[PATCH v3] iio: adc: xilinx-xadc: Push interrupts into threaded context

[xander.huff@ni.com (Xander Huff)]

The driver currently registers a pair of irq handlers using
request_threaded_irq(), however the synchronization mechanism between the
hardirq and the threadedirq handler is a regular spinlock.

Unfortunately, this breaks PREEMPT_RT builds, where a spinlock can sleep,
and is thus not able to be acquired from a hardirq handler. This patch gets
rid of the hardirq handler and pushes all interrupt handling into the
threaded context.

To validate that this change has no impact on RT performance, here are
cyclictest values with no processes running:

$ sudo cyclictest -S -m -p 98
# /dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 0.05 0.04 0.05 1/237 828
T: 0 ( 1861) P:98 I:1000 C:56925046 Min: 9 Act: 12 Avg: 12 Max: 71
T: 1 ( 1862) P:98 I:1500 C:37950030 Min: 9 Act: 12 Avg: 13 Max: 74

Then, all xadc raw handles were accessed in a continuous loop via
/sys/bus/iio/devices/iio:device0:

$ sudo cyclictest -S -m -p 98
# /dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 7.81 7.64 7.541 2/247 5751
T: 0 ( 1568) P:98 I:1000 C:23845515 Min: 11 Act: 22 Avg: 21 Max: 71
T: 1 ( 1569) P:98 I:1500 C:15897239 Min: 11 Act: 21 Avg: 22 Max: 68

Signed-off-by: Xander Huff <xander.huff@ni.com>
---
 drivers/iio/adc/xilinx-xadc-core.c | 37 ++++++++-----------------------------
 1 file changed, 8 insertions(+), 29 deletions(-)

diff --git a/drivers/iio/adc/xilinx-xadc-core.c b/drivers/iio/adc/xilinx-xadc-core.c
index ce93bd8..e16afdb 100644
--- a/drivers/iio/adc/xilinx-xadc-core.c
+++ b/drivers/iio/adc/xilinx-xadc-core.c
@@ -267,40 +267,15 @@ static void xadc_zynq_unmask_worker(struct work_struct *work)
 	xadc_zynq_update_intmsk(xadc, 0, 0);
 
 	spin_unlock_irq(&xadc->lock);
-
-	/* if still pending some alarm re-trigger the timer */
-	if (xadc->zynq_masked_alarm) {
-		schedule_delayed_work(&xadc->zynq_unmask_work,
-				msecs_to_jiffies(XADC_ZYNQ_UNMASK_TIMEOUT));
-	}
 }
 
 static irqreturn_t xadc_zynq_threaded_interrupt_handler(int irq, void *devid)
 {
 	struct iio_dev *indio_dev = devid;
 	struct xadc *xadc = iio_priv(indio_dev);
-	unsigned int alarm;
-
-	spin_lock_irq(&xadc->lock);
-	alarm = xadc->zynq_alarm;
-	xadc->zynq_alarm = 0;
-	spin_unlock_irq(&xadc->lock);
-
-	xadc_handle_events(indio_dev, xadc_zynq_transform_alarm(alarm));
-
-	/* unmask the required interrupts in timer. */
-	schedule_delayed_work(&xadc->zynq_unmask_work,
-			msecs_to_jiffies(XADC_ZYNQ_UNMASK_TIMEOUT));
-
-	return IRQ_HANDLED;
-}
-
-static irqreturn_t xadc_zynq_interrupt_handler(int irq, void *devid)
-{
-	struct iio_dev *indio_dev = devid;
-	struct xadc *xadc = iio_priv(indio_dev);
 	irqreturn_t ret = IRQ_HANDLED;
 	uint32_t status;
+	unsigned int alarm;
 
 	xadc_read_reg(xadc, XADC_ZYNQ_REG_INTSTS, &status);
 
@@ -312,7 +287,6 @@ static irqreturn_t xadc_zynq_interrupt_handler(int irq, void *devid)
 	spin_lock(&xadc->lock);
 
 	xadc_write_reg(xadc, XADC_ZYNQ_REG_INTSTS, status);
-
 	if (status & XADC_ZYNQ_INT_DFIFO_GTH) {
 		xadc_zynq_update_intmsk(xadc, XADC_ZYNQ_INT_DFIFO_GTH,
 			XADC_ZYNQ_INT_DFIFO_GTH);
@@ -330,8 +304,14 @@ static irqreturn_t xadc_zynq_interrupt_handler(int irq, void *devid)
 		xadc_zynq_update_intmsk(xadc, 0, 0);
 		ret = IRQ_WAKE_THREAD;
 	}
+
+	alarm = xadc->zynq_alarm;
+	xadc->zynq_alarm = 0;
+
 	spin_unlock(&xadc->lock);
 
+	xadc_handle_events(indio_dev, xadc_zynq_transform_alarm(alarm));
+
 	return ret;
 }
 
@@ -436,7 +416,6 @@ static const struct xadc_ops xadc_zynq_ops = {
 	.write = xadc_zynq_write_adc_reg,
 	.setup = xadc_zynq_setup,
 	.get_dclk_rate = xadc_zynq_get_dclk_rate,
-	.interrupt_handler = xadc_zynq_interrupt_handler,
 	.threaded_interrupt_handler = xadc_zynq_threaded_interrupt_handler,
 	.update_alarm = xadc_zynq_update_alarm,
 };
@@ -1225,7 +1204,7 @@ static int xadc_probe(struct platform_device *pdev)
 	if (ret)
 		goto err_free_samplerate_trigger;
 
-	ret = request_threaded_irq(irq, xadc->ops->interrupt_handler,
+	ret = request_threaded_irq(irq, NULL,
 				xadc->ops->threaded_interrupt_handler,
 				0, dev_name(&pdev->dev), indio_dev);
 	if (ret)
-- 
1.9.1