On 5P49V6965, when an output is enabled we enable the corresponding
FOD. When this happens for the first time, and specifically when writing
register VC5_OUT_DIV_CONTROL in vc5_clk_out_prepare(), all other outputs
are stopped for a short time and then restarted.

According to Renesas support this is intended: "The reason for that is VC6E
has synced up all output function".

This behaviour can be disabled at least on VersaClock 6E devices, of which
only the 5P49V6965 is currently implemented by this driver. This requires
writing bit 7 (bypass_sync{1..4}) in register 0x20..0x50.  Those registers
are named "Unused Factory Reserved Register", and the bits are documented
as "Skip VDDO<N> verification", which does not clearly explain the relation
to FOD sync. However according to Renesas support as well as my testing
setting this bit does prevent disabling of all clock outputs when enabling
a FOD.

See "VersaClock ® 6E Family Register Descriptions and Programming Guide"
(August 30, 2018), Table 116 "Power Up VDD check", page 58:
https://www.renesas.com/us/en/document/mau/versaclock-6e-family-register-descriptions-and-programming-guideSigned-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/20210527211647.1520720-1-luca@lucaceresoli.net
Fixes: 2bda748e ("clk: vc5: Add support for IDT VersaClock 5P49V6965")
Signed-off-by: Stephen Boyd <sboyd@kernel.org>