Date:   Tue, 14 Feb 2023 09:34:09 +0000
From:   Wei Fang <wei.fang@....com>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
CC:     Shenwei Wang <shenwei.wang@....com>,
        Clark Wang <xiaoning.wang@....com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "simon.horman@...igine.com" <simon.horman@...igine.com>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH V2 net-next] net: fec: add CBS offload support

> -----Original Message-----
> From: Alexander Lobakin <alexandr.lobakin@...el.com>
> Sent: 2023年2月14日 0:05
> To: Wei Fang <wei.fang@....com>
> Cc: Shenwei Wang <shenwei.wang@....com>; Clark Wang
> <xiaoning.wang@....com>; davem@...emloft.net; edumazet@...gle.com;
> kuba@...nel.org; pabeni@...hat.com; simon.horman@...igine.com;
> andrew@...n.ch; netdev@...r.kernel.org; dl-linux-imx <linux-imx@....com>;
> linux-kernel@...r.kernel.org
> Subject: Re: [PATCH V2 net-next] net: fec: add CBS offload support
> 
> From: Wei Fang <wei.fang@....com>
> Date: Mon, 13 Feb 2023 17:29:12 +0800
> 
> > From: Wei Fang <wei.fang@....com>
> >
> > The FEC hardware supports the Credit-based shaper (CBS) which control
> > the bandwidth distribution between normal traffic and time-sensitive
> > traffic with respect to the total link bandwidth available.
> > But notice that the bandwidth allocation of hardware is restricted to
> > certain values. Below is the equation which is used to calculate the
> > BW (bandwidth) fraction for per class:
> > 	BW fraction = 1 / (1 + 512 / idle_slope)
> 
> [...]
> 
> > @@ -571,6 +575,12 @@ struct fec_stop_mode_gpr {
> >  	u8 bit;
> >  };
> >
> > +struct fec_cbs_params {
> > +	bool enable[FEC_ENET_MAX_TX_QS];
> 
> Maybe smth like
> 
> 	DECLARE_BITMAP(enabled, FEC_ENET_MAX_TX_QS);
> 
> ?
I think it's a matter of personal habit.

> 
> > +	int idleslope[FEC_ENET_MAX_TX_QS];
> > +	int sendslope[FEC_ENET_MAX_TX_QS];
> 
> Can they actually be negative? (probably I'll see it below)
> 
idleslope and sendslope are used to save the parameters passed in from user space.
And the sendslope should always be negative.

> > +};
> > +
> >  /* The FEC buffer descriptors track the ring buffers.  The rx_bd_base and
> >   * tx_bd_base always point to the base of the buffer descriptors.  The
> >   * cur_rx and cur_tx point to the currently available buffer.
> > @@ -679,6 +689,9 @@ struct fec_enet_private {
> >  	/* XDP BPF Program */
> >  	struct bpf_prog *xdp_prog;
> >
> > +	/* CBS parameters */
> > +	struct fec_cbs_params cbs;
> > +
> >  	u64 ethtool_stats[];
> >  };
> >
> > diff --git a/drivers/net/ethernet/freescale/fec_main.c
> > b/drivers/net/ethernet/freescale/fec_main.c
> > index c73e25f8995e..91394ad05121 100644
> > --- a/drivers/net/ethernet/freescale/fec_main.c
> > +++ b/drivers/net/ethernet/freescale/fec_main.c
> > @@ -66,6 +66,7 @@
> >  #include <linux/mfd/syscon.h>
> >  #include <linux/regmap.h>
> >  #include <soc/imx/cpuidle.h>
> > +#include <net/pkt_sched.h>
> 
> Some alphabetic order? At least for new files :D
> 
I just want to keep the reverse Christmas tree style, although the whole #include
order is already out of the style.

