[PATCH weston v11 03/13] compositor-drm: Add DRM property cache

[Daniel Stone]

Hi,

On 25 July 2017 at 11:28, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> On Mon, 24 Jul 2017 18:13:39 +0100
> Daniel Stone <daniel at fooishbar.org> wrote:
>> > I believe this should shorten to: "Call this for every DRM object
>> > individually once."
>> >
>> > The language about status changes is probably a remnant of the value
>> > caching days.
>>
>> When a connector is hotplugged, that can invalidate the enum
>> properties, so we still need to re-scan.
>
> Does that not mean that a new DRM connector object appears? Or that an
> existing DRM connector object magically becomes rewritten in which case
> what do we use for triggering the re-scan?
>
> Do we need to re-scan all DRM connector objects every time we get a
> hotplug event?
>
> I mean, now that you said that, and I have read the doc comment a dozen
> times more, I'm getting a feeling of what it's trying to say in a very
> very compressed form. Expand a bit, please. :-)

Well, not every panel is identical. Some are going to have different
capabilities to others. None of these apply to EDID/DPMS, but here are
a few examples:

There are patches on dri-devel@ for HDCP support at the moment. On a
non-HDCP-capable sink, you might want to have a single 'unavailable'
enum available, but then when you hotplug to a HDCP-capable sink,
suddenly you have 'enabled' and 'disabled' enum.

TV outputs have a surplus of available connector properties, including
which connector type to use (e.g. component vs. S-Video), 'flicker
reduction' / filtering, overscan, etc.

For scaling, you can control a surprising amount through, e.g., HDMI
infoframes. Again, whether or not this is available - and what's
variable - depends on your sink. So just plugging in a different
device might give you different options for scaling.

Those are just the ones I could find with a quick scan, but they are
all examples of enum properties whose values differ depending on the
currently-connected device. Which is why we need to rescan on hotplug.

Cheers,
Daniel