[PATCH v16 2/7] spi: pxa2xx: introduce suspended flag for interrupt synchronization

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

When a shared interrupt line is used, the interrupt handler ssp_int()
can be triggered by other devices sharing the line. The handler must
ensure it does not access the SSP controller registers via MMIO when
the device is powered down or when its clock is gated; otherwise, it
will cause PCIe Completion Timeouts and system hangs.

Currently, ssp_int() guards MMIO access using:
    if (pm_runtime_suspended(drv_data->ssp->dev)) return IRQ_NONE;

However, during PM transitions (such as system suspend or runtime PM
autosuspend), device callbacks execute to disable the hardware and gate the
clock, but the PM state machine does not mark the device as RPM_SUSPENDED
until after the suspend callback returns. During this transitional state
(RPM_SUSPENDING), pm_runtime_suspended() returns false. If a shared
interrupt fires after the clock has been gated but before the PM state has
transitioned, ssp_int() will execute, attempt MMIO reads on the unclocked
register space, and hang the system.

Formal verification using Spin/PROMELA confirms that a scheduling window
exists where the interrupt thread accesses MMIO when clk_enabled is false,
violating safety properties.

Introduce a custom 'suspended' boolean flag in struct driver_data to
track the device's suspended state across all PM transitions. Check
both 'drv_data->suspended' and '!drv_data->clk_enabled' in ssp_int()
to return IRQ_NONE immediately before any MMIO access is attempted.

This closes the state transition race condition and mathematically guarantees
deadlock-free, safe shared interrupt handling during power transitions.

Signed-off-by: Shih-Yuan Lee <fourdollars@debian.org>
---
 drivers/spi/spi-pxa2xx.c | 51 ++++++++++++++++++++++++++--------------
 drivers/spi/spi-pxa2xx.h |  1 +
 2 files changed, 35 insertions(+), 17 deletions(-)

diff --git a/drivers/spi/spi-pxa2xx.c b/drivers/spi/spi-pxa2xx.c
index d50152aad348..c44349ab2b52 100644
--- a/drivers/spi/spi-pxa2xx.c
+++ b/drivers/spi/spi-pxa2xx.c
@@ -730,8 +730,8 @@ static int pxa2xx_spi_clk_enable(struct driver_data *drv_data)
 static void pxa2xx_spi_clk_disable(struct driver_data *drv_data)
 {
 	if (drv_data->clk_enabled) {
-		clk_disable_unprepare(drv_data->ssp->clk);
 		drv_data->clk_enabled = false;
+		clk_disable_unprepare(drv_data->ssp->clk);
 	}
 }
 
@@ -743,12 +743,12 @@ static irqreturn_t ssp_int(int irq, void *dev_id)
 	u32 status;
 
 	/*
-	 * The IRQ might be shared with other peripherals so we must first
-	 * check that are we RPM suspended or not. If we are we assume that
-	 * the IRQ was not for us (we shouldn't be RPM suspended when the
-	 * interrupt is enabled).
+	 * The IRQ might be shared with other peripherals or trigger during
+	 * power state transitions. First check if device is suspended or if
+	 * clock is disabled; if so, return IRQ_NONE immediately to avoid
+	 * unclocked MMIO reads.
 	 */
-	if (pm_runtime_suspended(drv_data->ssp->dev))
+	if (drv_data->suspended || !drv_data->clk_enabled)
 		return IRQ_NONE;
 
 	/*
@@ -1310,6 +1310,7 @@ int pxa2xx_spi_probe(struct device *dev, struct ssp_device *ssp,
 	drv_data->controller = controller;
 	drv_data->controller_info = platform_info;
 	drv_data->ssp = ssp;
+	drv_data->suspended = true;
 
 	/* The spi->mode bits understood by this driver: */
 	controller->mode_bits = SPI_CPOL | SPI_CPHA | SPI_CS_HIGH | SPI_LOOP;
@@ -1352,11 +1353,6 @@ int pxa2xx_spi_probe(struct device *dev, struct ssp_device *ssp,
 						| SSSR_ROR | SSSR_TUR;
 	}
 
