NULL pointer exception in drivers/memory/mtk-smi.c

[Uwe Kleine-König]

Hello,

[expanding the audience a bit according to the drivers that are involved
now that the problem is better understood]

On Tue, Jun 17, 2025 at 05:18:30PM +0200, Uwe Kleine-König wrote:
> on a 6.16-rc2 kernel running on an mt8365-evk I occasionally hit the
> following during boot:

I invested some time now to understand the issue. So here comes what I
understood, maybe that helps someone to spot the fix to the described
problem. 

With a configuration that has all drivers built-in but

	CONFIG_DRM_MEDIATEK=m
	CONFIG_MTK_IOMMU=m

and 

diff --git a/drivers/iommu/mtk_iommu.c b/drivers/iommu/mtk_iommu.c
index cb95fecf6016..d4320db7cd2d 100644
--- a/drivers/iommu/mtk_iommu.c
+++ b/drivers/iommu/mtk_iommu.c
@@ -1387,14 +1387,15 @@ static int mtk_iommu_probe(struct platform_device *pdev)
 		goto out_list_del;
 
 	ret = iommu_device_register(&data->iommu, &mtk_iommu_ops, dev);
 	if (ret)
 		goto out_sysfs_remove;
 
 	if (MTK_IOMMU_IS_TYPE(data->plat_data, MTK_IOMMU_TYPE_MM)) {
+		msleep(10000);
 		ret = component_master_add_with_match(dev, &mtk_iommu_com_ops, match);
 		if (ret)
 			goto out_device_unregister;
 	}
 	return ret;
 
 out_device_unregister:

I can reliably trigger the race.

With that sleep in place iommu_device_register() completes quickly which
enables probing of the devices with drivers contained in the
drm_mediatek module (because the modules are loaded in parallel on
a different CPU).

Then generic driver code calls resume on all suppliers for devices to
bind, among them the four larb devices. However due to
component_master_add_with_match() not being called yet, the larb devices
are not yet bound to the iommu device and so larb->mmu is still NULL.
The latter is a problem in mtk_smi_larb_config_port_gen2_general() which
is called from mtk_smi_larb_resume().

I don't know what the right fix here is, but maybe someone else does?

Best regards
Uwe
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