[PATCH] drm/doc: Document ioctl errno value patterns

[Daniel Vetter]

We're not super-consistent about these, but I think it's worth to
document at least the commmon patterns.

Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: "Zhang, Tina" <tina.zhang at intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 Documentation/gpu/drm-uapi.rst | 53 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
index 679373b4a03f..f3cc829467b4 100644
--- a/Documentation/gpu/drm-uapi.rst
+++ b/Documentation/gpu/drm-uapi.rst
@@ -177,6 +177,59 @@ IOCTL Support on Device Nodes
 .. kernel-doc:: drivers/gpu/drm/drm_ioc32.c
    :export:
 
+Recommended IOCTL Return Values
+===============================
+
+In theory a driver's IOCTL callback is only allowed to return very few error
+codes. In practice it's good to abuse a few more. This section documents common
+practice within the DRM subsystem:
+
+ENOENT:
+        Strictly speaking only when you try to open isn't there. We reuse that
+        to signal any kind of object lookup failure, e.g. for unknown GEM buffer
+        object handles, unknown KMS object handles and similar cases.
+
+ENOSPC:
+        Some drivers use this to differiante "out of kernel memory" from "out
+        of VRAM". Sometimes also applies to other limited gpu resources used for
+        rendering (e.g. when you have a special limited compression buffer).
+        Sometimes resource allocation/reservation issues in command submission
+        IOCTLs are also signalled through EDEADLK.
+
+        Simply running out of kernel/system memory is signalled through ENOMEM.
+
+EPERM/EACCESS:
+        Returned for an operation that is valid, but needs more priviledges.
+        E.g. root-only or much more common, DRM master-only operations return
+        this when when called by unpriviledges clients. There's no clear
+        difference between EACCESS and EPERM.
+
+ENODEV:
+        Feature (like PRIME, modesetting, GEM) is not supported by the driver.
+
+ENXIO:
+        Remote failure, either a hardware transaction (like i2c), but also used
+        when the exporting driver of a shared dma-buf or fence doesn't support a
+        feature needed.
+
+EINTR:
+        DRM drivers assume that userspace restarts all IOCTLs. Any DRM IOCTL can
+        return EINTR and in such a case should be restarted with the IOCTL
+        parameters left unchanged.
+
+EIO:
+        The GPU died and couldn't be resurrected through a reset. Modesetting
+        hardware failures are signalled through the "link status" connector
+        property.
+
+EINVAL:
+        Catch-all for anything that is an invalid argument combination which
+        cannot work.
+
+IOCTL also use other error codes like ETIME, EFAULT, EBUSY, but their usage is
+in line with the common meanings. The above list tries to just document DRM
+specific patterns.
+
 Testing and validation
 ======================
 
-- 
2.13.3