[PATCH v4] iio: adc: xilinx-xadc: Push interrupts into hardirq context

[Xander Huff <xander.huff@ni.com>]

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 threaded handler and pushes all interrupt handling into the
hardirq context, and uses request_irq().

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: 0.00 0.01 0.05 1/174 2539
T: 0 ( 1405) P:98 I:1000 C:167010520 Min: 9 Act: 12 Avg: 12 Max: 75
T: 1 ( 1862) P:98 I:1500 C:111340339 Min: 9 Act: 12 Avg: 12 Max: 73

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: 7.84 7.70 7.63 3/182 4260
T: 0 ( 2559) P:98 I:1000 C:241557018 Min: 11 Act: 18 Avg: 21 Max: 74
T: 1 ( 2560) P:98 I:1500 C:161038006 Min: 10 Act: 21 Avg: 20 Max: 73

Signed-off-by: Xander Huff <xander.huff@ni.com>
---
 drivers/iio/adc/xilinx-xadc-core.c | 37 ++++++++++---------------------------
 drivers/iio/adc/xilinx-xadc.h      |  2 --
 2 files changed, 10 insertions(+), 29 deletions(-)

diff --git a/drivers/iio/adc/xilinx-xadc-core.c b/drivers/iio/adc/xilinx-xadc-core.c
index ce93bd8..0370624 100644
--- a/drivers/iio/adc/xilinx-xadc-core.c
+++ b/drivers/iio/adc/xilinx-xadc-core.c
@@ -273,33 +273,13 @@ static void xadc_zynq_unmask_worker(struct work_struct *work)
 		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;
 
 	xadc_read_reg(xadc, XADC_ZYNQ_REG_INTSTS, &status);
@@ -321,18 +301,23 @@ static irqreturn_t xadc_zynq_interrupt_handler(int irq, void *devid)
 
 	status &= XADC_ZYNQ_INT_ALARM_MASK;
 	if (status) {
-		xadc->zynq_alarm |= status;
 		xadc->zynq_masked_alarm |= status;
 		/*
 		 * mask the current event interrupt,
 		 * unmask it when the interrupt is no more active.
 		 */
 		xadc_zynq_update_intmsk(xadc, 0, 0);
-		ret = IRQ_WAKE_THREAD;
+
+		xadc_handle_events(indio_dev,
+				xadc_zynq_transform_alarm(status));
+
+		/* unmask the required interrupts in timer. */
+		schedule_delayed_work(&xadc->zynq_unmask_work,
+				msecs_to_jiffies(XADC_ZYNQ_UNMASK_TIMEOUT));
 	}
 	spin_unlock(&xadc->lock);
 
-	return ret;
+	return IRQ_HANDLED;
 }
 
 #define XADC_ZYNQ_TCK_RATE_MAX 50000000
@@ -437,7 +422,6 @@ static const struct xadc_ops xadc_zynq_ops = {
 	.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,
 };
 
@@ -1225,9 +1209,8 @@ static int xadc_probe(struct platform_device *pdev)
 	if (ret)
 		goto err_free_samplerate_trigger;
 
-	ret = request_threaded_irq(irq, xadc->ops->interrupt_handler,
-				xadc->ops->threaded_interrupt_handler,
-				0, dev_name(&pdev->dev), indio_dev);
+	ret = request_irq(irq, xadc->ops->interrupt_handler, 0,
+			dev_name(&pdev->dev), indio_dev);
 	if (ret)
 		goto err_clk_disable_unprepare;
 
diff --git a/drivers/iio/adc/xilinx-xadc.h b/drivers/iio/adc/xilinx-xadc.h
index 54adc50..f6f0819 100644
--- a/drivers/iio/adc/xilinx-xadc.h
+++ b/drivers/iio/adc/xilinx-xadc.h
@@ -60,7 +60,6 @@ struct xadc {
 
 	enum xadc_external_mux_mode external_mux_mode;
 
-	unsigned int zynq_alarm;
 	unsigned int zynq_masked_alarm;
 	unsigned int zynq_intmask;
 	struct delayed_work zynq_unmask_work;
@@ -79,7 +78,6 @@ struct xadc_ops {
 	void (*update_alarm)(struct xadc *, unsigned int);
 	unsigned long (*get_dclk_rate)(struct xadc *);
 	irqreturn_t (*interrupt_handler)(int, void *);
-	irqreturn_t (*threaded_interrupt_handler)(int, void *);
 
 	unsigned int flags;
 };
-- 
1.9.1