Subject: Re: [PATCH v5 00/39] i.MX Media Driver
To: Russell King - ARM Linux <linux@armlinux.org.uk>
References: <1489121599-23206-1-git-send-email-steve_longerbeam@mentor.com>
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Cc: mark.rutland@arm.com, andrew-ct.chen@mediatek.com,
        minghsiu.tsai@mediatek.com, sakari.ailus@linux.intel.com,
        nick@shmanahar.org, songjun.wu@microchip.com, hverkuil@xs4all.nl,
        Steve Longerbeam <steve_longerbeam@mentor.com>, pavel@ucw.cz,
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        sudipm.mukherjee@gmail.com
From: Steve Longerbeam <slongerbeam@gmail.com>
Message-ID: <c6eda3b3-52b8-8560-8f46-a6e2d6303bbd@gmail.com>
Date: Sun, 12 Mar 2017 13:16:02 -0700
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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