drm/i915: taint kernel when force probing unsupported devices

For development and testing purposes, the i915.force_probe module
parameter and DRM_I915_FORCE_PROBE kconfig option allow probing of
devices that aren't supported by the driver.

The i915.force_probe module parameter is "unsafe" and setting it taints
the kernel. However, using the kconfig option does not.

Always taint the kernel when force probing a device that is not
supported.

v2: Drop "depends on EXPERT" to avoid build breakage (kernel test robot)

Fixes: 7ef5ef5c ("drm/i915: add force_probe module parameter to replace alpha_support")
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Dave Airlie <airlied@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230504103508.1818540-1-jani.nikula@intel.com

drivers/gpu/drm/i915/Kconfig

@@ -62,10 +62,11 @@ config DRM_I915_FORCE_PROBE
 	  This is the default value for the i915.force_probe module
 	  parameter. Using the module parameter overrides this option.
 
-	  Force probe the i915 for Intel graphics devices that are
-	  recognized but not properly supported by this kernel version. It is
-	  recommended to upgrade to a kernel version with proper support as soon
-	  as it is available.
+	  Force probe the i915 driver for Intel graphics devices that are
+	  recognized but not properly supported by this kernel version. Force
+	  probing an unsupported device taints the kernel. It is recommended to
+	  upgrade to a kernel version with proper support as soon as it is
+	  available.
 
 	  It can also be used to block the probe of recognized and fully
 	  supported devices.
@@ -75,7 +76,8 @@ config DRM_I915_FORCE_PROBE
 	  Use "<pci-id>[,<pci-id>,...]" to force probe the i915 for listed
 	  devices. For example, "4500" or "4500,4571".
 
-	  Use "*" to force probe the driver for all known devices.
+	  Use "*" to force probe the driver for all known devices. Not
+	  recommended.
 
 	  Use "!" right before the ID to block the probe of the device. For
 	  example, "4500,!4571" forces the probe of 4500 and blocks the probe of

drivers/gpu/drm/i915/i915_pci.c

@@ -1345,6 +1345,12 @@ static int i915_pci_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 		return -ENODEV;
 	}
 
+	if (intel_info->require_force_probe) {
+		dev_info(&pdev->dev, "Force probing unsupported Device ID %04x, tainting kernel\n",
+			 pdev->device);
+		add_taint(TAINT_USER, LOCKDEP_STILL_OK);
+	}
+
 	/* Only bind to function 0 of the device. Early generations
 	 * used function 1 as a placeholder for multi-head. This causes
 	 * us confusion instead, especially on the systems where both