Have a great day,
If the SerDes are unused, the SD1_REF_CLK inputs can be connected to GND, however AVDD_SD1_PLL1, AVDD_SD1_PLL2, S1VDD, X1VDD must remain powered. In the RCW configuration the following RCW bits have to be configured:
SRDS_PLL_PD_S1 = 2'b11 (both PLLs configured as powered down)
SRDS_PLL_REF_CLK_SEL_S1 = 2'b00
SRDS_PRTCL_S1=8'hFF (no other values are permitted when both PLLs are powered down)
As you can see in the https://community.nxp.com/thread/430180 LS1021 can boot with that settings. So the LS1020 should boot too.
It is not clear what do you mean as boot fail. Notice if SerDes in use then after completing reset, software should check the SerDesx_PLLnRSTCTL[RST_DONE] field to make sure that each active SerDes PLL on the device has locked. SerDes PLL will not lock if the corresponding reference clock is not provided. May your software fails due to that checking.
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