-	status = request_irq(ssp->irq, ssp_int, IRQF_SHARED, dev_name(dev),
-			drv_data);
-	if (status < 0)
-		return dev_err_probe(dev, status, "cannot get IRQ %d\n", ssp->irq);
-
 	/* Setup DMA if requested */
 	if (platform_info->enable_dma) {
 		status = pxa2xx_spi_dma_setup(drv_data);
@@ -1376,7 +1372,16 @@ int pxa2xx_spi_probe(struct device *dev, struct ssp_device *ssp,
 	/* Enable SOC clock */
 	status = pxa2xx_spi_clk_enable(drv_data);
 	if (status)
-		goto out_error_dma_irq_alloc;
+		goto out_error_dma_alloc;
+
+	drv_data->suspended = false;
+
+	status = request_irq(ssp->irq, ssp_int, IRQF_SHARED, dev_name(dev),
+			drv_data);
+	if (status < 0) {
+		status = dev_err_probe(dev, status, "cannot get IRQ %d\n", ssp->irq);
+		goto out_error_clock_enabled;
+	}
 
 	controller->max_speed_hz = clk_get_rate(ssp->clk);
 	/*
@@ -1456,7 +1461,7 @@ int pxa2xx_spi_probe(struct device *dev, struct ssp_device *ssp,
 						"ready", GPIOD_OUT_LOW);
 		if (IS_ERR(drv_data->gpiod_ready)) {
 			status = PTR_ERR(drv_data->gpiod_ready);
-			goto out_error_clock_enabled;
+			goto out_error_irq_alloc;
 		}
 	}
 
@@ -1465,17 +1470,19 @@ int pxa2xx_spi_probe(struct device *dev, struct ssp_device *ssp,
 	status = spi_register_controller(controller);
 	if (status) {
 		dev_err_probe(dev, status, "problem registering SPI controller\n");
-		goto out_error_clock_enabled;
+		goto out_error_irq_alloc;
 	}
 
 	return status;
 
+out_error_irq_alloc:
+	free_irq(ssp->irq, drv_data);
+
 out_error_clock_enabled:
 	pxa2xx_spi_clk_disable(drv_data);
 
-out_error_dma_irq_alloc:
+out_error_dma_alloc:
 	pxa2xx_spi_dma_release(drv_data);
-	free_irq(ssp->irq, drv_data);
 
 	return status;
 }
@@ -1511,6 +1518,7 @@ static int pxa2xx_spi_suspend(struct device *dev)
 	if (status)
 		return status;
 
+	drv_data->suspended = true;
 	pxa_ssp_disable(ssp);
 
 	if (!pm_runtime_suspended(dev))
@@ -1531,6 +1539,8 @@ static int pxa2xx_spi_resume(struct device *dev)
 			return status;
 	}
 
+	drv_data->suspended = false;
+
 	/* Start the queue running */
 	return spi_controller_resume(drv_data->controller);
 }
@@ -1539,6 +1549,7 @@ static int pxa2xx_spi_runtime_suspend(struct device *dev)
 {
 	struct driver_data *drv_data = dev_get_drvdata(dev);
 
+	drv_data->suspended = true;
 	pxa2xx_spi_clk_disable(drv_data);
 	return 0;
 }
@@ -1546,8 +1557,14 @@ static int pxa2xx_spi_runtime_suspend(struct device *dev)
 static int pxa2xx_spi_runtime_resume(struct device *dev)
 {
 	struct driver_data *drv_data = dev_get_drvdata(dev);
+	int ret;
 
-	return pxa2xx_spi_clk_enable(drv_data);
+	ret = pxa2xx_spi_clk_enable(drv_data);
+	if (ret)
+		return ret;
+
+	drv_data->suspended = false;
+	return 0;
 }
 
 EXPORT_NS_GPL_DEV_PM_OPS(pxa2xx_spi_pm_ops, SPI_PXA2xx) = {
diff --git a/drivers/spi/spi-pxa2xx.h b/drivers/spi/spi-pxa2xx.h
index 820e573a3c60..44f37bf9c519 100644
--- a/drivers/spi/spi-pxa2xx.h
+++ b/drivers/spi/spi-pxa2xx.h
@@ -72,6 +72,7 @@ struct driver_data {
 
 	void __iomem *lpss_base;
 
+	bool suspended;
 	bool clk_enabled;
 
 	/* Optional slave FIFO ready signal */
-- 
2.39.5