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

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Message-ID: <c6eda3b3-52b8-8560-8f46-a6e2d6303bbd@gmail.com>
Date:   Sun, 12 Mar 2017 13:16:02 -0700
From:   Steve Longerbeam <slongerbeam@...il.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     mark.rutland@....com, andrew-ct.chen@...iatek.com,
        minghsiu.tsai@...iatek.com, sakari.ailus@...ux.intel.com,
        nick@...anahar.org, songjun.wu@...rochip.com, hverkuil@...all.nl,
        Steve Longerbeam <steve_longerbeam@...tor.com>, pavel@....cz,
        robert.jarzmik@...e.fr, devel@...verdev.osuosl.org,
        markus.heiser@...marIT.de,
        laurent.pinchart+renesas@...asonboard.com, shuah@...nel.org,
        geert@...ux-m68k.org, linux-media@...r.kernel.org,
        devicetree@...r.kernel.org, kernel@...gutronix.de, arnd@...db.de,
        mchehab@...nel.org, bparrot@...com, robh+dt@...nel.org,
        horms+renesas@...ge.net.au, tiffany.lin@...iatek.com,
        linux-arm-kernel@...ts.infradead.org,
        niklas.soderlund+renesas@...natech.se, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, jean-christophe.trotin@...com,
        p.zabel@...gutronix.de, fabio.estevam@....com, shawnguo@...nel.org,
        sudipm.mukherjee@...il.com
Subject: Re: [PATCH v5 00/39] i.MX Media Driver



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