[RFC] clk: inherit display clocks enabled by bootloader

[Geert Uytterhoeven]

On Tue, Jul 4, 2017 at 7:51 PM, Rob Clark <robdclark at gmail.com> 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.

Cfr. Documentation/devicetree/bindings/display/simple-framebuffer.txt?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds