linux-kernel - [ 36/42] mfd: Clear twl6030 IRQ status register only once

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Date:	Wed, 11 Apr 2012 16:13:43 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Nishanth Menon <nm@...com>,
	Samuel Ortiz <sameo@...ux.intel.com>
Subject: [ 36/42] mfd: Clear twl6030 IRQ status register only once

3.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Nishanth Menon <nm@...com>

commit 3f8349e6e98ba0455437724589072523865eae5e upstream.

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@...com>
Signed-off-by: Nishanth Menon <nm@...com>
Signed-off-by: Samuel Ortiz <sameo@...ux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/mfd/twl6030-irq.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

--- a/drivers/mfd/twl6030-irq.c
+++ b/drivers/mfd/twl6030-irq.c
@@ -145,8 +145,17 @@ 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");

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