We can't muck with
 * a descriptor chain on the fly, since the controller buffers it
 * internally.  So we have to actually stop and restart; Marvell
 * says this is the way to do it.
 *

...and, indeed, at the time, I was told by somebody at Marvell that I
needed to stop the controller before I could store a new descriptor into
the chain.  I don't see how it could work otherwise, really?

I'd be happy to see this code go, it always felt a bit hacky.  But the
controller buffers the descriptor chain deep inside its unreachable guts,
so one has to mess with it carefully.

Thanks,

jon