linux-kernel - [PATCH v5 2/3] drm: Expand max DRM device number to full MINORBITS

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Date:   Sun, 11 Sep 2022 23:14:42 +0200
From:   Michał Winiarski <michal.winiarski@...el.com>
To:     <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
        <intel-gfx@...ts.freedesktop.org>
CC:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        "Maarten Lankhorst" <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Simon Ser <contact@...rsion.fr>,
        Matthew Wilcox <willy@...radead.org>,
        Michał Winiarski <michal.winiarski@...el.com>
Subject: [PATCH v5 2/3] 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@...el.com>
---
 drivers/gpu/drm/drm_drv.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 61d24cdcd0f8..3718bd6bbef6 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -106,7 +106,8 @@ static void drm_minor_alloc_release(struct drm_device *dev, void *data)
 	xa_erase(&drm_minors_xa, minor->index);
 }
 
-#define DRM_MINOR_LIMIT(t) ({ typeof(t) _t = (t); XA_LIMIT(64 * _t, 64 * _t + 63); })
+#define DRM_LEGACY_MINOR_LIMIT(t) ({ typeof(t) _t = (t); XA_LIMIT(64 * _t, 64 * _t + 63); })
+#define DRM_MINOR_LIMIT XA_LIMIT(192, (1 << MINORBITS) - 1)
 
 static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
 {
@@ -120,7 +121,15 @@ static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
 	minor->type = type;
 	minor->dev = dev;
 
-	r = xa_alloc(&drm_minors_xa, &minor->index, NULL, DRM_MINOR_LIMIT(type), GFP_KERNEL);
+	/*
+	 * 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.
+	 */
+	r = xa_alloc(&drm_minors_xa, &minor->index, NULL, DRM_LEGACY_MINOR_LIMIT(type), GFP_KERNEL);
+	if (r == -EBUSY)
+		r = xa_alloc(&drm_minors_xa, &minor->index, NULL, DRM_MINOR_LIMIT, GFP_KERNEL);
 	if (r < 0)
 		return r;
 
-- 
2.37.3