mfd: Clear twl6030 IRQ status register only once

TWL6030 family of PMIC use a shadow interrupt status register
while kernel processes the current interrupt event.
However, any write(0 or 1) to register INT_STS_A, INT_STS_B or
INT_STS_C clears all 3 interrupt status registers.

Since clear of the interrupt is done on 32k clk, depending on I2C
bus speed, we could in-adverently clear the status of a interrupt
status pending on shadow register in the current implementation.
This is due to the fact that multi-byte i2c write operation into
three seperate status register could result in multiple load
and clear of status and result in lost interrupts.

Instead, doing a single byte write to INT_STS_A register with 0x0
will clear all three interrupt status registers without the related
risk.
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/twl6030-irq.c

@@ -185,8 +185,17 @@ static int twl6030_irq_thread(void *data)
 			}
 		local_irq_enable();
 		}
-		ret = twl_i2c_write(TWL_MODULE_PIH, sts.bytes,
-				REG_INT_STS_A, 3); /* clear INT_STS_A */
+
+		/*
+		 * NOTE:
+		 * Simulation confirms that documentation is wrong w.r.t the
+		 * interrupt status clear operation. A single *byte* write to
+		 * any one of STS_A to STS_C register results in all three
+		 * STS registers being reset. Since it does not matter which
+		 * value is written, all three registers are cleared on a
+		 * single byte write, so we just use 0x0 to clear.
+		 */
+		ret = twl_i2c_write_u8(TWL_MODULE_PIH, 0x00, REG_INT_STS_A);
 		if (ret)
 			pr_warning("twl6030: I2C error in clearing PIH ISR\n");