Input: iqs7222 - acknowledge reset before writing registers

If the device suffers a spurious reset while reacting to a previous
spurious reset, the second reset interrupt is preempted because the
ACK_RESET bit is written last.

To solve this problem, write the ACK_RESET bit prior to writing any
other registers. This ensures that any registers written before the
second spurious reset will be rewritten.

Last but not least, the order in which the ACK_RESET bit is written
relative to the second filter beta register is important for select
variants of silicon. Enforce the correct order so as to not clobber
the system status register.

Fixes: e505eda ("Input: add support for Azoteq IQS7222A/B/C")
Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Link: https://lore.kernel.org/r/20220626072412.475211-5-jeff@labundy.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/misc/iqs7222.c

@@ -94,11 +94,11 @@ enum iqs7222_reg_key_id {
 
 enum iqs7222_reg_grp_id {
 	IQS7222_REG_GRP_STAT,
+	IQS7222_REG_GRP_FILT,
 	IQS7222_REG_GRP_CYCLE,
 	IQS7222_REG_GRP_GLBL,
 	IQS7222_REG_GRP_BTN,
 	IQS7222_REG_GRP_CHAN,
-	IQS7222_REG_GRP_FILT,
 	IQS7222_REG_GRP_SLDR,
 	IQS7222_REG_GRP_GPIO,
 	IQS7222_REG_GRP_SYS,
@@ -1348,6 +1348,34 @@ static int iqs7222_dev_init(struct iqs7222_private *iqs7222, int dir)
 	int comms_offset = dev_desc->comms_offset;
 	int error, i, j, k;
 
+	/*
+	 * Acknowledge reset before writing any registers in case the device
+	 * suffers a spurious reset during initialization. Because this step
+	 * may change the reserved fields of the second filter beta register,
+	 * its cache must be updated.
+	 *
+	 * Writing the second filter beta register, in turn, may clobber the
+	 * system status register. As such, the filter beta register pair is
+	 * written first to protect against this hazard.
+	 */
+	if (dir == WRITE) {
+		u16 reg = dev_desc->reg_grps[IQS7222_REG_GRP_FILT].base + 1;
+		u16 filt_setup;
+
+		error = iqs7222_write_word(iqs7222, IQS7222_SYS_SETUP,
+					   iqs7222->sys_setup[0] |
+					   IQS7222_SYS_SETUP_ACK_RESET);
+		if (error)
+			return error;
+
+		error = iqs7222_read_word(iqs7222, reg, &filt_setup);
+		if (error)
+			return error;
+
+		iqs7222->filt_setup[1] &= GENMASK(7, 0);
+		iqs7222->filt_setup[1] |= (filt_setup & ~GENMASK(7, 0));
+	}
+
 	/*
 	 * Take advantage of the stop-bit disable function, if available, to
 	 * save the trouble of having to reopen a communication window after
@@ -2254,8 +2282,6 @@ static int iqs7222_parse_all(struct iqs7222_private *iqs7222)
 			return error;
 	}
 
-	sys_setup[0] |= IQS7222_SYS_SETUP_ACK_RESET;
-
 	return iqs7222_parse_props(iqs7222, NULL, 0, IQS7222_REG_GRP_SYS,
 				   IQS7222_REG_KEY_NONE);
 }