An idea for extending uABI inspired by Vulkan's extension chains.
Instead of expanding the data struct for each ioctl every time we need
to add a new feature, define an extension chain instead. As we add
optional interfaces to control the ioctl, we define a new extension
struct that can be linked into the ioctl data only when required by the
user. The key advantage being able to ignore large control structs for
optional interfaces/extensions, while being able to process them in a
consistent manner.
In comparison to other extensible ioctls, the key difference is the
use of a linked chain of extension structs vs an array of tagged
pointers. For example,
struct drm_amdgpu_cs_chunk {
__u32 chunk_id;
__u32 length_dw;
__u64 chunk_data;
};
struct drm_amdgpu_cs_in {
__u32 ctx_id;
__u32 bo_list_handle;
__u32 num_chunks;
__u32 _pad;
__u64 chunks;
};
allows userspace to pass in array of pointers to extension structs, but
must therefore keep constructing that array along side the command stream.
In dynamic situations like that, a linked list is preferred and does not
similar from extra cache line misses as the extension structs themselves
must still be loaded separate to the chunks array.
v2: Apply the tail call optimisation directly to nip the worry of stack
overflow in the bud.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/Makefile | 1 +
drivers/gpu/drm/i915/i915_user_extensions.c | 37 +++++++++++++++++++++
drivers/gpu/drm/i915/i915_user_extensions.h | 20 +++++++++++
include/uapi/drm/i915_drm.h | 20 +++++++++++
4 files changed, 78 insertions(+)
create mode 100644 drivers/gpu/drm/i915/i915_user_extensions.c
create mode 100644 drivers/gpu/drm/i915/i915_user_extensions.h
diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
index 4654ef481b45..6ffeb60392f5 100644
--- a/drivers/gpu/drm/i915/Makefile
+++ b/drivers/gpu/drm/i915/Makefile
@@ -45,6 +45,7 @@ i915-y := i915_drv.o \
i915_syncmap.o \
i915_sw_fence.o \
i915_sysfs.o \
+ i915_user_extensions.o \
intel_csr.o \
intel_device_info.o \
intel_pm.o \
diff --git a/drivers/gpu/drm/i915/i915_user_extensions.c b/drivers/gpu/drm/i915/i915_user_extensions.c
new file mode 100644
index 000000000000..00622f8ac468
--- /dev/null
+++ b/drivers/gpu/drm/i915/i915_user_extensions.c
@@ -0,0 +1,37 @@
+/*
+ * SPDX-License-Identifier: MIT
+ *
+ * Copyright © 2018 Intel Corporation
+ */
+
+#include <linux/uaccess.h>
+#include <uapi/drm/i915_drm.h>
+
+#include "i915_user_extensions.h"
+
+int i915_user_extensions(struct i915_user_extension __user *ext,
+ const i915_user_extension_fn *tbl,
+ unsigned long count,
+ void *data)
+{
+ while (ext) {
+ int err;
+ u64 x;
+
+ if (get_user(x, &ext->name))
+ return -EFAULT;
+
+ err = -EINVAL;
+ if (x < count && tbl[x])
+ err = tbl[x](ext, data);
+ if (err)
+ return err;
+
+ if (get_user(x, &ext->next_extension))
+ return -EFAULT;
+
+ ext = u64_to_user_ptr(x);
+ }
+
+ return 0;
+}
diff --git a/drivers/gpu/drm/i915/i915_user_extensions.h b/drivers/gpu/drm/i915/i915_user_extensions.h
new file mode 100644
index 000000000000..313a510b068a
--- /dev/null
+++ b/drivers/gpu/drm/i915/i915_user_extensions.h
@@ -0,0 +1,20 @@
+/*
+ * SPDX-License-Identifier: MIT
+ *
+ * Copyright © 2018 Intel Corporation
+ */
+
+#ifndef I915_USER_EXTENSIONS_H
+#define I915_USER_EXTENSIONS_H
+
+struct i915_user_extension;
+
+typedef int (*i915_user_extension_fn)(struct i915_user_extension __user *ext,
+ void *data);
+
+int i915_user_extensions(struct i915_user_extension __user *ext,
+ const i915_user_extension_fn *tbl,
+ unsigned long count,
+ void *data);
+
+#endif /* I915_USER_EXTENSIONS_H */
diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index a4446f452040..4fa7e54501cc 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -62,6 +62,26 @@ extern "C" {
#define I915_ERROR_UEVENT "ERROR"
#define I915_RESET_UEVENT "RESET"
+/*
+ * i915_user_extension: Base class for defining a chain of extensions
+ *
+ * Many interfaces need to grow over time. In most cases we can simply
+ * extend the struct and have userspace pass in more data. Another option,
+ * as demonstrated by Vulkan's approach to providing extensions for forward
+ * and backward compatibilty, is to use a list of optional structs to
+ * provide those extra details.
+ *
+ * The key advantage to using an extension chain is that it allows us to
+ * redefine the interface more easily than an ever growing struct of
+ * increasing complexity, and for large parts of that interface to be
+ * entirely optional. The downside is more pointer chasing; chasing across
+ * the __user boundary with pointers encapsulated inside u64.
+ */
+struct i915_user_extension {
+ __u64 next_extension;
+ __u64 name;
+};
+
/*
* MOCS indexes used for GPU surfaces, defining the cacheability of the
* surface data and the coherency for this data wrt. CPU vs. GPU accesses.
--
2.19.0
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