[PATCH v2] ASoC: meson: axg-fifo: fix irq scheduling issue with PREEMPT_RT

[Jerome Brunet]

With PREEMPT_RT enabled a spinlock_t becomes a sleeping lock.

This is usually not a problem with spinlocks used in IRQ context since
IRQ handlers get threaded. However, if IRQF_ONESHOT is set, the primary
handler won't be force-threaded and runs always in hardirq context. This is
a problem because spinlock_t requires a preemptible context on PREEMPT_RT.

In this particular instance, regmap mmio uses spinlock_t to protect the
register access and IRQF_ONESHOT is set on the IRQ. In this case, it is
actually better to do everything in threaded handler and it solves the
problem with PREEMPT_RT.

Reported-by: Arseniy Krasnov <avkrasnov at salutedevices.com>
Closes: https://lore.kernel.org/linux-amlogic/20240729131652.3012327-1-avkrasnov@salutedevices.com
Suggested-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
Fixes: b11d26660dff ("ASoC: meson: axg-fifo: use threaded irq to check periods")
Signed-off-by: Jerome Brunet <jbrunet at baylibre.com>
---
 sound/soc/meson/axg-fifo.c | 26 ++++++++++----------------
 1 file changed, 10 insertions(+), 16 deletions(-)

diff --git a/sound/soc/meson/axg-fifo.c b/sound/soc/meson/axg-fifo.c
index 7e6090af720b..75909196b769 100644
--- a/sound/soc/meson/axg-fifo.c
+++ b/sound/soc/meson/axg-fifo.c
@@ -207,25 +207,18 @@ static irqreturn_t axg_fifo_pcm_irq_block(int irq, void *dev_id)
 	status = FIELD_GET(STATUS1_INT_STS, status);
 	axg_fifo_ack_irq(fifo, status);
 
-	/* Use the thread to call period elapsed on nonatomic links */
-	if (status & FIFO_INT_COUNT_REPEAT)
-		return IRQ_WAKE_THREAD;
+	if (status & ~FIFO_INT_COUNT_REPEAT)
+		dev_dbg(axg_fifo_dev(ss), "unexpected irq - STS 0x%02x\n",
+			status);
 
-	dev_dbg(axg_fifo_dev(ss), "unexpected irq - STS 0x%02x\n",
-		status);
+	if (status & FIFO_INT_COUNT_REPEAT) {
+		snd_pcm_period_elapsed(ss);
+		return IRQ_HANDLED;
+	}
 
 	return IRQ_NONE;
 }
 
-static irqreturn_t axg_fifo_pcm_irq_block_thread(int irq, void *dev_id)
-{
-	struct snd_pcm_substream *ss = dev_id;
-
-	snd_pcm_period_elapsed(ss);
-
-	return IRQ_HANDLED;
-}
-
 int axg_fifo_pcm_open(struct snd_soc_component *component,
 		      struct snd_pcm_substream *ss)
 {
@@ -251,8 +244,9 @@ int axg_fifo_pcm_open(struct snd_soc_component *component,
 	if (ret)
 		return ret;
 
-	ret = request_threaded_irq(fifo->irq, axg_fifo_pcm_irq_block,
-				   axg_fifo_pcm_irq_block_thread,
+	/* Use the threaded irq handler only with non-atomic links */
+	ret = request_threaded_irq(fifo->irq, NULL,
+				   axg_fifo_pcm_irq_block,
 				   IRQF_ONESHOT, dev_name(dev), ss);
 	if (ret)
 		return ret;
-- 
2.43.0