Re: [PATCH 14/18] drm/mediatek: dpi: add support for dpi clock

[AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>]

Il 23/10/23 16:40, amergnat@baylibre.com ha scritto:
> From: Fabien Parent <fparent@baylibre.com>
> 
> MT8365 requires an additional clock for DPI. Add support for that
> additional clock.
> 
> Signed-off-by: Fabien Parent <fparent@baylibre.com>
> Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>

I'm not convinced that this is right... at all.

 From a fast check of the MT8365 DPI clocks, I can see that the DPI0 clock declares
parent VPLL_DPIX (a fixed clock), but nothing ever has VPLL_DPIX_EN (which is the
GATE clock, enabling output of DPIx VPLL?).

But then, there's even more: no clock ever references the CLK_TOP_DPI0_SEL nor the
CLK_TOP_DPI1_SEL gate, which is a PLL parent selector... in other platforms, that
is muxing through the TVDPLL, but on MT8365 that is LVDSPLL?!

I have many questions now:
* Two PLLs are apparently brought up, but which one is the right one?!
   * Is the LVDS PLL really used for DisplayPort? (dpi0_sel)
   * Is the VPLL_DPIx PLL used for DisplayPort instead? (dpi0_dpi0)
* Why is the LVDSTX_PXL clock using the same PLL as DPI0?!
   * Why is the VPLL_DPIx gate never enabled?
* Are you sure that CLK_MM_DPI0_DPI0's parent shouldn't be dpi0_sel instead?
* Where is DPI1 in this SoC? Why is there a dpi1_sel clock, but no MM clock
   for the DPI1 controller? Is there any DPI1 controller, even?!
   * Why is there a DPI1 MUX, if there's no DPI1 controller?!

Answering all those questions will lead you to the right change, which I believe
to be in the clock drivers, not here in mtk_dpi.c.

Cheers!
Angelo