[PATCH v17 4/6] spi: pxa2xx: overhaul teardown and suspend sequence to synchronize IRQ before clock gating

[Shih-Yuan Lee <fourdollars@debian.org>]

When removing the driver or suspending the device, the clock must not
be disabled while shared interrupts are still active. Gating the clock
before waiting for in-flight interrupt handlers to complete results
in race conditions where the handler performs unclocked MMIO accesses,
causing PCIe Completion Timeouts.

Overhaul the remove, suspend, and runtime_suspend paths to use a strict
synchronized teardown order:
1. In remove, call free_irq() (which internally synchronizes any in-flight
   handlers) before disabling the clock via pxa2xx_spi_clk_disable().
2. In runtime_suspend, under clk_lock and only when the clock is enabled,
   disable the SSP peripheral via pxa_ssp_disable(), clear 'clk_enabled'
   via WRITE_ONCE() so that new interrupts immediately bail out with
   IRQ_NONE, drain in-flight handlers via synchronize_irq() while the
   clock is still running, and finally gate the clock with
   clk_disable_unprepare().
3. In system suspend, suspend the controller queue and use
   pm_runtime_force_suspend() to invoke runtime_suspend, ensuring
   in-flight interrupts are drained before the clock is gated. If
   pm_runtime_force_suspend() fails, resume the controller queue so
   the controller remains operational since the system will stay awake.
4. In system resume, restore the device state using
   pm_runtime_force_resume() before restarting the controller queue.
   If pm_runtime_force_resume() fails, return the error immediately
   without calling spi_controller_resume(), keeping the queue stopped
   to prevent transferring messages against unclocked or unpowered
   hardware. If the device was already runtime-suspended prior to
   system sleep, pm_runtime_force_resume() leaves the clock gated until
   the next transfer resumes it, optimizing idle power.

Throughout suspended states, 'clk_enabled' being false serves as the
primary invariant ensuring that any subsequent interrupt handler
invocation safely returns IRQ_NONE without accessing hardware registers.

Assisted-by: Antigravity:gemini-3.8-flash spin sparse
Signed-off-by: Shih-Yuan Lee <fourdollars@debian.org>
---
 drivers/spi/spi-pxa2xx.c | 39 +++++++++++++++++++++++----------------
 1 file changed, 23 insertions(+), 16 deletions(-)

diff --git a/drivers/spi/spi-pxa2xx.c b/drivers/spi/spi-pxa2xx.c
index b091434977af..2a3fa9ca7213 100644
--- a/drivers/spi/spi-pxa2xx.c
+++ b/drivers/spi/spi-pxa2xx.c
@@ -1520,31 +1520,33 @@ void pxa2xx_spi_remove(struct device *dev)
 
 	/* Disable the SSP at the peripheral and SOC level */
 	pxa_ssp_disable(ssp);
+
+	/* Release IRQ before gating the SOC clock */
+	free_irq(ssp->irq, drv_data);
+
+	/* Safe to disable the SSP clock now */
 	pxa2xx_spi_clk_disable(drv_data);
 
 	/* Release DMA */
 	if (drv_data->controller_info->enable_dma)
 		pxa2xx_spi_dma_release(drv_data);
-
-	/* Release IRQ */
-	free_irq(ssp->irq, drv_data);
 }
 EXPORT_SYMBOL_NS_GPL(pxa2xx_spi_remove, "SPI_PXA2xx");
 
 static int pxa2xx_spi_suspend(struct device *dev)
 {
 	struct driver_data *drv_data = dev_get_drvdata(dev);
-	struct ssp_device *ssp = drv_data->ssp;
 	int ret;
 
 	ret = spi_controller_suspend(drv_data->controller);
 	if (ret)
 		return ret;
 
-	pxa_ssp_disable(ssp);
-
-	if (!pm_runtime_suspended(dev))
-		pxa2xx_spi_clk_disable(drv_data);
+	ret = pm_runtime_force_suspend(dev);
+	if (ret) {
+		spi_controller_resume(drv_data->controller);
+		return ret;
+	}
 
 	return 0;
 }
@@ -1554,14 +1556,10 @@ static int pxa2xx_spi_resume(struct device *dev)
 	struct driver_data *drv_data = dev_get_drvdata(dev);
 	int ret;
 
-	/* Enable the SSP clock */
-	if (!pm_runtime_suspended(dev)) {
-		ret = pxa2xx_spi_clk_enable(drv_data);
-		if (ret)
-			return ret;
-	}
+	ret = pm_runtime_force_resume(dev);
+	if (ret)
+		return ret;
 
-	/* Start the queue running */
 	return spi_controller_resume(drv_data->controller);
 }
 
@@ -1569,7 +1567,16 @@ static int pxa2xx_spi_runtime_suspend(struct device *dev)
 {
 	struct driver_data *drv_data = dev_get_drvdata(dev);
 
-	pxa2xx_spi_clk_disable(drv_data);
+	mutex_lock(&drv_data->clk_lock);
+	if (drv_data->clk_enabled) {
+		pxa_ssp_disable(drv_data->ssp);
+		WRITE_ONCE(drv_data->clk_enabled, false);
+		mutex_unlock(&drv_data->clk_lock);
+		synchronize_irq(drv_data->ssp->irq);
+		clk_disable_unprepare(drv_data->ssp->clk);
+	} else {
+		mutex_unlock(&drv_data->clk_lock);
+	}
 	return 0;
 }
 
-- 
2.39.5