Date:   Sat, 7 Jul 2018 14:15:28 -0700
From:   Steve Longerbeam <slongerbeam@...il.com>
To:     Sakari Ailus <sakari.ailus@....fi>,
        Steve Longerbeam <slongerbeam@...il.com>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 00/17] media: imx: Switch to subdev notifiers

Hi Sakari,


On 07/02/2018 02:51 AM, Sakari Ailus wrote:
> Hi Steve,
>
> On Fri, Jun 29, 2018 at 11:49:44AM -0700, Steve Longerbeam wrote:
>> This patchset converts the imx-media driver and its dependent
>> subdevs to use subdev notifiers.
>>
>> There are a couple shortcomings in v4l2-core that prevented
>> subdev notifiers from working correctly in imx-media:
>>
>> 1. v4l2_async_notifier_fwnode_parse_endpoint() treats a fwnode
>>     endpoint that is not connected to a remote device as an error.
>>     But in the case of the video-mux subdev, this is not an error,
>>     it is OK if some of the mux inputs have no connection. Also,
>>     Documentation/devicetree/bindings/media/video-interfaces.txt explicitly
>>     states that the 'remote-endpoint' property is optional. So the first
>>     patch is a small modification to ignore empty endpoints in
>>     v4l2_async_notifier_fwnode_parse_endpoint() and allow
>>     __v4l2_async_notifier_parse_fwnode_endpoints() to continue to
>>     parse the remaining port endpoints of the device.
>>
>> 2. In the imx-media graph, multiple subdevs will encounter the same
>>     upstream subdev (such as the imx6-mipi-csi2 receiver), and so
>>     v4l2_async_notifier_parse_fwnode_endpoints() will add imx6-mipi-csi2
>>     multiple times. This is treated as an error by
>>     v4l2_async_notifier_register() later.
>>
>>     To get around this problem, add an v4l2_async_notifier_add_subdev()
>>     which first verifies the provided asd does not already exist in the
>>     given notifier asd list or in other registered notifiers. If the asd
>>     exists, the function returns -EEXIST and it's up to the caller to
>>     decide if that is an error (in imx-media case it is never an error).
>>
>>     Patches 2-5 deal with adding that support.
>>
>> 3. Patch 6 adds v4l2_async_register_fwnode_subdev(), which is a
>>     convenience function for parsing a subdev's fwnode port endpoints
>>     for connected remote subdevs, registering a subdev notifier, and
>>     then registering the sub-device itself.
>>
>> 4. Patches 7-14 update the subdev drivers to register a subdev notifier
>>     with endpoint parsing, and the changes to imx-media to support that.
>>
>> 5. Finally, the last 3 patches endeavor to completely remove support for
>>     the notifier->subdevs[] array in platform drivers and v4l2 core. All
>>     platform drivers are modified to make use of
>>     v4l2_async_notifier_add_subdev() and its related convenience functions
>>     to add asd's to the notifier @asd_list, and any allocation or reference
>>     to the notifier->subdevs[] array removed. After that large patch,
>>     notifier->subdevs[] array is stripped from v4l2-async and v4l2-subdev
>>     docs are updated to reflect the new method of adding asd's to notifiers.
>>
>>
> Thanks for the update! This is beginning to look really nice. A few notes
> on the entire set. I'll separately review some of the patches; I mainly
> wanted to see how the async/fwnode framework changes end up:
>
> - The reason V4L2_MAX_SUBDEVS exists is to avoid drivers accidentally
>    allocating more space than intended. Now that the subdevs array will
>    disappear, the checks as well as the macro can be removed. I think the
>    num_subdevs field also becomes redundant as a result. Could you do this
>    in the patch that removes the subdevs array?

Yes, good idea, done! It was easy to do.

>
> - The notifier has register, unregister and cleanup operations. Now that
>    there's an obvious need to initialise it, it'd make sense to show that to
>    the drivers as an init operation --- rather than silently initialise it
>    based on the need.

That was actually my plan initially, but at the time, it meant modifying
all platform drivers to call v4l2_async_notifier_init(). But since this 
patchset
now touches all platform drivers anyway, might as well do it.

Of course doing this adds more risk, since I have to call 
v4l2_async_notifier_init()
in the correct places. But cross-my-fingers, I think I've done this 
correctly.

>
> - I'd assign j in its declaration in
>    v4l2_async_notifier_has_async_subdev().

Done.

>
> - No need to explicitly check that the notifier's asd_list is empty in
>    __v4l2_async_notifier_register --- list_for_each_entry() over the same
>    list will be nop in that case.
>

Done.

I'm submitting v8 shortly.

Steve

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