[Intel-gfx] [PATCH] drm: Add fake controlD* symlinks for backwards compat

[Daniel Vetter]

We thought that no userspace is using them, but oops libdrm is using
them to figure out whether a driver supports modesetting. Check out
drmCheckModesettingSupported but maybe don't because it's horrible and
totally runs counter to where we want to go with libdrm device
handling. The function looks in the device hierarchy for whether
controlD* exist using the following format string:

/sys/bus/pci/devices/%04x:%02x:%02x.%d/drm/controlD%d

The "/drm" subdirectory is the glue directory from the sysfs class
stuff, and the only way to get at it seems to through
kdev->kobj.parent (when kdev is represents e.g. the card0 chardev
instance in sysfs). Git grep says we're not the only ones touching
that, so I hope it's ok we dig into such internals - I couldn't find a
proper interface for getting at the glue directory.

Quick git grep shows that at least -amdgpu, -ati and UXA in -intel are
using this. -modesetting and SNA in -intel do not, which is why this
didn't blow up earlier.

Oh well, we need to keep it working, and the simplest way is to add a
symlink at the right place in debugfs from controlD* to card*.

Fixes: 8a357d10043c ("drm: Nerf DRM_CONTROL nodes")
Cc: Dave Airlie <airlied at gmail.com>
Reported-by: Alex Deucher <alexander.deucher at amd.com>
Cc: Alex Deucher <alexander.deucher at amd.com>
Cc: David Herrmann <dh.herrmann at gmail.com>
Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 drivers/gpu/drm/drm_drv.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 4ec61ac27477..5baec7202558 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -650,6 +650,47 @@ void drm_dev_unref(struct drm_device *dev)
 }
 EXPORT_SYMBOL(drm_dev_unref);
 
+static int create_compat_control_link(struct drm_device *dev)
+{
+	struct drm_minor *minor;
+	char *name;
+	int ret;
+
+	if (!drm_core_check_feature(dev, DRIVER_MODESET))
+		return 0;
+
+	minor = *drm_minor_get_slot(dev, DRM_MINOR_PRIMARY);
+	name = kasprintf(GFP_KERNEL, "controlD%d", minor->index);
+	if (!name)
+		return -ENOMEM;
+
+	ret = sysfs_create_link(minor->kdev->kobj.parent,
+				&minor->kdev->kobj,
+				name);
+
+	kfree(name);
+
+	return ret;
+}
+
+static void remove_compat_control_link(struct drm_device *dev)
+{
+	struct drm_minor *minor;
+	char *name;
+
+	if (!drm_core_check_feature(dev, DRIVER_MODESET))
+		return;
+
+	minor = *drm_minor_get_slot(dev, DRM_MINOR_PRIMARY);
+	name = kasprintf(GFP_KERNEL, "controlD%d", minor->index);
+	if (!name)
+		return;
+
+	sysfs_remove_link(minor->kdev->kobj.parent, name);
+
+	kfree(name);
+}
+
 /**
  * drm_dev_register - Register DRM device
  * @dev: Device to register
@@ -688,6 +729,10 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
 	if (ret)
 		goto err_minors;
 
+	ret = create_compat_control_link(dev);
+	if (ret)
+		goto err_minors;
+
 	if (dev->driver->load) {
 		ret = dev->driver->load(dev, flags);
 		if (ret)
@@ -701,6 +746,7 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
 	goto out_unlock;
 
 err_minors:
+	remove_compat_control_link(dev);
 	drm_minor_unregister(dev, DRM_MINOR_PRIMARY);
 	drm_minor_unregister(dev, DRM_MINOR_RENDER);
 	drm_minor_unregister(dev, DRM_MINOR_CONTROL);
@@ -741,6 +787,7 @@ void drm_dev_unregister(struct drm_device *dev)
 	list_for_each_entry_safe(r_list, list_temp, &dev->maplist, head)
 		drm_legacy_rmmap(dev, r_list->map);
 
+	remove_compat_control_link(dev);
 	drm_minor_unregister(dev, DRM_MINOR_PRIMARY);
 	drm_minor_unregister(dev, DRM_MINOR_RENDER);
 	drm_minor_unregister(dev, DRM_MINOR_CONTROL);
-- 
2.10.2