diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
index 6da1e77e55fa..a1c6f0103bc4 100644
--- a/Documentation/gpu/drm-uapi.rst
+++ b/Documentation/gpu/drm-uapi.rst
@@ -14,6 +14,12 @@ management, and output management.
 Cover generic ioctls and sysfs layout here. We only need high-level
 info, since man pages should cover the rest.
 
+libdrm Device Lookup
+====================
+
+.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
+   :doc: getunique and setversion story
+
 Render nodes
 ============
 
diff --git a/drivers/gpu/drm/armada/armada_drv.c b/drivers/gpu/drm/armada/armada_drv.c
index cb21c0b6374a..f5ebdd681445 100644
--- a/drivers/gpu/drm/armada/armada_drv.c
+++ b/drivers/gpu/drm/armada/armada_drv.c
@@ -189,7 +189,6 @@ static struct drm_driver armada_drm_driver = {
 	.load			= armada_drm_load,
 	.lastclose		= armada_drm_lastclose,
 	.unload			= armada_drm_unload,
-	.set_busid		= drm_platform_set_busid,
 	.get_vblank_counter	= drm_vblank_no_hw_counter,
 	.enable_vblank		= armada_drm_enable_vblank,
 	.disable_vblank		= armada_drm_disable_vblank,
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 1fa7619face3..ffd6a2795230 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -37,6 +37,64 @@
 #include <linux/pci.h>
 #include <linux/export.h>
 
+/**
+ * DOC: getunique and setversion story
+ *
+ * BEWARE THE DRAGONS! MIND THE TRAPDOORS!
+ *
+ * In an attempt to warn anyone else who's trying to figure out what's going
+ * on here, I'll try to summarize the story. First things first, let's clear up
+ * the names, because the kernel internals, libdrm and the ioctls are all named
+ * differently:
+ *
+ *  - GET_UNIQUE ioctl, implemented by drm_getunique is wrapped up in libdrm
+ *    through the drmGetBusid function.
+ *  - The libdrm drmSetBusid function is backed by the SET_UNIQUE ioctl. All
+ *    that code is nerved in the kernel with drm_invalid_op().
+ *  - The internal set_busid kernel functions and driver callbacks are
+ *    exclusively use by the SET_VERSION ioctl, because only drm 1.0 (which is
+ *    nerved) allowed userspace to set the busid through the above ioctl.
+ *  - Other ioctls and functions involved are named consistently.
+ *
+ * For anyone wondering what's the difference between drm 1.1 and 1.4: Correctly
+ * handling pci domains in the busid on ppc. Doing this correctly was only
+ * implemented in libdrm in 2010, hence can't be nerved yet. No one knows what's
+ * special with drm 1.2 and 1.3.
+ *
+ * Now the actual horror story of how device lookup in drm works. At large,
+ * there's 2 different ways, either by busid, or by device driver name.
+ *
+ * Opening by busid is fairly simple:
+ *
+ * 1. First call SET_VERSION to make sure pci domains are handled properly. As a
+ *    side-effect this fills out the unique name in the master structure.
+ * 2. Call GET_UNIQUE to read out the unique name from the master structure,
+ *    which matches the busid thanks to step 1. If it doesn't, proceed to try
+ *    the next device node.
+ *
+ * Opening by name is slightly different:
+ *
+ * 1. Directly call VERSION to get the version and to match against the driver
+ *    name returned by that ioctl. Note that SET_VERSION is not called, which
+ *    means the the unique name for the master node just opening is _not_ filled
+ *    out. This despite that with current drm device nodes are always bound to
+ *    one device, and can't be runtime assigned like with drm 1.0.
+ * 2. Match driver name. If it mismatches, proceed to the next device node.
+ * 3. Call GET_UNIQUE, and check whether the unique name has length zero (by
+ *    checking that the first byte in the string is 0). If that's not the case
+ *    libdrm skips and proceeds to the next device node. Probably this is just
+ *    copypasta from drm 1.0 times where a set unique name meant that the driver
+ *    was in use already, but that's just conjecture.
+ *
+ * Long story short: To keep the open by name logic working, GET_UNIQUE must
+ * _not_ return a unique string when SET_VERSION hasn't been called yet,
+ * otherwise libdrm breaks. Even when that unique string can't ever change, and
+ * is totally irrelevant for actually opening the device because runtime
+ * assignable device instances were only support in drm 1.0, which is long dead.
+ * But the libdrm code in drmOpenByName somehow survived, hence this can't be
+ * broken.
+ */
+
 static int drm_version(struct drm_device *dev, void *data,
 		       struct drm_file *file_priv);
 
diff --git a/drivers/gpu/drm/drm_platform.c b/drivers/gpu/drm/drm_platform.c
index 644169e1a029..2c819ef90090 100644
--- a/drivers/gpu/drm/drm_platform.c
+++ b/drivers/gpu/drm/drm_platform.c
@@ -68,24 +68,6 @@ err_free:
 	return ret;
 }
 
