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New reference manuals show that there is actually a status bit for the ADB400 handshake. Add a poll loop to wait for the ADB400 to acknowledge our request. [Peng Fan: i.MX8MM has blk ctl module, the handshake can only finish after setting blk ctl. The blk ctl driver will set the bus clk bit and the handshake will finish there. we just add a delay and suppose the handshake will finish after that.] Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de> Signed-off-by: Lucas Stach <l.stach@pengutronix.de> Signed-off-by: Peng Fan <peng.fan@nxp.com> Signed-off-by: Shawn Guo <shawnguo@kernel.org>