iio: adc: xilinx-xadc: Push interrupts into hardirq context

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
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
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>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

1 files changed, 0 insertions, 2 deletions

diff --git a/drivers/iio/adc/xilinx-xadc.h b/drivers/iio/adc/xilinx-xadc.h
index 54adc5087210..f6f081965647 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;
 };