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

[Daniel Vetter]

On Thu, Dec 01, 2016 at 02:22:18PM +0100, Andrzej Hajda wrote:
> 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].

That sounds horrible. We load our entire driver in that time, and it has 3
hdmi ports. What exactly is that thing doing for 3 seconds?! Until we know
what's going on I'm not sure it's just a driver that has a dead-slow init
function ...

> [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.

Well, to do that (at least with drm) you need the driver loaded, or at
least the stuff it supports registered.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch