i2c: mv64xxx: Fix reading invalid status value in atomic mode

There seems to be a bug within the mv64xxx I2C controller, wherein the
status register may not necessarily contain valid value immediately
after the IFLG flag is set in the control register.

My theory is that the controller:
- first sets the IFLG in control register
- then updates the status register
- then raises an interrupt

This may sometime cause weird bugs when in atomic mode, since in this
mode we do not wait for an interrupt, but instead we poll the control
register for IFLG and read status register immediately after.

I encountered -ENXIO from mv64xxx_i2c_fsm() due to this issue when using
this driver in atomic mode.

Note that I've only seen this issue on Armada 385, I don't know whether
other SOCs with this controller are also affected. Also note that this
fix has been in U-Boot for over 4 years [1] without anybody complaining,
so it should not cause regressions.

[1] https://source.denx.de/u-boot/u-boot/-/commit/d50e29662f78

Fixes: 544a8d75 ("i2c: mv64xxx: Add atomic_xfer method to driver")
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/busses/i2c-mv64xxx.c

@@ -520,6 +520,17 @@ mv64xxx_i2c_intr(int irq, void *dev_id)
 
 	while (readl(drv_data->reg_base + drv_data->reg_offsets.control) &
 						MV64XXX_I2C_REG_CONTROL_IFLG) {
+		/*
+		 * It seems that sometime the controller updates the status
+		 * register only after it asserts IFLG in control register.
+		 * This may result in weird bugs when in atomic mode. A delay
+		 * of 100 ns before reading the status register solves this
+		 * issue. This bug does not seem to appear when using
+		 * interrupts.
+		 */
+		if (drv_data->atomic)
+			ndelay(100);
+
 		status = readl(drv_data->reg_base + drv_data->reg_offsets.status);
 		mv64xxx_i2c_fsm(drv_data, status);
 		mv64xxx_i2c_do_action(drv_data);