iommu/mediatek: fix NULL pointer dereference when printing dev_name

When larbdev is NULL (in the case I hit, the node is incorrectly set
iommus = <&iommu NUM>), it will cause device_link_add() fail and
the kernel crashes when we try to print dev_name(larbdev).

Fix it by adding a NULL pointer check before
device_link_add/device_link_remove.

It should work for normal correct setting and avoid the crash caused
by my incorrect setting.

Error log:
[   18.189042][  T301] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000050
[   18.190247][  T301] Mem abort info:
[   18.190255][  T301]   ESR = 0x96000005
[   18.190263][  T301]   EC = 0x25: DABT (current EL), IL = 32 bits
[   18.192142][  T301]   SET = 0, FnV = 0
[   18.192151][  T301]   EA = 0, S1PTW = 0
[   18.194710][  T301]   FSC = 0x05: level 1 translation fault
[   18.195424][  T301] Data abort info:
[   18.195888][  T301]   ISV = 0, ISS = 0x00000005
[   18.196500][  T301]   CM = 0, WnR = 0
[   18.196977][  T301] user pgtable: 4k pages, 39-bit VAs, pgdp=0000000104f9e000
[   18.197889][  T301] [0000000000000050] pgd=0000000000000000, p4d=0000000000000000, pud=0000000000000000
[   18.199220][  T301] Internal error: Oops: 96000005 [#1] PREEMPT SMP
[   18.343152][  T301] Kernel Offset: 0x1444080000 from 0xffffffc008000000
[   18.343988][  T301] PHYS_OFFSET: 0x40000000
[   18.344519][  T301] pstate: a0400005 (NzCv daif +PAN -UAO)
[   18.345213][  T301] pc : mtk_iommu_probe_device+0xf8/0x118 [mtk_iommu]
[   18.346050][  T301] lr : mtk_iommu_probe_device+0xd0/0x118 [mtk_iommu]
[   18.346884][  T301] sp : ffffffc00a5635e0
[   18.347392][  T301] x29: ffffffc00a5635e0 x28: ffffffd44a46c1d8
[   18.348156][  T301] x27: ffffff80c39a8000 x26: ffffffd44a80cc38
[   18.348917][  T301] x25: 0000000000000000 x24: ffffffd44a80cc38
[   18.349677][  T301] x23: ffffffd44e4da4c6 x22: ffffffd44a80cc38
[   18.350438][  T301] x21: ffffff80cecd1880 x20: 0000000000000000
[   18.351198][  T301] x19: ffffff80c439f010 x18: ffffffc00a50d0c0
[   18.351959][  T301] x17: ffffffffffffffff x16: 0000000000000004
[   18.352719][  T301] x15: 0000000000000004 x14: ffffffd44eb5d420
[   18.353480][  T301] x13: 0000000000000ad2 x12: 0000000000000003
[   18.354241][  T301] x11: 00000000fffffad2 x10: c0000000fffffad2
[   18.355003][  T301] x9 : a0d288d8d7142d00 x8 : a0d288d8d7142d00
[   18.355763][  T301] x7 : ffffffd44c2bc640 x6 : 0000000000000000
[   18.356524][  T301] x5 : 0000000000000080 x4 : 0000000000000001
[   18.357284][  T301] x3 : 0000000000000000 x2 : 0000000000000005
[   18.358045][  T301] x1 : 0000000000000000 x0 : 0000000000000000
[   18.360208][  T301] Hardware name: MT6873 (DT)
[   18.360771][  T301] Call trace:
[   18.361168][  T301]  dump_backtrace+0xf8/0x1f0
[   18.361737][  T301]  dump_stack_lvl+0xa8/0x11c
[   18.362305][  T301]  dump_stack+0x1c/0x2c
[   18.362816][  T301]  mrdump_common_die+0x184/0x40c [mrdump]
[   18.363575][  T301]  ipanic_die+0x24/0x38 [mrdump]
[   18.364230][  T301]  atomic_notifier_call_chain+0x128/0x2b8
[   18.364937][  T301]  die+0x16c/0x568
[   18.365394][  T301]  __do_kernel_fault+0x1e8/0x214
[   18.365402][  T301]  do_page_fault+0xb8/0x678
[   18.366934][  T301]  do_translation_fault+0x48/0x64
[   18.368645][  T301]  do_mem_abort+0x68/0x148
[   18.368652][  T301]  el1_abort+0x40/0x64
[   18.368660][  T301]  el1h_64_sync_handler+0x54/0x88
[   18.368668][  T301]  el1h_64_sync+0x68/0x6c
[   18.368673][  T301]  mtk_iommu_probe_device+0xf8/0x118 [mtk_iommu]
[   18.369840][  T301]  __iommu_probe_device+0x12c/0x358
[   18.370880][  T301]  iommu_probe_device+0x3c/0x31c
[   18.372026][  T301]  of_iommu_configure+0x200/0x274
[   18.373587][  T301]  of_dma_configure_id+0x1b8/0x230
[   18.375200][  T301]  platform_dma_configure+0x24/0x3c
[   18.376456][  T301]  really_probe+0x110/0x504
[   18.376464][  T301]  __driver_probe_device+0xb4/0x188
[   18.376472][  T301]  driver_probe_device+0x5c/0x2b8
[   18.376481][  T301]  __driver_attach+0x338/0x42c
[   18.377992][  T301]  bus_add_driver+0x218/0x4c8
[   18.379389][  T301]  driver_register+0x84/0x17c
[   18.380580][  T301]  __platform_driver_register+0x28/0x38
...

