drm/i915: Plumb 'dsb' all way to the plane hooks

[ Upstream commit 01389846 ]

We need to be able to do both MMIO and DSB based pipe/plane
programming. To that end plumb the 'dsb' all way from the top
into the plane commit hooks.

The compiler appears smart enough to combine the branches from
all the back-to-back register writes into a single branch.
So the generated asm ends up looking more or less like this:
plane_hook()
{
	if (dsb) {
		intel_dsb_reg_write();
		intel_dsb_reg_write();
		...
	} else {
		intel_de_write_fw();
		intel_de_write_fw();
		...
	}
}
which seems like a reasonably efficient way to do this.

An alternative I was also considering is some kind of closure
(register write function + display vs. dsb pointer passed to it).
That does result is smaller code as there are no branches anymore,
but having each register access go via function pointer sounds
less efficient.

Not that I actually measured the overhead of either approach yet.
Also the reg_rw tracepoint seems to be making a huge mess of the
generated code for the mmio path. And additionally there's some
kind of IS_GSI_REG() hack in __raw_uncore_read() which ends up
generating a pointless branch for every mmio register access.
So looks like there might be quite a bit of room for improvement
in the mmio path still.

Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930170415.23841-12-ville.syrjala@linux.intel.com


Stable-dep-of: 30bfc151 ("drm/xe: Remove double pageflip")
Signed-off-by: Sasha Levin <sashal@kernel.org>