[PATCH 2/2] spi: pxa2xx: restore LPSS private and IDMA registers on S3 resume

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

Intel LPSS SPI controllers lose all private register state across S3
suspend because the LPSS power domain is fully removed.  On resume the
driver only re-enables the SSP clock but leaves the LPSS private
registers (BAR0 0x200-0x2ff) and the IDMA registers (0x800-0x814) in
their power-on-reset state, which causes two separate problems:

1. LPSS_PRIV_RESETS (0x204) stays zero, keeping the functional block
   and IDMA in reset.  Writing 7 to de-assert both resets before any
   register access is mandatory; accessing MMIO while in reset causes a
   PCIe Completion Timeout and a watchdog-triggered system reset.

2. The IDMA block shares the SPI interrupt line.  With its registers
   zeroed the IDMA asserts a spurious interrupt that masks the real SPI
   interrupt, causing every subsequent SPI transfer to time out (-110).

3. The LPSS software chip-select control register (0x224) must *not* be
   blindly restored from its suspend-time snapshot: if CS was asserted
   at the moment of suspend, restoring that state corrupts the first
   post-resume SPI transaction.  Instead, call lpss_ssp_setup() which
   unconditionally writes SW_MODE | CS_HIGH (idle/deasserted), matching
   the state established at probe time.

Fix all three issues by:
  - Saving the four LPSS clock/LTR/SSP private registers and six IDMA
    registers in pxa2xx_spi_suspend().
  - In pxa2xx_spi_resume(), writing LPSS_PRIV_RESETS first, then
    restoring the saved private registers (excluding 0x204 and 0x224),
    then calling lpss_ssp_setup() to re-initialise CS to idle-high,
    and finally restoring the IDMA registers.

Signed-off-by: Shih-Yuan Lee <fourdollars@debian.org>
---
 drivers/spi/spi-pxa2xx.c | 57 ++++++++++++++++++++++++++++++++++++++++
 drivers/spi/spi-pxa2xx.h |  4 +++
 2 files changed, 61 insertions(+)

diff --git a/drivers/spi/spi-pxa2xx.c b/drivers/spi/spi-pxa2xx.c
index 6291d7c2e06f..e8a83c818328 100644
--- a/drivers/spi/spi-pxa2xx.c
+++ b/drivers/spi/spi-pxa2xx.c
@@ -1479,12 +1479,35 @@ void pxa2xx_spi_remove(struct device *dev)
 }
 EXPORT_SYMBOL_NS_GPL(pxa2xx_spi_remove, "SPI_PXA2xx");
 
+/*
+ * LPSS private registers to save across S3 suspend.
+ * NOTE: 0x224 (CS control) is intentionally excluded - it is re-initialised
+ * by lpss_ssp_setup() on resume to ensure CS starts deasserted (idle-high).
+ */
+static const unsigned int lpss_saved_regs[] = {
+	0x200,
+	0x204,
+	0x220,
+	0x238,
+};
+
 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 status;
 
+	if (is_lpss_ssp(drv_data) && !pm_runtime_suspended(dev)) {
+		struct pxa2xx_spi_controller *pdata = drv_data->controller_info;
+		int i;
+
+		for (i = 0; i < ARRAY_SIZE(lpss_saved_regs); i++)
+			pdata->lpss_priv_ctx[i] = readl(ssp->mmio_base + lpss_saved_regs[i]);
+
+		for (i = 0; i < 6; i++)
+			pdata->lpss_idma_ctx[i] = readl(ssp->mmio_base + 0x800 + i * 4);
+	}
+
 	status = spi_controller_suspend(drv_data->controller);
 	if (status)
 		return status;
@@ -1508,6 +1531,40 @@ static int pxa2xx_spi_resume(struct device *dev)
 		status = clk_prepare_enable(ssp->clk);
 		if (status)
 			return status;
+
+		if (is_lpss_ssp(drv_data)) {
+			struct pxa2xx_spi_controller *pdata = drv_data->controller_info;
+			int i;
+
+			/* First de-assert resets by writing 7 to 0x204 (LPSS_PRIV_RESETS) */
+			writel(7, ssp->mmio_base + 0x204);
+
+			/*
+			 * Restore clock/LTR/SSP private registers.
+			 * 0x204 (resets) is skipped - already written above.
+			 * 0x224 (CS control) is skipped - re-initialised by
+			 * lpss_ssp_setup() below to ensure CS starts idle-high.
+			 */
+			for (i = 0; i < ARRAY_SIZE(lpss_saved_regs); i++) {
+				if (lpss_saved_regs[i] == 0x204)
+					continue;
+				writel(pdata->lpss_priv_ctx[i],
+				       ssp->mmio_base + lpss_saved_regs[i]);
+			}
+
+			/*
+			 * Re-initialise SW chip-select control so CS starts
+			 * deasserted (SW_MODE | CS_HIGH) regardless of the
+			 * state it was in at suspend time.
+			 */
+			lpss_ssp_setup(drv_data);
+
+			/* Restore IDMA registers */
+			for (i = 0; i < 6; i++) {
+				writel(pdata->lpss_idma_ctx[i],
+				       ssp->mmio_base + 0x800 + i * 4);
+			}
+		}
 	}
 
 	/* Start the queue running */
diff --git a/drivers/spi/spi-pxa2xx.h b/drivers/spi/spi-pxa2xx.h
index 447be0369384..423cef5118e7 100644
--- a/drivers/spi/spi-pxa2xx.h
+++ b/drivers/spi/spi-pxa2xx.h
@@ -34,6 +34,10 @@ struct pxa2xx_spi_controller {
 
 	/* For non-PXA arches */
 	struct ssp_device ssp;
+
+	/* LPSS private registers context */
+	u32 lpss_priv_ctx[4];
+	u32 lpss_idma_ctx[6];
 };
 
 struct spi_controller;
-- 
2.39.5