NULL pointer exception in drivers/memory/mtk-smi.c
Yong Wu (吴勇)
Yong.Wu at mediatek.com
Mon Jul 7 00:24:09 PDT 2025
On Thu, 2025-07-03 at 18:41 +0200, Uwe Kleine-König wrote:
> 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().
Hi Uwe,
Thanks for your help. In this case, it looks like the disp probe occurs
before the smi_larb_bind operation, we need to let disp wait for the
bind to complete.
--- a/drivers/memory/mtk-smi.c
+++ b/drivers/memory/mtk-smi.c
@@ -666,6 +666,10 @@ static int __maybe_unused
mtk_smi_larb_resume(struct device *dev)
if (MTK_SMI_CAPS(larb->larb_gen->flags_general,
MTK_SMI_FLAG_SLEEP_CTL))
mtk_smi_larb_sleep_ctrl_disable(larb);
+ /* The larb_bind operation may be later than the master probe. */
+ if (!larb->mmu)
+ return -EPROBE_DEFER;
+
/* Configure the basic setting for this larb */
Hi Angelo,
Do you have any suggestion?
Thanks.
>
> I don't know what the right fix here is, but maybe someone else does?
>
> Best regards
> Uwe
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