drm/xe/pxp: Clarify PXP queue creation behavior if PXP is not ready

The expected flow of operations when using PXP is to query the PXP
status and wait for it to transition to "ready" before attempting to
create an exec_queue. This flow is followed by the Mesa driver, but
there is no guarantee that an incorrectly coded (or malicious) app
will not attempt to create the queue first without querying the status.
Therefore, we need to clarify what the expected behavior of the queue
creation ioctl is in this scenario.

Currently, the ioctl always fails with an -EBUSY code no matter the
error, but for consistency it is better to distinguish between "failed
to init" (-EIO) and "not ready" (-EBUSY), the same way the query ioctl
does. Note that, while this is a change in the return code of an ioctl,
the behavior of the ioctl in this particular corner case was not clearly
spec'd, so no one should have been relying on it (and we know that Mesa,
which is the only known userspace for this, didn't).

v2: Minor rework of the doc (Rodrigo)

Fixes: 72d4796 ("drm/xe/pxp/uapi: Add userspace and LRC support for PXP-using queues")
Signed-off-by: Daniele Ceraolo Spurio <[email protected]>
Cc: John Harrison <[email protected]>
Cc: José Roberto de Souza <[email protected]>
Reviewed-by: José Roberto de Souza <[email protected]>
Reviewed-by: John Harrison <[email protected]>
Acked-by: Rodrigo Vivi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
(cherry picked from commit 21784ca)
Signed-off-by: Thomas Hellström <[email protected]>

Author: dceraolo

drivers/gpu/drm/xe/xe_pxp.c

@@ -541,10 +541,14 @@ int xe_pxp_exec_queue_add(struct xe_pxp *pxp, struct xe_exec_queue *q)
 	 */
 	xe_pm_runtime_get(pxp->xe);
 
-	if (!pxp_prerequisites_done(pxp)) {
-		ret = -EBUSY;
+	/* get_readiness_status() returns 0 for in-progress and 1 for done */
+	ret = xe_pxp_get_readiness_status(pxp);
+	if (ret <= 0) {
+		if (!ret)
+			ret = -EBUSY;
 		goto out;
 	}
+	ret = 0;
 
 wait_for_idle:
 	/*

include/uapi/drm/xe_drm.h

@@ -1210,6 +1210,11 @@ struct drm_xe_vm_bind {
  *    there is no need to explicitly set that. When a queue of type
  *    %DRM_XE_PXP_TYPE_HWDRM is created, the PXP default HWDRM session
  *    (%XE_PXP_HWDRM_DEFAULT_SESSION) will be started, if isn't already running.
+ *    The user is expected to query the PXP status via the query ioctl (see
+ *    %DRM_XE_DEVICE_QUERY_PXP_STATUS) and to wait for PXP to be ready before
+ *    attempting to create a queue with this property. When a queue is created
+ *    before PXP is ready, the ioctl will return -EBUSY if init is still in
+ *    progress or -EIO if init failed.
  *    Given that going into a power-saving state kills PXP HWDRM sessions,
  *    runtime PM will be blocked while queues of this type are alive.
  *    All PXP queues will be killed if a PXP invalidation event occurs.