[PATCH] drm/amd/amdgpu: Prevent null pointer dereference in GPU bandwidth calculation

[Srinivasan Shanmugam]

This commit adds an early return if no upstream bridge is found, setting
the speed and width to PCI_SPEED_UNKNOWN and PCIE_LNK_WIDTH_UNKNOWN,
respectively. This ensures that the function handles the absence of an
upstream bridge gracefully.

Fixes the below:
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:6193 amdgpu_device_gpu_bandwidth()
	error: we previously assumed 'parent' could be null (see line 6180)

drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
    6170 static void amdgpu_device_gpu_bandwidth(struct amdgpu_device *adev,
    6171                                         enum pci_bus_speed *speed,
    6172                                         enum pcie_link_width *width)
    6173 {
    6174         struct pci_dev *parent = adev->pdev;
    6175
    6176         if (!speed || !width)
    6177                 return;
    6178
    6179         parent = pci_upstream_bridge(parent);
    6180         if (parent && parent->vendor == PCI_VENDOR_ID_ATI) {
                     ^^^^^^
If parent is NULL

    6181                 /* use the upstream/downstream switches internal to dGPU */
    6182                 *speed = pcie_get_speed_cap(parent);
    6183                 *width = pcie_get_width_cap(parent);
    6184                 while ((parent = pci_upstream_bridge(parent))) {
    6185                         if (parent->vendor == PCI_VENDOR_ID_ATI) {
    6186                                 /* use the upstream/downstream switches internal to dGPU */
    6187                                 *speed = pcie_get_speed_cap(parent);
    6188                                 *width = pcie_get_width_cap(parent);
    6189                         }
    6190                 }
    6191         } else {
    6192                 /* use the device itself */
--> 6193                 *speed = pcie_get_speed_cap(parent);
                                                     ^^^^^^ Then we are toasted here.

    6194                 *width = pcie_get_width_cap(parent);
    6195         }
    6196 }

Fixes: 9e424a5d9087 ("drm/amdgpu: cache gpu pcie link width")
Cc: Christian König <christian.koenig at amd.com>
Cc: Alex Deucher <alexander.deucher at amd.com>
Reported-by: Dan Carpenter <dan.carpenter at linaro.org>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam at amd.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_device.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
index 46af07faf8c8..749791a486bd 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
@@ -6174,6 +6174,14 @@ static void amdgpu_device_gpu_bandwidth(struct amdgpu_device *adev,
 		return;
 
 	parent = pci_upstream_bridge(parent);
+
+	if (!parent) {
+		/*Return early if no upstream bridge is found */
+		*speed = PCI_SPEED_UNKNOWN;
+		*width = PCIE_LNK_WIDTH_UNKNOWN;
+		return;
+	}
+
 	if (parent && parent->vendor == PCI_VENDOR_ID_ATI) {
 		/* use the upstream/downstream switches internal to dGPU */
 		*speed = pcie_get_speed_cap(parent);
-- 
2.34.1