bus: ti-sysc: Flush posted write only after srst_udelay

Commit 34539b442b3b ("bus: ti-sysc: Flush posted write on enable before
reset") caused a regression reproducable on omap4 duovero where the ISS
target module can produce interconnect errors on boot. Turns out the
registers are not accessible until after a delay for devices needing
a ti,sysc-delay-us value.

Let's fix this by flushing the posted write only after the reset delay.
We do flushing also for ti,sysc-delay-us using devices as that should
trigger an interconnect error if the delay is not properly configured.

Let's also add some comments while at it.

Fixes: 34539b442b3b ("bus: ti-sysc: Flush posted write on enable before reset")
Cc: [e-mail]
Signed-off-by: Tony Lindgren <[e-mail]>

drivers/bus/ti-sysc.c

···
 		sysc_val = sysc_read_sysconfig(ddata);
 		sysc_val |= sysc_mask;
 		sysc_write(ddata, sysc_offset, sysc_val);
-		/* Flush posted write */
+
+		/*
+		 * Some devices need a delay before reading registers
+		 * after reset. Presumably a srst_udelay is not needed
+		 * for devices that use a rstctrl register reset.
+		 */
+		if (ddata->cfg.srst_udelay)
+			fsleep(ddata->cfg.srst_udelay);
+
+		/*
+		 * Flush posted write. For devices needing srst_udelay
+		 * this should trigger an interconnect error if the
+		 * srst_udelay value is needed but not configured.
+		 */
 		sysc_val = sysc_read_sysconfig(ddata);
 	}
-
-	if (ddata->cfg.srst_udelay)
-		fsleep(ddata->cfg.srst_udelay);
 
 	if (ddata->post_reset_quirk)
 		ddata->post_reset_quirk(ddata);