[PATCH] drm/drm_bridge: adjust bridge module's refcount

[Andrzej Hajda]

On 01.12.2016 08:18, Daniel Vetter wrote:
> On Thu, Dec 01, 2016 at 08:07:29AM +0100, Andrzej Hajda wrote:
>> On 30.11.2016 14:09, Daniel Vetter wrote:
>>> On Wed, Nov 30, 2016 at 01:03:20PM +0200, Laurent Pinchart wrote:
>>>> On Wednesday 30 Nov 2016 11:55:20 Daniel Vetter wrote:
>>>>> Why exactly do you want to hotplug encoders? Or bridges fwiw ... since at
>>>>> least only making those hotpluggable will make the uabi story easier since
>>>>> they're not exposed.
>>>> Ideally to avoid disabling the whole display engine when one encoder isn't 
>>>> available/operational. Right now we're waiting for all pieces to be available 
>>>> (using deferred probing or the component framework) before registering the DRM 
>>>> device. This means that if one bridge can't be probed successfully for any 
>>>> reason we'll end up having not display at all. This includes the case where 
>>>> the driver for the bridge is not available. If we could support dynamic 
>>>> hotplug of bridges, we could start with a display engine that supports a 
>>>> subset of the outputs, and add new outputs as they become operational.
>>>>
>>>> We have a similar issue when unbinding bridge devices from their driver. They 
>>>> obviously can't be used anymore, but we have no solution to handle that apart 
>>>> from unregistering the DRM device completely, as otherwise rebinding the 
>>>> bridge to the driver later can't be handled.
>>> This all sounds pretty cool, but does anyone care? Like what's the
>>> real-world use-case here? Some cosmic ray destroyed the bridge driver on
>>> your android phone and now you want it to magically fall back to hdmi that
>>> no one ever plugs in? Or someone misconfigures their kernel and gets
>>> greeted with a black screen, instead of a ... black screen?
>> Real use case is that we need to always load hdmi path drivers at phone
>> startup just in case somebody will use it.
>> This way we are wasting space and more importantly boot time, for code
>> which won't be used by 99% users of phones.
>> Putting them into modules an loading on MHL/HDMI cable plug-in would be
>> more optimal, I guess.
> Do we have numbers for this?

For number of HDMI/MHL users in mobiles, I have no stats :)
For display boot delay due to deferring hdmi driver is 2.5-3.5 seconds
on peach-pi board for example [1].

[1]:
https://storage.kernelci.org/ulfh/v4.9-rc7-120-g38cdf7e0bfee/arm-exynos_defconfig/lab-baylibre-seattle/boot-exynos5800-peach-pi.html


>  What part of the overhead is the edid probing
> and reading, which we probably should optimize either way ... optimize as
> in make sure we never ever stall anything for edid reads.

As EDID probing should be performed only after detecting sink it seems
irrelevant here.

>
> And if you never load the hdmi driver, how do you know when to load it
> because the user plugged in the cable?

Mobiles often have detection which cable is plugged in. However I am not
sure if kernel sends such events to userspace,
but this should be simple to do.

Regards
Andrzej