firmware: qcom: scm: Cleanup global '__scm' on probe failures

If SCM driver fails the probe, it should not leave global '__scm'
variable assigned, because external users of this driver will assume the
probe finished successfully.  For example TZMEM parts ('__scm->mempool')
are initialized later in the probe, but users of it (__scm_smc_call())
rely on the '__scm' variable.

This fixes theoretical NULL pointer exception, triggered via introducing
probe deferral in SCM driver with call trace:

  qcom_tzmem_alloc+0x70/0x1ac (P)
  qcom_tzmem_alloc+0x64/0x1ac (L)
  qcom_scm_assign_mem+0x78/0x194
  qcom_rmtfs_mem_probe+0x2d4/0x38c
  platform_probe+0x68/0xc8

Fixes: 40289e3 ("firmware: qcom: scm: enable the TZ mem allocator")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20241209-qcom-scm-missing-barriers-and-all-sort-of-srap-v2-4-9061013c8d92@linaro.org
Signed-off-by: Bjorn Andersson <andersson@kernel.org>

drivers/firmware/qcom/qcom_scm.c

@@ -2038,13 +2038,17 @@ static int qcom_scm_probe(struct platform_device *pdev)
 
 	irq = platform_get_irq_optional(pdev, 0);
 	if (irq < 0) {
-		if (irq != -ENXIO)
-			return irq;
+		if (irq != -ENXIO) {
+			ret = irq;
+			goto err;
+		}
 	} else {
 		ret = devm_request_threaded_irq(__scm->dev, irq, NULL, qcom_scm_irq_handler,
 						IRQF_ONESHOT, "qcom-scm", __scm);
-		if (ret < 0)
-			return dev_err_probe(scm->dev, ret, "Failed to request qcom-scm irq\n");
+		if (ret < 0) {
+			dev_err_probe(scm->dev, ret, "Failed to request qcom-scm irq\n");
+			goto err;
+		}
 	}
 
 	__get_convention();
@@ -2063,24 +2067,30 @@ static int qcom_scm_probe(struct platform_device *pdev)
 		qcom_scm_disable_sdi();
 
 	ret = of_reserved_mem_device_init(__scm->dev);
-	if (ret && ret != -ENODEV)
-		return dev_err_probe(__scm->dev, ret,
-				     "Failed to setup the reserved memory region for TZ mem\n");
+	if (ret && ret != -ENODEV) {
+		dev_err_probe(__scm->dev, ret,
+			      "Failed to setup the reserved memory region for TZ mem\n");
+		goto err;
+	}
 
 	ret = qcom_tzmem_enable(__scm->dev);
-	if (ret)
-		return dev_err_probe(__scm->dev, ret,
-				     "Failed to enable the TrustZone memory allocator\n");
+	if (ret) {
+		dev_err_probe(__scm->dev, ret,
+			      "Failed to enable the TrustZone memory allocator\n");
+		goto err;
+	}
 
 	memset(&pool_config, 0, sizeof(pool_config));
 	pool_config.initial_size = 0;
 	pool_config.policy = QCOM_TZMEM_POLICY_ON_DEMAND;
 	pool_config.max_size = SZ_256K;
 
 	__scm->mempool = devm_qcom_tzmem_pool_new(__scm->dev, &pool_config);
-	if (IS_ERR(__scm->mempool))
-		return dev_err_probe(__scm->dev, PTR_ERR(__scm->mempool),
-				     "Failed to create the SCM memory pool\n");
+	if (IS_ERR(__scm->mempool)) {
+		dev_err_probe(__scm->dev, PTR_ERR(__scm->mempool),
+			      "Failed to create the SCM memory pool\n");
+		goto err;
+	}
 
 	/*
 	 * Initialize the QSEECOM interface.
@@ -2096,6 +2106,12 @@ static int qcom_scm_probe(struct platform_device *pdev)
 	WARN(ret < 0, "failed to initialize qseecom: %d\n", ret);
 
 	return 0;
+
+err:
+	/* Paired with smp_load_acquire() in qcom_scm_is_available(). */
+	smp_store_release(&__scm, NULL);
+
+	return ret;
 }
 
 static void qcom_scm_shutdown(struct platform_device *pdev)