drm: rcar-du: lvds: Fix LVDS PLL disable on D3/E3

drm: rcar-du: lvds: Fix LVDS PLL disable on D3/E3

On R-Car D3 and E3, the LVDS encoder provides the dot (pixel) clock to
the DU, regardless of whether the LVDS output is used or not. When using
the DPAD (RGB) output, the DU driver thus enables and disables the LVDS
PLL manually, while when using the LVDS output, it lets the LVDS bridge
driver handle the PLL configuration internally as part of the atomic
enable and disable operations.

This causes an issue when using the LVDS output. As bridges are disabled
before CRTCs, the current implementation violates the enable/disable
sequences documented in the hardware datasheet, which requires the dot
clock to be enabled before the CRTC is started and disabled after it
gets stopped.

Fix the problem by enabling/disabling the LVDS PLL manually from the DU
regardless of which output is used, and skipping the PLL handling in the
LVDS bridge atomic enable and disable operations.

This is however not enough. Disabling the LVDS encoder while leaving the
PLL on still results in a vertical blanking wait timeout when disabling
the DU. Investigation showed that the culprit is the LVEN bit. For an
unclear reason, clearing the bit when disabling the LVDS encoder blocks
vertical blanking interrupts. We thus have to delay disabling the whole
LVDS encoder, not just disabling the PLL, until the DU is disabled.

We could split the LVDS disable sequence by clearing the LVRES bit in
the LVDS bridge atomic disable handler, and delaying the rest of the
operations, in order to disable the LVDS output at bridge atomic disable
time, before stopping the CRTC. This would make the code more complex,
without a clear benefit, so keep the implementation simple(r).

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>