Re: [PATCH v5 00/39] i.MX Media Driver

[Steve Longerbeam]

On 03/12/2017 12:44 PM, Steve Longerbeam wrote:
> On 03/12/2017 12:29 PM, Russell King - ARM Linux wrote:
> > On Sun, Mar 12, 2017 at 12:21:45PM -0700, Steve Longerbeam wrote:
> > > There's actually nothing preventing userland from disabling a link
> > > multiple times, and imx_media_link_notify() complies, and so
> > > csi_s_power(OFF) gets called multiple times, and so that WARN_ON()
> > > in there is silly, I borrowed this from other MC driver examples,
> > > but it makes no sense to me, I'll remove it and prevent the power
> > > count from going negative.
> >
> > Hmm.  So what happens if one of the CSI's links is enabled, and we
> > disable a different link from the CSI several times?  Doesn't that
> > mean the power count will go to zero despite there being an enabled
> > link?
>
> Yes, the CSI will be powered off even if it still has an enabled link.
> But one of its other links has been disabled, meaning the pipeline as
> a whole is disabled. So I think it makes sense to power down the CSI,
> the pipeline isn't usable at that point.
>
> And remember that the CSI does not allow both output pads to be enabled
> at the same time. If that were so then indeed there would be a problem,
> because it would mean there is another active pipeline that requires the
> CSI being powered on, but that's not the case.
>
> I think this is consistent with the other entities as well, but I will
> double check.


At first I thought this could be a problem for one entity, the csi-2
receiver.

It can enable all four of its output pads at once (if the input stream
contains all 4 virtual channels, the csi-2 receiver must support
demuxing all of them onto all 4 of its output pads).

But after more review, this should not be an issue. If a csi-2 sink
(a CSI or a CSI mux) link is disabled, the csi-2 receiver is no longer
reachable from that sink, so attempts to disable the csi-2 via that
path again is not possible. The other potential problem is disabling
from the csi-2's own sink pad, but in that case the csi-2 no longer
has a source, so again it makes sense to power off the csi-2 even
if it has enabled output pads.


Steve