[PATCH 1/2] drm: Add DRM_CAP_PRIME_SCANOUT.

[raof@ubuntu.com]

From: Christopher James Halse Rogers <raof@ubuntu.com>

Until recently, on (at least) nouveau, radeon, and amdgpu attempting to scanout of an
imported dma-buf would silently result in the dma-buf sharing being broken.

While the hardware is capable of scanning out of imported dma-bufs (at least in some circumstances),
these drivers do not currently implement it, so attempts to scan out of such buffers will never succeed.

Add a userspace-visible drm capability and associated driver_feature so that userspace can discover
when scanning out of an imported dma-buf can work.

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
---
 drivers/gpu/drm/drm_ioctl.c |  3 +++
 include/drm/drm_drv.h       |  1 +
 include/uapi/drm/drm.h      | 21 +++++++++++++++++++++
 3 files changed, 25 insertions(+)

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index a7c61c23685a..79ccf638668e 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -285,6 +285,9 @@ static int drm_getcap(struct drm_device *dev, void *data, struct drm_file *file_
 	case DRM_CAP_ADDFB2_MODIFIERS:
 		req->value = dev->mode_config.allow_fb_modifiers;
 		break;
+	case DRM_CAP_PRIME_SCANOUT:
+		req->value = drm_core_check_feature(dev, DRIVER_PRIME_SCANOUT);
+		break;
 	default:
 		return -EINVAL;
 	}
diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
index 5699f42195fe..32cc0d956d7e 100644
--- a/include/drm/drm_drv.h
+++ b/include/drm/drm_drv.h
@@ -53,6 +53,7 @@ struct drm_mode_create_dumb;
 #define DRIVER_RENDER			0x8000
 #define DRIVER_ATOMIC			0x10000
 #define DRIVER_KMS_LEGACY_CONTEXT	0x20000
+#define DRIVER_PRIME_SCANOUT		0x40000
 
 /**
  * struct drm_driver - DRM driver structure
diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h
index b2c52843bc70..c57213cdb702 100644
--- a/include/uapi/drm/drm.h
+++ b/include/uapi/drm/drm.h
@@ -647,6 +647,27 @@ struct drm_gem_open {
 #define DRM_CAP_CURSOR_HEIGHT		0x9
 #define DRM_CAP_ADDFB2_MODIFIERS	0x10
 #define DRM_CAP_PAGE_FLIP_TARGET	0x11
+/**
+ * DRM_CAP_PRIME_SCANOUT
+ *
+ * The PRIME_SCANOUT capability returns whether the driver might be capable
+ * of scanning out of an imported PRIME buffer.
+ *
+ * This does not guarantee that any imported buffer can be scanned out, as
+ * there may be specific requirements that a given buffer might not satisfy.
+ *
+ * If this capability is false then imported PRIME buffers will definitely
+ * never be suitable for scanout.
+ *
+ * Note: Prior to the introduction of this capability, bugs in drivers would
+ * allow userspace to attempt to scan out of imported PRIME buffers. This would
+ * work once, but move the buffer into an inconsistent state where rendering from
+ * the exporting GPU would no longer be seen by the importing GPU.
+ *
+ * It is unsafe for driver-agnostic code to attempt to scan out of an imported
+ * PRIME buffer in the absense of this capability.
+ */
+#define DRM_CAP_PRIME_SCANOUT		0x12
 
 /** DRM_IOCTL_GET_CAP ioctl argument type */
 struct drm_get_cap {
-- 
2.11.0

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