Commit 316bcffe4479 ("net: dsa: felix: disable always guard band bit for
TAS config") disabled the guard band and broke 802.3Qbv compliance.
There are two issues here:
(1) Without the guard band the end of the scheduling window could be
overrun by a frame in transit.
(2) Frames that don't fit into a configured window will still be sent.
The reason for both issues is that the switch will schedule the _start_
of a frame transmission inside the predefined window without taking the
length of the frame into account. Thus, we'll need the guard band which
will close the gate early, so that a complete frame can still be sent.
Revert the commit and add a note.
For a lengthy discussion see [1].
[1] https://lore.kernel.org/netdev/[email protected]/
Fixes: 316bcffe4479 ("net: dsa: felix: disable always guard band bit for TAS config")
Signed-off-by: Michael Walle <[email protected]>
---
drivers/net/dsa/ocelot/felix_vsc9959.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)
diff --git a/drivers/net/dsa/ocelot/felix_vsc9959.c b/drivers/net/dsa/ocelot/felix_vsc9959.c
index 2473bebe48e6..f966a253d1c7 100644
--- a/drivers/net/dsa/ocelot/felix_vsc9959.c
+++ b/drivers/net/dsa/ocelot/felix_vsc9959.c
@@ -1227,12 +1227,17 @@ static int vsc9959_qos_port_tas_set(struct ocelot *ocelot, int port,
if (taprio->num_entries > VSC9959_TAS_GCL_ENTRY_MAX)
return -ERANGE;
- /* Set port num and disable ALWAYS_GUARD_BAND_SCH_Q, which means set
- * guard band to be implemented for nonschedule queues to schedule
- * queues transition.
+ /* Enable guard band. The switch will schedule frames without taking
+ * their length into account. Thus we'll always need to enable the
+ * guard band which reserves the time of a maximum sized frame at the
+ * end of the time window.
+ *
+ * Although the ALWAYS_GUARD_BAND_SCH_Q bit is global for all ports, we
+ * need to set PORT_NUM, because subsequent writes to PARAM_CFG_REG_n
+ * operate on the port number.
*/
- ocelot_rmw(ocelot,
- QSYS_TAS_PARAM_CFG_CTRL_PORT_NUM(port),
+ ocelot_rmw(ocelot, QSYS_TAS_PARAM_CFG_CTRL_PORT_NUM(port) |
+ QSYS_TAS_PARAM_CFG_CTRL_ALWAYS_GUARD_BAND_SCH_Q,
QSYS_TAS_PARAM_CFG_CTRL_PORT_NUM_M |
QSYS_TAS_PARAM_CFG_CTRL_ALWAYS_GUARD_BAND_SCH_Q,
QSYS_TAS_PARAM_CFG_CTRL);
--
2.20.1
On Mon, May 10, 2021 at 01:07:08PM +0200, Michael Walle wrote:
> Commit 316bcffe4479 ("net: dsa: felix: disable always guard band bit for
> TAS config") disabled the guard band and broke 802.3Qbv compliance.
>
> There are two issues here:
> (1) Without the guard band the end of the scheduling window could be
> overrun by a frame in transit.
> (2) Frames that don't fit into a configured window will still be sent.
>
> The reason for both issues is that the switch will schedule the _start_
> of a frame transmission inside the predefined window without taking the
> length of the frame into account. Thus, we'll need the guard band which
> will close the gate early, so that a complete frame can still be sent.
> Revert the commit and add a note.
>
> For a lengthy discussion see [1].
>
> [1] https://lore.kernel.org/netdev/[email protected]/
>
> Fixes: 316bcffe4479 ("net: dsa: felix: disable always guard band bit for TAS config")
> Signed-off-by: Michael Walle <[email protected]>
> ---
Reviewed-by: Vladimir Oltean <[email protected]>
Hello:
This patch was applied to netdev/net.git (refs/heads/master):
On Mon, 10 May 2021 13:07:08 +0200 you wrote:
> Commit 316bcffe4479 ("net: dsa: felix: disable always guard band bit for
> TAS config") disabled the guard band and broke 802.3Qbv compliance.
>
> There are two issues here:
> (1) Without the guard band the end of the scheduling window could be
> overrun by a frame in transit.
> (2) Frames that don't fit into a configured window will still be sent.
>
> [...]
Here is the summary with links:
- [net] net: dsa: felix: re-enable TAS guard band mode
https://git.kernel.org/netdev/net/c/297c4de6f780
You are awesome, thank you!
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