[PATCH] drm/vgem: Allow root to inject hanging fences onto dmabufs

[Chris Wilson]

When performing driver testing, one factor we want to test is how we
handle a foreign fence that is never signaled. We can wait on that fence
indefinitely, in which case the driver appears hung, or we can take some
remedial action (with risks regarding the state of any shared content).
Whatever the action choosen by the driver, in order to perform the test
we want to disable the safety feature of vgem fence (which is then used
to test implicit dma-buf fencing). This is regarded as a highly
dangerous feature and so hidden behind an expert config option and only
available to root when enabled.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/Kconfig           | 13 +++++++++++++
 drivers/gpu/drm/vgem/vgem_fence.c | 14 ++++++++++++--
 include/uapi/drm/vgem_drm.h       |  1 +
 3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index fc357319de35..2b25bc38fad2 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -228,6 +228,19 @@ config DRM_VGEM
 	  as used by Mesa's software renderer for enhanced performance.
 	  If M is selected the module will be called vgem.
 
+config DRM_VGEM_FENCE_HANG
+	bool "Enable fence hang testing (DANGEROUS)"
+	depends on DRM_VGEM
+	depends on EXPERT
+	depends on !COMPILE_TEST
+	default n
+	help
+	  Enables support for root to use indefinite fences.
+	  Do not enable this unless you are testing DRM drivers.
+
+	  Recommended for driver developers only.
+
+	  If in doubt, say "N".
 
 source "drivers/gpu/drm/exynos/Kconfig"
 
diff --git a/drivers/gpu/drm/vgem/vgem_fence.c b/drivers/gpu/drm/vgem/vgem_fence.c
index 5c57c1ffa1f9..91d3d75fc9c5 100644
--- a/drivers/gpu/drm/vgem/vgem_fence.c
+++ b/drivers/gpu/drm/vgem/vgem_fence.c
@@ -107,7 +107,8 @@ static struct fence *vgem_fence_create(struct vgem_file *vfile,
 	setup_timer(&fence->timer, vgem_fence_timeout, (unsigned long)fence);
 
 	/* We force the fence to expire within 10s to prevent driver hangs */
-	mod_timer(&fence->timer, jiffies + VGEM_FENCE_TIMEOUT);
+	if ((flags & VGEM_FENCE_NOTIMEOUT) == 0)
+		mod_timer(&fence->timer, jiffies + VGEM_FENCE_TIMEOUT);
 
 	return &fence->base;
 }
@@ -160,9 +161,18 @@ int vgem_fence_attach_ioctl(struct drm_device *dev,
 	struct fence *fence;
 	int ret;
 
-	if (arg->flags & ~VGEM_FENCE_WRITE)
+	if (arg->flags & ~(VGEM_FENCE_WRITE | VGEM_FENCE_NOTIMEOUT))
 		return -EINVAL;
 
+	if (arg->flags & VGEM_FENCE_NOTIMEOUT) {
+		if (config_enabled(CONFIG_DRM_VGEM_FENCE_HANG)) {
+			if (!capable(CAP_SYS_ADMIN))
+				return -EPERM;
+		} else {
+			return -EINVAL;
+		}
+	}
+
 	if (arg->pad)
 		return -EINVAL;
 
diff --git a/include/uapi/drm/vgem_drm.h b/include/uapi/drm/vgem_drm.h
index bf66f5db6da8..55fd08750773 100644
--- a/include/uapi/drm/vgem_drm.h
+++ b/include/uapi/drm/vgem_drm.h
@@ -46,6 +46,7 @@ struct drm_vgem_fence_attach {
 	__u32 handle;
 	__u32 flags;
 #define VGEM_FENCE_WRITE	0x1
+#define VGEM_FENCE_NOTIMEOUT	0x2
 	__u32 out_fence;
 	__u32 pad;
 };
-- 
2.8.1