-int drm_platform_set_busid(struct drm_device *dev, struct drm_master *master)
-{
-	int id;
-
-	id = dev->platformdev->id;
-	if (id < 0)
-		id = 0;
-
-	master->unique = kasprintf(GFP_KERNEL, "platform:%s:%02d",
-						dev->platformdev->name, id);
-	if (!master->unique)
-		return -ENOMEM;
-
-	master->unique_len = strlen(master->unique);
-	return 0;
-}
-EXPORT_SYMBOL(drm_platform_set_busid);
-
 /**
  * drm_platform_init - Register a platform device with the DRM subsystem
  * @driver: DRM device driver
diff --git a/drivers/gpu/drm/etnaviv/etnaviv_drv.c b/drivers/gpu/drm/etnaviv/etnaviv_drv.c
index 3d4f56df8359..340d390306d8 100644
--- a/drivers/gpu/drm/etnaviv/etnaviv_drv.c
+++ b/drivers/gpu/drm/etnaviv/etnaviv_drv.c
@@ -496,7 +496,6 @@ static struct drm_driver etnaviv_drm_driver = {
 				DRIVER_RENDER,
 	.open               = etnaviv_open,
 	.preclose           = etnaviv_preclose,
-	.set_busid          = drm_platform_set_busid,
 	.gem_free_object_unlocked = etnaviv_gem_free_object,
 	.gem_vm_ops         = &vm_ops,
 	.prime_handle_to_fd = drm_gem_prime_handle_to_fd,
diff --git a/drivers/gpu/drm/exynos/exynos_drm_drv.c b/drivers/gpu/drm/exynos/exynos_drm_drv.c
index 4a679fb9bb02..13d28d4229e2 100644
--- a/drivers/gpu/drm/exynos/exynos_drm_drv.c
+++ b/drivers/gpu/drm/exynos/exynos_drm_drv.c
@@ -407,7 +407,6 @@ static struct drm_driver exynos_drm_driver = {
 	.preclose		= exynos_drm_preclose,
 	.lastclose		= exynos_drm_lastclose,
 	.postclose		= exynos_drm_postclose,
-	.set_busid		= drm_platform_set_busid,
 	.get_vblank_counter	= drm_vblank_no_hw_counter,
 	.enable_vblank		= exynos_drm_crtc_enable_vblank,
 	.disable_vblank		= exynos_drm_crtc_disable_vblank,
diff --git a/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c b/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c
index ef2c32ec1616..1edd9bc80294 100644
--- a/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c
+++ b/drivers/gpu/drm/hisilicon/kirin/kirin_drm_drv.c
@@ -171,7 +171,6 @@ static struct drm_driver kirin_drm_driver = {
 	.driver_features	= DRIVER_GEM | DRIVER_MODESET | DRIVER_PRIME |
 				  DRIVER_ATOMIC | DRIVER_HAVE_IRQ,
 	.fops			= &kirin_drm_fops,
-	.set_busid		= drm_platform_set_busid,
 
 	.gem_free_object_unlocked = drm_gem_cma_free_object,
 	.gem_vm_ops		= &drm_gem_cma_vm_ops,
diff --git a/drivers/gpu/drm/imx/imx-drm-core.c b/drivers/gpu/drm/imx/imx-drm-core.c
index 82656654fb21..7746418a4c08 100644
--- a/drivers/gpu/drm/imx/imx-drm-core.c
+++ b/drivers/gpu/drm/imx/imx-drm-core.c
@@ -407,7 +407,6 @@ static struct drm_driver imx_drm_driver = {
 	.load			= imx_drm_driver_load,
 	.unload			= imx_drm_driver_unload,
 	.lastclose		= imx_drm_driver_lastclose,
-	.set_busid		= drm_platform_set_busid,
 	.gem_free_object_unlocked = drm_gem_cma_free_object,
 	.gem_vm_ops		= &drm_gem_cma_vm_ops,
 	.dumb_create		= drm_gem_cma_dumb_create,
diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
index 568fcc328f27..a02dc2b27739 100644
--- a/drivers/gpu/drm/msm/msm_drv.c
+++ b/drivers/gpu/drm/msm/msm_drv.c
@@ -722,7 +722,6 @@ static struct drm_driver msm_driver = {
 	.open               = msm_open,
 	.preclose           = msm_preclose,
 	.lastclose          = msm_lastclose,
-	.set_busid          = drm_platform_set_busid,
 	.irq_handler        = msm_irq,
 	.irq_preinstall     = msm_irq_preinstall,
 	.irq_postinstall    = msm_irq_postinstall,
diff --git a/drivers/gpu/drm/nouveau/nouveau_drm.c b/drivers/gpu/drm/nouveau/nouveau_drm.c
index c00ff6e786a3..295e7621cc68 100644
--- a/drivers/gpu/drm/nouveau/nouveau_drm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_drm.c
@@ -1070,7 +1070,6 @@ nouveau_drm_init(void)
 	driver_pci = driver_stub;
 	driver_pci.set_busid = drm_pci_set_busid;
 	driver_platform = driver_stub;
-	driver_platform.set_busid = drm_platform_set_busid;
 
 	nouveau_display_options();
 
diff --git a/drivers/gpu/drm/omapdrm/omap_drv.c b/drivers/gpu/drm/omapdrm/omap_drv.c
index 6b97011154bf..26c6134eb744 100644
--- a/drivers/gpu/drm/omapdrm/omap_drv.c
+++ b/drivers/gpu/drm/omapdrm/omap_drv.c
@@ -801,7 +801,6 @@ static struct drm_driver omap_drm_driver = {
 	.unload = dev_unload,
 	.open = dev_open,
 	.lastclose = dev_lastclose,
-	.set_busid = drm_platform_set_busid,
 	.get_vblank_counter = drm_vblank_no_hw_counter,
 	.enable_vblank = omap_irq_enable_vblank,
 	.disable_vblank = omap_irq_disable_vblank,
diff --git a/drivers/gpu/drm/shmobile/shmob_drm_drv.c b/drivers/gpu/drm/shmobile/shmob_drm_drv.c
index ee79264b5b6a..f0492603ea88 100644
--- a/drivers/gpu/drm/shmobile/shmob_drm_drv.c
+++ b/drivers/gpu/drm/shmobile/shmob_drm_drv.c
@@ -259,7 +259,6 @@ static struct drm_driver shmob_drm_driver = {
 				| DRIVER_PRIME,
 	.load			= shmob_drm_load,
 	.unload			= shmob_drm_unload,
-	.set_busid		= drm_platform_set_busid,
 	.irq_handler		= shmob_drm_irq,
 	.get_vblank_counter	= drm_vblank_no_hw_counter,
 	.enable_vblank		= shmob_drm_enable_vblank,
diff --git a/drivers/gpu/drm/tilcdc/tilcdc_drv.c b/drivers/gpu/drm/tilcdc/tilcdc_drv.c
index 308e197908fc..d27809372d54 100644
--- a/drivers/gpu/drm/tilcdc/tilcdc_drv.c
+++ b/drivers/gpu/drm/tilcdc/tilcdc_drv.c
@@ -541,7 +541,6 @@ static struct drm_driver tilcdc_driver = {
 	.load               = tilcdc_load,
 	.unload             = tilcdc_unload,
 	.lastclose          = tilcdc_lastclose,
-	.set_busid          = drm_platform_set_busid,
 	.irq_handler        = tilcdc_irq,
 	.irq_preinstall     = tilcdc_irq_preinstall,
 	.irq_postinstall    = tilcdc_irq_postinstall,
diff --git a/drivers/gpu/drm/virtio/virtgpu_drm_bus.c b/drivers/gpu/drm/virtio/virtgpu_drm_bus.c
index 88a39165edd5..7f0e93f87a55 100644
--- a/drivers/gpu/drm/virtio/virtgpu_drm_bus.c
+++ b/drivers/gpu/drm/virtio/virtgpu_drm_bus.c
@@ -27,16 +27,6 @@
 
 #include "virtgpu_drv.h"
 
-int drm_virtio_set_busid(struct drm_device *dev, struct drm_master *master)
-{
-	struct pci_dev *pdev = dev->pdev;
-
-	if (pdev) {
-		return drm_pci_set_busid(dev, master);
-	}
-	return 0;
-}
-
 static void virtio_pci_kick_out_firmware_fb(struct pci_dev *pci_dev)
 {
 	struct apertures_struct *ap;
diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.c b/drivers/gpu/drm/virtio/virtgpu_drv.c
index 5820b7020ae5..c13f70cfc461 100644
--- a/drivers/gpu/drm/virtio/virtgpu_drv.c
+++ b/drivers/gpu/drm/virtio/virtgpu_drv.c
@@ -117,7 +117,6 @@ static const struct file_operations virtio_gpu_driver_fops = {
 
 static struct drm_driver driver = {
 	.driver_features = DRIVER_MODESET | DRIVER_GEM | DRIVER_PRIME | DRIVER_RENDER | DRIVER_ATOMIC,
-	.set_busid = drm_virtio_set_busid,
 	.load = virtio_gpu_driver_load,
 	.unload = virtio_gpu_driver_unload,
 	.open = virtio_gpu_driver_open,
diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h b/drivers/gpu/drm/virtio/virtgpu_drv.h
index acf556a35cb2..b18ef3111f0c 100644
--- a/drivers/gpu/drm/virtio/virtgpu_drv.h
+++ b/drivers/gpu/drm/virtio/virtgpu_drv.h
@@ -49,7 +49,6 @@
 #define DRIVER_PATCHLEVEL 1
 
 /* virtgpu_drm_bus.c */
-int drm_virtio_set_busid(struct drm_device *dev, struct drm_master *master);
 int drm_virtio_init(struct drm_driver *driver, struct virtio_device *vdev);
 
 struct virtio_gpu_object {
diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index fd4e4391aee5..babec2e80760 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -1132,7 +1132,6 @@ extern int drm_pcie_get_max_link_width(struct drm_device *dev, u32 *mlw);
 
 /* platform section */
 extern int drm_platform_init(struct drm_driver *driver, struct platform_device *platform_device);
-extern int drm_platform_set_busid(struct drm_device *d, struct drm_master *m);
 
 /* returns true if currently okay to sleep */
 static __inline__ bool drm_can_sleep(void)