FAILED: patch "[PATCH] iio: imu: st_lsm6dsx: fix edge-trigger interrupts" failed to apply to 4.19-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 3f9bce7a22a3f8ac9d885c9d75bc45569f24ac8b Mon Sep 17 00:00:00 2001
From: Lorenzo Bianconi <lorenzo@kernel.org>
Date: Sat, 14 Nov 2020 19:39:05 +0100
Subject: [PATCH] iio: imu: st_lsm6dsx: fix edge-trigger interrupts

If we are using edge IRQs, new samples can arrive while processing
current interrupt since there are no hw guarantees the irq line
stays "low" long enough to properly detect the new interrupt.
In this case the new sample will be missed.
Polling FIFO status register in st_lsm6dsx_handler_thread routine
allow us to read new samples even if the interrupt arrives while
processing previous data and the timeslot where the line is "low"
is too short to be properly detected.

Fixes: 89ca88a7cdf2 ("iio: imu: st_lsm6dsx: support active-low interrupts")
Fixes: 290a6ce11d93 ("iio: imu: add support to lsm6dsx driver")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/5e93cda7dc1e665f5685c53ad8e9ea71dbae782d.1605378871.git.lorenzo@kernel.org
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

diff --git a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
index 467214e2e77c..7cedaab096a7 100644
--- a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
+++ b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
@@ -2069,19 +2069,35 @@ st_lsm6dsx_report_motion_event(struct st_lsm6dsx_hw *hw)
 static irqreturn_t st_lsm6dsx_handler_thread(int irq, void *private)
 {
 	struct st_lsm6dsx_hw *hw = private;
+	int fifo_len = 0, len;
 	bool event;
-	int count;
 
 	event = st_lsm6dsx_report_motion_event(hw);
 
 	if (!hw->settings->fifo_ops.read_fifo)
 		return event ? IRQ_HANDLED : IRQ_NONE;
 
-	mutex_lock(&hw->fifo_lock);
-	count = hw->settings->fifo_ops.read_fifo(hw);
-	mutex_unlock(&hw->fifo_lock);
+	/*
+	 * If we are using edge IRQs, new samples can arrive while
+	 * processing current interrupt since there are no hw
+	 * guarantees the irq line stays "low" long enough to properly
+	 * detect the new interrupt. In this case the new sample will
+	 * be missed.
+	 * Polling FIFO status register allow us to read new
+	 * samples even if the interrupt arrives while processing
+	 * previous data and the timeslot where the line is "low" is
+	 * too short to be properly detected.
+	 */
+	do {
+		mutex_lock(&hw->fifo_lock);
+		len = hw->settings->fifo_ops.read_fifo(hw);
+		mutex_unlock(&hw->fifo_lock);
+
+		if (len > 0)
+			fifo_len += len;
+	} while (len > 0);
 
-	return count || event ? IRQ_HANDLED : IRQ_NONE;
+	return fifo_len || event ? IRQ_HANDLED : IRQ_NONE;
 }
 
 static int st_lsm6dsx_irq_setup(struct st_lsm6dsx_hw *hw)