On 25-08-2025 15:14, Jani Nikula wrote:
On Sat, 23 Aug 2025, "Murthy, Arun R"<arun.r.mur...@intel.com> wrote:
On 22-08-2025 16:07, Jani Nikula wrote:
On Fri, 22 Aug 2025, Arun R Murthy<arun.r.mur...@intel.com> wrote:
There can be multiple reasons for a failure in atomic_ioctl. Most often
in these error conditions -EINVAL is returned. User/Compositor would
have to blindly take a call on failure of this ioctl so as to use
ALLOW_MODESET or any. It would be good if user/compositor gets a
readable error code on failure so they can take proper corrections in
the next commit.
The struct drm_mode_atomic is being passed by the user/compositor which
holds the properties for modeset/flip. Reusing the same struct for
returning the error code in case of failure can save by creating a new
uapi/interface for returning the error code.
The element 'reserved' in the struct drm_mode_atomic is used for
returning the user readable error code. This points to the struct
drm_mode_atomic_err_code. Failure reasons have been initialized in
DRM_MODE_ATOMIC_FAILURE_REASON.
At the moment, I'm not (yet) convinced any of this will work, even as a
concept.
km_flip test in IGT has been run and is working fine with these patch
series. Also
the existing kms_atomic test with atomic_invalid_test has been modified to
test this error code(https://patchwork.freedesktop.org/series/153330/)
Am I missing anything over here?
Having a few tests in IGT does not mean a non-trivial userspace
(compositor) can use the information in a useful way.
Even so, I'll comment on some of the choices in the series.
Signed-off-by: Arun R Murthy<arun.r.mur...@intel.com>
---
include/uapi/drm/drm_mode.h | 42 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)
diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index
a122bea2559387576150236e3a88f99c24ad3138..f0986a3fe9a7d61e57e9a9a5ec01a604343f6930
100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -45,6 +45,7 @@ extern "C" {
#define DRM_CONNECTOR_NAME_LEN 32
#define DRM_DISPLAY_MODE_LEN 32
#define DRM_PROP_NAME_LEN 32
+#define DRM_MODE_ATOMIC_FAILURE_STRING_LEN 64
#define DRM_MODE_TYPE_BUILTIN (1<<0) /* deprecated */
#define DRM_MODE_TYPE_CLOCK_C ((1<<1) | DRM_MODE_TYPE_BUILTIN) /*
deprecated */
@@ -1157,6 +1158,47 @@ struct drm_mode_destroy_dumb {
DRM_MODE_ATOMIC_NONBLOCK |\
DRM_MODE_ATOMIC_ALLOW_MODESET)
+#define DRM_MODE_ATOMIC_FAILURE_REASON \
+ FAILURE_REASON(DRM_MODE_ATOMIC_CAP_NOT_ENABLED, "DRM_ATOMIC capability not
enabled") \
Please don't add macros that expect other macros in the context. They
become very confusing. Just pass in the other macro to use. See MACRO__
in include/drm/intel/pciids.h for an example.
Here we are mapping an error_code defined as enum with a corresponding
string
using the X macros.
Anyway will have a look at the macro in include/drm/intel/pciids.h
+ FAILURE_REASON(DRM_MODE_ATOMIC_INVALID_FLAG, "invalid flag") \
+ FAILURE_REASON(DRM_MODE_ATOMIC_PAGE_FLIP_ASYNC, "Legacy
DRM_MODE_PAGE_FLIP_ASYNC not to be used in atomic ioctl") \
+ FAILURE_REASON(DRM_MODE_ATOMIC_FLIP_EVENT_WITH_CHECKONLY, "requesting
page-flip event with TEST_ONLY") \
+ FAILURE_REASON(DRM_MODE_ATOMIC_CRTC_NEED_FULL_MODESET, "Need full modeset on
this crtc") \
+ FAILURE_REASON(DRM_MODE_ATOMIC_NEED_FULL_MODESET, "Need full modeset on all
the connected crtc's") \
+ FAILURE_REASON(DRM_MODE_ATOMIC_ASYNC_NOT_SUP_PLANE, "Async flip not
supported on this plane") \
+ FAILURE_REASON(DRM_MODE_ATOMIC_ASYNC_MODIFIER_NOT_SUPPORTED, "Modifier not
supported on this plane with async flip") \
+ FAILURE_REASON(DRM_MODE_ATOMIC_ASYNC_PROP_CHANGED, "No property change
allowed when async flip is enabled")
+
+#define FAILURE_REASON(flag, reason) flag,
+typedef enum {
+ DRM_MODE_ATOMIC_FAILURE_REASON
+} drm_mode_atomic_failure_flag;
+#undef FAILURE_REASON
This is a uapi header. Do we really want all of that macro trickery here
in the first place? What's wrong with just a plain enum? (Or macros.)
This is defined as enum. We have two separate list one for enum and
other for error message(string) but upon user adding a new
element/error_code in the enum, there can be a possibility of missing
adding the error string. So have linked enum with the corresponding
error message/string so both are at a single place and upon adding new
entry will be easy for the user.
I think the overall point is this:
a) If the messages are fixed for each enum, there is no point in passing
that message across the ioctl. Userspace can map the enums and
messages directly.
b) If the messages are not fixed, I see no point in having the above
fixed messages at all.
c) If the messages are not fixed, but you want to have default messages,
the messages do not belong in the uapi header at all. Hide them in
the kernel.
My intention was option C, wherein have a default message and the driver
is free
to overwrite the default message string if required.
I agree in that case doesn't make sense having it on uapi, will move this
inside the driver.
+
+#define FAILURE_REASON(flag, reason) #reason,
+extern const char *drm_mode_atomic_failure_string[];
+#undef FAILURE_REASON
Data is not an interface. Don't expose arrays like this. Who is even
going to know what its size is? Again, uapi header, where does it point,
what address space is it?
Sorry missed adding documentation for this. Will add in my next series.
Documentation does not fix the issue that data is not an interface.
As pointed out above will move to option C, and will get rid of this
failure messages
in the uapi.
+
+/**
+ * drm_mode_atomic_err_code - struct to store the error code
+ *
+ * pointer to this struct will be stored in reserved variable of
+ * struct drm_mode_atomic to report the failure cause to the user.
+ *
+ * @failure_flags: error codes defined in drm_atomic_failure.failure_flag
Flags? As in a mask of multiple things? Is 64 going to be enough for
everyone, all conditions in all drivers?
Should be more that sufficient, this is an enum.
Then don't call it "flags".
Done!
OTOH, what's the promise wrt multiple reasons? Practically all drivers
bail out at the sight of first issue, and it's usually pretty much
meaningless to continue to figure out *other* reasons after that.
And this brings us to one of my main concerns on making this fly:
You bail out on the first thing, but you can't expect all drivers to
check stuff in the same order. It's not practical. So different drivers
will likely return different reasons for virtually the same
condition. So how is userspace going to handle that?
Most of the error check is in the drm-core and these are common across
the drivers. Upon drivers sending driver specific error code, the
compositor may not be able to handle the error code. But in cases
required compositor changes can be added so as to handle them. In
general the generic error conditions that are met by compositor and
for which compositor can take corrective measurements are listed down
first.
I think we're going to need a handful of error codes for starters, along
with compositor changes that make meaningful and benefitial usage of the
error codes.
Sure will narrow down the list in my next version.
Thanks and Regards,
Arun R Murthy
-------------------
