check_cmd() is checking whether a command adheres to certain restrictions that ensure it's safe to execute within a privileged batch buffer. Returning false implies a privilege problem, not that the command is invalid.
The distinction makes the difference between allowing the buffer to be executed as an unprivileged batch buffer or returning an EINVAL error to userspace without executing anything.
In a case where userspace may want to test whether it can successfully write to a register that needs privileges the distinction may be important and an EINVAL error may be considered fatal.
In particular this is currently true for Mesa, which includes a test for whether OACONTROL can be written too, but Mesa treats any error when flushing a batch buffer as fatal, calling exit(1).
As it is currently Mesa can gracefully handle a failure to write to OACONTROL if the command parser is disabled, but if we were to remove OACONTROL from the parser's whitelist then the returned EINVAL would break Mesa applications as they attempt an OACONTROL write.
Signed-off-by: Robert Bragg robert@sixbynine.org --- drivers/gpu/drm/i915/i915_cmd_parser.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/i915/i915_cmd_parser.c b/drivers/gpu/drm/i915/i915_cmd_parser.c index 7269fe8..5ad02dc 100644 --- a/drivers/gpu/drm/i915/i915_cmd_parser.c +++ b/drivers/gpu/drm/i915/i915_cmd_parser.c @@ -1272,7 +1272,7 @@ int intel_engine_cmd_parser(struct intel_engine_cs *engine,
if (!check_cmd(engine, desc, cmd, length, is_master, &oacontrol_set)) { - ret = -EINVAL; + ret = -EACCES; break; }