On 19/09/2018 20:55, Chris Wilson wrote:
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;
};
Since verbose example of what this is better from, or at least different
than, I think an example of itself in the commit message would be preferred.
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
s/similar/suffer/ ?
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)
Should data be called base or something like that? To signify it is not
per-extension but points back to the base ioctl?
+{
+ while (ext) {
+ int err;
+ u64 x;
s/x/name/ or id?
+
+ if (get_user(x, &ext->name))
+ return -EFAULT;
+
+ err = -EINVAL;
+ if (x < count && tbl[x])
+ err = tbl[x](ext, data);
+ if (err)
+ return err;
Stopping on unknown extension or error, hm.. probably makes sense given
the context of extension use cases. Which is not like for instance the
list of query ioctls, but something really specific to each individual
base ioctl.
+
+ 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;
next_extension_ptr ?
+ __u64 name;
Would id be better instead of name? Name is too much a string to me.
Regards,
Tvrtko
+};
+
/*
* MOCS indexes used for GPU surfaces, defining the cacheability of the
* surface data and the coherency for this data wrt. CPU vs. GPU accesses.
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