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SANTINO(-LT): Switch display clock to PLL2 to enable spread spectrum

Dmitry Petrov requested to merge linux-5.15.32-seco-spread-spectrum into linux-5.15.32-seco

On the i.MX6, only PLL2 supports SSC (spread spectrum clocking), the other 6 PLLs do not support SSC. At the moment, SCC is not active for the display clock on SANTINO(-LT), because the clock is derived from PLL3 (instead of PLL2).

The patches update clock initialization on i.MX6 platform to check:

  • device-tree to identify systems with parallel LCD-interface
  • presence of enabled spread-spectrum settings in device-tree

If both conditions are met, clock driver switches IPU1 to PLL2 to allow spread-spectrum clocking of parallel LCD-displays

Also fixed parsing of "forbid_int_pixclk" from device tree. This property was set in runtime by old kernel, but now missing.

Issue-Id: 746-001441

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