Fixes: 635319a ("media: iommu/mediatek: Add device_link between the consumer and the larb devices")
Signed-off-by: Miles Chen <miles.chen@mediatek.com>

drivers/iommu/mtk_iommu.c

@@ -581,10 +581,12 @@ static struct iommu_device *mtk_iommu_probe_device(struct device *dev)
 		}
 	}
 	larbdev = data->larb_imu[larbid].dev;
-	link = device_link_add(dev, larbdev,
-			       DL_FLAG_PM_RUNTIME | DL_FLAG_STATELESS);
-	if (!link)
-		dev_err(dev, "Unable to link %s\n", dev_name(larbdev));
+	if (larbdev) {
+		link = device_link_add(dev, larbdev,
+				       DL_FLAG_PM_RUNTIME | DL_FLAG_STATELESS);
+		if (!link)
+			dev_err(dev, "Unable to link %s\n", dev_name(larbdev));
+	}
 	return &data->iommu;
 }
 
@@ -600,8 +602,10 @@ static void mtk_iommu_release_device(struct device *dev)
 
 	data = dev_iommu_priv_get(dev);
 	larbid = MTK_M4U_TO_LARB(fwspec->ids[0]);
-	larbdev = data->larb_imu[larbid].dev;
-	device_link_remove(dev, larbdev);
+	if (larbdev) {
+		larbdev = data->larb_imu[larbid].dev;
+		device_link_remove(dev, larbdev);
+	}
 
 	iommu_fwspec_free(dev);
 }

drivers/iommu/mtk_iommu_v1.c

@@ -467,10 +467,12 @@ static struct iommu_device *mtk_iommu_probe_device(struct device *dev)
 	}
 
 	larbdev = data->larb_imu[larbid].dev;
-	link = device_link_add(dev, larbdev,
-			       DL_FLAG_PM_RUNTIME | DL_FLAG_STATELESS);
-	if (!link)
-		dev_err(dev, "Unable to link %s\n", dev_name(larbdev));
+	if (larbdev) {
+		link = device_link_add(dev, larbdev,
+				       DL_FLAG_PM_RUNTIME | DL_FLAG_STATELESS);
+		if (!link)
+			dev_err(dev, "Unable to link %s\n", dev_name(larbdev));
+	}
 
 	return &data->iommu;
 }
@@ -501,8 +503,10 @@ static void mtk_iommu_release_device(struct device *dev)
 
 	data = dev_iommu_priv_get(dev);
 	larbid = mt2701_m4u_to_larb(fwspec->ids[0]);
-	larbdev = data->larb_imu[larbid].dev;
-	device_link_remove(dev, larbdev);
+	if (larbdev) {
+		larbdev = data->larb_imu[larbid].dev;
+		device_link_remove(dev, larbdev);
+	}
 
 	iommu_fwspec_free(dev);
 }