The CAN bus clock (CLK_CAN_APB) can be enabled and disabled in the Main Clock module, and the default state of CLK_CAN_APB can be found in the Peripheral Clock Masking section.
A generic clock (GCLK_CAN) is required to clock the CAN. This clock must be configured and enabled in the generic clock controller before using the CAN.
This generic clock is asynchronous to the bus clock (CLK_CAN_APB). Due to this asynchronicity, writes to certain registers will require synchronization between the clock domains.