rpmsg: glink: Handle rejected intent request better

GLINK operates using pre-allocated buffers, aka intents, where incoming
messages are aggregated before being passed up the stack. In the case
that no suitable intents have been announced by the receiver, the sender
can request an intent to be allocated.

The initial implementation of the response to such request dealt
with two outcomes; granted allocations, and all other cases being
considered -ECANCELLED (likely from "cancelling the operation as the
remote is going down").

But on some channels intent allocation is not supported, instead the
remote will pre-allocate and announce a fixed number of intents for the
sender to use. If for such channels an rpmsg_send() is being invoked
before any channels have been announced, an intent request will be
issued and as this comes back rejected the call fails with -ECANCELED.

Given that this is reported in the same way as the remote being shut
down, there's no way for the client to differentiate the two cases.

In line with the original GLINK design, change the return value to
-EAGAIN for the case where the remote rejects an intent allocation
request.

It's tempting to handle this case in the GLINK core, as we expect
intents to show up in this case. But there's no way to distinguish
between this case and a rejection for a too big allocation, nor is it
possible to predict if a currently used (and seemingly suitable) intent
will be returned for reuse or not. As such, returning the error to the
client and allow it to react seems to be the only sensible solution.

In addition to this, commit 'c05dfce0b89e ("rpmsg: glink: Wait for
intent, not just request ack")' changed the logic such that the code
always wait for an intent request response and an intent. This works out
in most cases, but in the event that an intent request is rejected and no
further intent arrives (e.g. client asks for a too big intent), the code
will stall for 10 seconds and then return -ETIMEDOUT; instead of a more
suitable error.

This change also resulted in intent requests racing with the shutdown of
the remote would be exposed to this same problem, unless some intent
happens to arrive. A patch for this was developed and posted by Sarannya
S [1], and has been incorporated here.

To summarize, the intent request can end in 4 ways:
- Timeout, no response arrived => return -ETIMEDOUT
- Abort TX, the edge is going away => return -ECANCELLED
- Intent request was rejected => return -EAGAIN
- Intent request was accepted, and an intent arrived => return 0

This patch was developed with input from Sarannya S, Deepak Kumar Singh,
and Chris Lew.

[1] https://lore.kernel.org/all/[email protected]/

Fixes: c05dfce ("rpmsg: glink: Wait for intent, not just request ack")
Cc: [email protected]
Tested-by: Johan Hovold <[email protected]>
Signed-off-by: Bjorn Andersson <[email protected]>
Reviewed-by: Chris Lew <[email protected]>
Link: https://lore.kernel.org/r/20241023-pmic-glink-ecancelled-v2-1-ebc268129407@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <[email protected]>

Author: quic-bjorande

drivers/rpmsg/qcom_glink_native.c

@@ -1440,14 +1440,18 @@ static int qcom_glink_request_intent(struct qcom_glink *glink,
 		goto unlock;
 
 	ret = wait_event_timeout(channel->intent_req_wq,
-				 READ_ONCE(channel->intent_req_result) >= 0 &&
-				 READ_ONCE(channel->intent_received),
+				 READ_ONCE(channel->intent_req_result) == 0 ||
+				 (READ_ONCE(channel->intent_req_result) > 0 &&
+				  READ_ONCE(channel->intent_received)) ||
+				 glink->abort_tx,
 				 10 * HZ);
 	if (!ret) {
 		dev_err(glink->dev, "intent request timed out\n");
 		ret = -ETIMEDOUT;
+	} else if (glink->abort_tx) {
+		ret = -ECANCELED;
 	} else {
-		ret = READ_ONCE(channel->intent_req_result) ? 0 : -ECANCELED;
+		ret = READ_ONCE(channel->intent_req_result) ? 0 : -EAGAIN;
 	}
 
 unlock: