Re: [net-next] net: dsa: felix: disable always guard band bit for TAS config

[Vladimir Oltean]

Hi Michael,

On Tue, May 04, 2021 at 07:05:14PM +0200, Michael Walle wrote:
> Hi,
> 
> > ALWAYS_GUARD_BAND_SCH_Q bit in TAS config register is descripted as
> > this:
> > 	0: Guard band is implemented for nonschedule queues to schedule
> > 	   queues transition.
> > 	1: Guard band is implemented for any queue to schedule queue
> > 	   transition.
> > 
> > The driver set guard band be implemented for any queue to schedule queue
> > transition before, which will make each GCL time slot reserve a guard
> > band time that can pass the max SDU frame. Because guard band time could
> > not be set in tc-taprio now, it will use about 12000ns to pass 1500B max
> > SDU. This limits each GCL time interval to be more than 12000ns.
> > 
> > This patch change the guard band to be only implemented for nonschedule
> > queues to schedule queues transition, so that there is no need to reserve
> > guard band on each GCL. Users can manually add guard band time for each
> > schedule queues in their configuration if they want.
> 
> 
> As explained in another mail in this thread, all queues are marked as
> scheduled. So this is actually a no-op, correct? It doesn't matter if
> it set or not set for now. Dunno why we even care for this bit then.

It matters because ALWAYS_GUARD_BAND_SCH_Q reduces the available throughput when set.

> > Signed-off-by: Xiaoliang Yang <xiaoliang.yang_1@xxxxxxx>
> > ---
> >  drivers/net/dsa/ocelot/felix_vsc9959.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/net/dsa/ocelot/felix_vsc9959.c b/drivers/net/dsa/ocelot/felix_vsc9959.c
> > index 789fe08cae50..2473bebe48e6 100644
> > --- a/drivers/net/dsa/ocelot/felix_vsc9959.c
> > +++ b/drivers/net/dsa/ocelot/felix_vsc9959.c
> > @@ -1227,8 +1227,12 @@ static int vsc9959_qos_port_tas_set(struct ocelot *ocelot, int port,
> >  	if (taprio->num_entries > VSC9959_TAS_GCL_ENTRY_MAX)
> >  		return -ERANGE;
> >  
> > -	ocelot_rmw(ocelot, QSYS_TAS_PARAM_CFG_CTRL_PORT_NUM(port) |
> > -		   QSYS_TAS_PARAM_CFG_CTRL_ALWAYS_GUARD_BAND_SCH_Q,
> > +	/* 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.
> > +	 */
> > +	ocelot_rmw(ocelot,
> > +		   QSYS_TAS_PARAM_CFG_CTRL_PORT_NUM(port),
> >  		   QSYS_TAS_PARAM_CFG_CTRL_PORT_NUM_M |
> >  		   QSYS_TAS_PARAM_CFG_CTRL_ALWAYS_GUARD_BAND_SCH_Q,
> >  		   QSYS_TAS_PARAM_CFG_CTRL);
> 
> Anyway, I don't think this the correct place for this:
>  (1) it isn't per port, but a global bit, but here its done per port.

I don't understand. According to the documentation, selecting the port
whose time-aware shaper you are configuring is done through
QSYS::TAS_PARAM_CFG_CTRL.PORT_NUM.

>  (2) rmw, I presume is read-modify-write. and there is one bit CONFIG_CHAGE
>      which is set by software and cleared by hardware. What happens if it
> 	 will be cleared right after we read it. Then it will be set again, no?
> 
> So if we really care about this bit, shouldn't this be moved to switch
> initialization then?

May I know what drew your attention to this patch? Is there something wrong?