[RFC] clk: inherit display clocks enabled by bootloader

[Rajendra Nayak]

On 07/04/2017 11:21 PM, Rob Clark wrote:
> The goal here is to support inheriting a display setup by bootloader,
> although there may also be some non-display related use-cases.
> 
> Rough idea is to add a flag for clks and power domains that might
> already be enabled when kernel starts, and make corresponding fixups
> to clk enable/prepare_count and power-domain state so that these are
> not automatically disabled late in boot.
> 
> If bootloader is enabling display, and kernel is using efifb before
> real display driver is loaded (potentially from kernel module after
> userspace starts, in a typical distro kernel), we don't want to kill
> the clocks and power domains that are used by the display before
> userspace starts.
> 
> Second part will be (*waves hands*) for drm/msm to check if display
> related clocks are enabled when it is loaded, and if so use drm
> atomic framework's hooks to read back hw state to sync existing
> display state w/ software state, and skip the initial clk_enable.
> Therefore inheriting the enable done by bootloader.
> 
> Obviously this should be split up into multiple patches and many
> TODOs addressed.  But I guess this is enough for now to start
> discussing the approach, and in particular how drm and clock/pd
> drivers work together to handle handover from bootloader.
> 
> The CLK_INHERIT_BOOTLOADER and related gsdc flag should only be set
> on leaf nodes.
> ---
> A bit hacky right now, but display survives clk_disable_unused()
> and genpd_power_off_unused().  It hangs just after that late in
> boot, which I'm still debugging (might be unrelated shenanigans).
> And haven't started on the drm/msm side of this.  But I figured
> it was half baked enough to send out for comments/ideas, or to
> see if anyone had some different idea about how to solve this.

Another RFC proposed around to handle similar situations is
https://lkml.org/lkml/2017/6/28/188

That one though, I guess deals with only regulator supplies for now.

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