netdev - Re: [PATCH net-next v3 6/8] igc: Add support for tuning frame preemption via ethtool

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Date:   Fri, 29 Jan 2021 13:27:28 -0800
From:   Vinicius Costa Gomes <vinicius.gomes@...el.com>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jhs@...atatu.com" <jhs@...atatu.com>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "Jose.Abreu@...opsys.com" <Jose.Abreu@...opsys.com>,
        Po Liu <po.liu@....com>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "anthony.l.nguyen@...el.com" <anthony.l.nguyen@...el.com>,
        "mkubecek@...e.cz" <mkubecek@...e.cz>
Subject: Re: [PATCH net-next v3 6/8] igc: Add support for tuning frame
 preemption via ethtool

Vladimir Oltean <vladimir.oltean@....com> writes:

> On Fri, Jan 22, 2021 at 02:44:51PM -0800, Vinicius Costa Gomes wrote:
>> The tc subsystem sets which queues are marked as preemptible, it's the
>> role of ethtool to control more hardware specific parameters. These
>> parameters include:
>> 
>>  - enabling the frame preemption hardware: As enabling frame
>>  preemption may have other requirements before it can be enabled, it's
>>  exposed via the ethtool API;
>> 
>>  - mininum fragment size multiplier: expressed in usually in the form
>>  of (1 + N)*64, this number indicates what's the size of the minimum
>>  fragment that can be preempted.
>
> And not one word has been said about the patch...

If I am undertanding this right. Will fix the commit message.

>
>> 
>> Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@...el.com>
>> ---
>>  drivers/net/ethernet/intel/igc/igc.h         | 12 +++++
>>  drivers/net/ethernet/intel/igc/igc_defines.h |  4 ++
>>  drivers/net/ethernet/intel/igc/igc_ethtool.c | 53 ++++++++++++++++++++
>>  drivers/net/ethernet/intel/igc/igc_tsn.c     | 25 +++++++--
>>  4 files changed, 91 insertions(+), 3 deletions(-)
>> 
>> diff --git a/drivers/net/ethernet/intel/igc/igc.h b/drivers/net/ethernet/intel/igc/igc.h
>> index 35baae900c1f..1067c46e0bc2 100644
>> --- a/drivers/net/ethernet/intel/igc/igc.h
>> +++ b/drivers/net/ethernet/intel/igc/igc.h
>> @@ -87,6 +87,7 @@ struct igc_ring {
>>  	u8 queue_index;                 /* logical index of the ring*/
>>  	u8 reg_idx;                     /* physical index of the ring */
>>  	bool launchtime_enable;         /* true if LaunchTime is enabled */
>> +	bool preemptible;		/* true if not express */
>
> Mixing tabs and spaces?

Ugh. Will fix. Thanks.

>
>> +static int igc_ethtool_set_preempt(struct net_device *netdev,
>> +				   struct ethtool_fp *fpcmd,
>> +				   struct netlink_ext_ack *extack)
>> +{
>> +	struct igc_adapter *adapter = netdev_priv(netdev);
>> +	int i;
>> +
>> +	if (fpcmd->add_frag_size < 68 || fpcmd->add_frag_size > 260) {
>> +		NL_SET_ERR_MSG_MOD(extack, "Invalid value for add-frag-size");
>> +		return -EINVAL;
>> +	}
>
> This check should belong in ethtool, since there's nothing unusual about
> this supported range.
>
> Also, I believe that Jakub requested the min-frag-size to be passed as
> 0, 1, 2, 3 as the standard specifies it, and not its multiplied
> version?

Later, Michal Kubechek suggested using the multiplied value, to be
future proof and less dependent on some specific standard version.

>
>> +
>> +	adapter->frame_preemption_active = fpcmd->enabled;
>> +	adapter->add_frag_size = fpcmd->add_frag_size;
>> +
>> +	if (!adapter->frame_preemption_active)
>> +		goto done;
>> +
>> +	/* Enabling frame preemption requires TSN mode to be enabled,
>> +	 * which requires a schedule to be active. So, if there isn't
>> +	 * a schedule already configured, configure a simple one, with
>> +	 * all queues open, with 1ms cycle time.
>> +	 */
>> +	if (adapter->base_time)
>> +		goto done;
>
> Unless I'm missing something, you are interpreting an adapter->base_time
> value of zero as "no Qbv schedule on port", as if it was invalid to have
> a base-time of zero, which it isn't.

This HW has specific limitations, it doesn't allow a base_time in the
past. So a base_time of zero can be used to signify "No Qbv".

>
>> @@ -115,6 +130,9 @@ static int igc_tsn_enable_offload(struct igc_adapter *adapter)
>>  		if (ring->launchtime_enable)
>>  			txqctl |= IGC_TXQCTL_QUEUE_MODE_LAUNCHT;
>>  
>> +		if (ring->preemptible)
>> +			txqctl |= IGC_TXQCTL_PREEMPTABLE;
>
> I think this is the only place in the series where you use PREEMPTABLE
> instead of PREEMPTIBLE.

Yeah, on the datasheet it's written PREEMPTABLE, I chose to use this
spelling to make it easier to search for this bit in the datasheet.

>
>> +
>>  		wr32(IGC_TXQCTL(i), txqctl);
>>  	}
>
> Out of curiosity, where is the ring to traffic class mapping configured
> in the igc driver? I suppose that you have more rings than traffic classes.

The driver follows the default behaviour, that netdev->queue[0] maps to
ring[0], queue[1] to ring[1], and so on. And by default ring[0] has
higher priority than ring[1], ring[1] higher than ring[2], and so on.

The HW only has 4 rings/queues.


Cheers,
-- 
Vinicius