linux-kernel - Re: [PATCH] platform/x86: dell-laptop: Add drm module soft dependency

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Date:   Wed, 7 Jun 2023 08:56:04 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Matthew Garrett <mjg59@...f.ucam.org>
Cc:     AceLan Kao <acelan.kao@...onical.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Mark Gross <markgross@...nel.org>,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kai-Heng Feng <kai.heng.feng@...onical.com>
Subject: Re: [PATCH] platform/x86: dell-laptop: Add drm module soft dependency

On Wednesday 07 June 2023 07:23:41 Matthew Garrett wrote:
> On Wed, Jun 07, 2023 at 02:13:31PM +0800, AceLan Kao wrote:
> > Matthew Garrett <mjg59@...f.ucam.org> 於 2023年6月7日 週三 下午1:27寫道：
> > >
> > > On Wed, Jun 07, 2023 at 01:19:40PM +0800, AceLan Kao wrote:
> > > > Gfx drivers(i915/amdgpu/nvidia) depend on the drm driver, so delaying
> > > > the loading of dell_laptop after drm can ease the issue the most.
> > > > Right, it's still possible to encounter the issue, unfortunately, we
> > > > do not have a better solution for it at the moment.
> > >
> > > We could unregister inappropriate backlight drivers when a more
> > > appropriate one is loaded, or the policy decision around which driver to
> > > use could be made in userland?
> > It's hard to decide which backlight driver is redundant, and it's kind of ugly
> > to unregister the backlight driver which is registered by other driver and maybe
> > problematic.
> 
> But you're relying on registering the working backlight first, which is 
> an inherently racy thing? We shouldn't be relying on order of 
> initialisation to make this work, either we should only export a working 
> interface or we should expose enough information for whatever is using 
> the interfaces to make an appropriate policy decision itself.

IIRC, drm drivers unregister redundant fbcon drivers (vesafb), so cannot
drm drivers use similar strategy also for backlight drivers and
unregister the redundant? If every backlight driver would have some
"flag" which say if it should be unregistered by drm then maybe it could
work? Or are there some other pitfalls?