[PATCH] ethdev: prohibit polling of a stopped queue

[Dmitry Kozlyuk <dkozlyuk@nvidia.com>]

Whether it is allowed to call Rx/Tx functions for a stopped queue
was undocumented. Some PMDs make this behavior a no-op
either by explicitly checking the queue state
or by the way how their routines are implemented or HW works.

No-op behavior may be convenient for application developers.
But it also means that pollers of stopped queues
would go all the way down to PMD Rx/Tx routines, wasting cycles.
Some PMDs would do a check for the queue state on data path,
even though it may never be needed for a particular application.
Also, use cases for stopping queues or starting them deferred
do not logically require polling stopped queues.

Use case 1: a secondary that was polling the queue has crashed,
the primary is doing a recovery to free all mbufs.
By definition the queue to be restarted is not polled.

Use case 2: deferred queue start or queue reconfiguration.
The polling thread must be synchronized anyway,
because queue start and stop are non-atomic.

Prohibit calling Rx/Tx functions on stopped queues.

Fixes: 0748be2cf9a2 ("ethdev: queue start and stop")
Cc: stable@dpdk.org

Signed-off-by: Dmitry Kozlyuk <dkozlyuk@nvidia.com>
---
This patch is was originally a part of the series:
http://patchwork.dpdk.org/project/dpdk/patch/20220307125351.697936-3-dkozlyuk@nvidia.com/
The discussion there is summarized in the commit message.

 lib/ethdev/rte_ethdev.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/ethdev/rte_ethdev.h b/lib/ethdev/rte_ethdev.h
index 04cff8ee10..435720a84e 100644
--- a/lib/ethdev/rte_ethdev.h
+++ b/lib/ethdev/rte_ethdev.h
@@ -74,7 +74,7 @@
  * rte_eth_rx_queue_setup()), it must call rte_eth_dev_stop() first to stop the
  * device and then do the reconfiguration before calling rte_eth_dev_start()
  * again. The transmit and receive functions should not be invoked when the
- * device is stopped.
+ * device is stopped or when the queue is stopped (for that queue).
  *
  * Please note that some configuration is not stored between calls to
  * rte_eth_dev_stop()/rte_eth_dev_start(). The following configuration will
-- 
2.25.1