drm: Expand max DRM device number to full MINORBITS

Having a limit of 64 DRM devices is not good enough for modern world
where we have multi-GPU servers, SR-IOV virtual functions and virtual
devices used for testing.
Let's utilize full minor range for DRM devices.
To avoid regressing the existing userspace, we're still maintaining the
numbering scheme where 0-63 is used for primary, 64-127 is reserved
(formerly for control) and 128-191 is used for render.
For minors >= 192, we're allocating minors dynamically on a first-come,
first-served basis.

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240823163048.2676257-4-michal.winiarski@intel.com
Acked-by: James Zhu <James.Zhu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>

drivers/gpu/drm/drm_drv.c

@@ -121,10 +121,19 @@ static void drm_minor_alloc_release(struct drm_device *dev, void *data)
 	xa_erase(drm_minor_get_xa(minor->type), minor->index);
 }
 
+/*
+ * DRM used to support 64 devices, for backwards compatibility we need to maintain the
+ * minor allocation scheme where minors 0-63 are primary nodes, 64-127 are control nodes,
+ * and 128-191 are render nodes.
+ * After reaching the limit, we're allocating minors dynamically - first-come, first-serve.
+ * Accel nodes are using a distinct major, so the minors are allocated in continuous 0-MAX
+ * range.
+ */
 #define DRM_MINOR_LIMIT(t) ({ \
 	typeof(t) _t = (t); \
 	_t == DRM_MINOR_ACCEL ? XA_LIMIT(0, ACCEL_MAX_MINORS) : XA_LIMIT(64 * _t, 64 * _t + 63); \
 })
+#define DRM_EXTENDED_MINOR_LIMIT XA_LIMIT(192, (1 << MINORBITS) - 1)
 
 static int drm_minor_alloc(struct drm_device *dev, enum drm_minor_type type)
 {
@@ -140,6 +149,9 @@ static int drm_minor_alloc(struct drm_device *dev, enum drm_minor_type type)
 
 	r = xa_alloc(drm_minor_get_xa(type), &minor->index,
 		     NULL, DRM_MINOR_LIMIT(type), GFP_KERNEL);
+	if (r == -EBUSY && (type == DRM_MINOR_PRIMARY || type == DRM_MINOR_RENDER))
+		r = xa_alloc(&drm_minors_xa, &minor->index,
+			     NULL, DRM_EXTENDED_MINOR_LIMIT, GFP_KERNEL);
 	if (r < 0)
 		return r;