> >  #include <linux/filter.h>
> >  #include <linux/bpf.h>
> >
> > @@ -1023,6 +1024,174 @@ static void fec_enet_reset_skb(struct
> net_device *ndev)
> >  	}
> >  }
> >
> > +static u32 fec_enet_get_idle_slope(u8 bw)
> 
> Just use `u32`, usually there's no reason to use types shorter than integer on
> the stack.
> 
> > +{
> > +	int msb, power;
> > +	u32 idle_slope;
> > +
> > +	if (bw >= 100)
> > +		return 0;
> > +
> > +	/* Convert bw to hardware idle slope */
> > +	idle_slope = (512 * bw) / (100 - bw);
> > +
> 
> Redundant newline. Also first pair of braces is optional and up to you.
> 
I will fix this nit, thanks!

> > +	if (idle_slope >= 128) {
> > +		/* For values greater than or equal to 128, idle_slope
> > +		 * rounded to the nearest multiple of 128.
> > +		 */
> 
> But you can just do
> 
> 	idle_slope = min(idle_slope, 128U);
> 
> and still use the "standard" path below, without the conditional return?
> Or even combine it with the code above:
> 
> 	idle_slope = min((512 * bw) / (100 - bw), 128U);
> 
If idles_slope is greater than or equal to 128, idle_slope should be rounded to the nearest
multiple of 128. For example, if idle_slope = 255, it should be set to 256. However 
min(idle_slope, 128U) is not as expected.

> > +		idle_slope = DIV_ROUND_CLOSEST(idle_slope, 128U) * 128U;
> > +
> > +		return idle_slope;
> > +	}
> > +
> > +	/* For values less than 128, idle_slope is rounded to
> > +	 * nearst power of 2.
> 
> Typo, 'nearest'.
> 
Yes, I'll fix it.

> > +	 */
> > +	if (idle_slope <= 1)
> > +		return 1;
> > +
> > +	msb = __fls(idle_slope);
> 
> I think `fls() - 1` is preferred over `__fls()` since it may go UB. And some checks
> wouldn't hurt.
> 
I used fls() at first, but later changed to __fls. Now that you pointed out its possible
risks, I'll change it back. Thanks.

> > +	power = BIT(msb);
> 
> Oh okay. Then rounddown_pow_of_two() is what you're looking for.
> 
> 	power = rounddown_pow_of_two(idle_slope);
> 
> Or even just use one variable, @idle_slope.
> 
Thanks for the reminder, I think I should use roundup_pow_of_two().

> > +	idle_slope = DIV_ROUND_CLOSEST(idle_slope, power) * power;
> > +
> > +	return idle_slope;
> 
> You can return DIV_ROUND_ ... right away, without assignning first.
> Also, I'm thinking of that this might be a generic helper. We have
> roundup() and rounddown(), this could be something like "round_closest()"?
> 
> > +}
> > +
> > +static void fec_enet_set_cbs_idle_slope(struct fec_enet_private *fep)
> > +{
> > +	u32 bw, val, idle_slope;
> > +	int speed = fep->speed;
> > +	int idle_slope_sum = 0;
> > +	int i;
> 
> Can any of them be negative?
> 
No.

> > +
> > +	if (!speed)
> > +		return;
> > +
> > +	for (i = 1; i < FEC_ENET_MAX_TX_QS; i++) {
> 
> So you don't use the zeroth array elements at all? Why having them then?
> 
Yes, the zeroth indicates queue 0, and queue 0 only support Non-AVB traffic.
Why having them then?
Firstly I think it more clear that the i indicates the index of queue. Secondly,
it is for more convenience. If you think it is inappropriate, I will amend it
later.

> > +		int port_tx_rate;
> 
> (same for type)
> 
> > +
> > +		/* As defined in IEEE 802.1Q-2014 Section 8.6.8.2 item:
> > +		 *       sendslope = idleslope -  port_tx_rate
> > +		 * So we need to check whether port_tx_rate is equal to
> > +		 * the current link rate.
> 
> [...]
> 
> > +	for (i = 1; i < FEC_ENET_MAX_TX_QS; i++) {
> > +		bw = fep->cbs.idleslope[i] / (speed * 10);
> > +		idle_slope = fec_enet_get_idle_slope(bw);
> > +
> > +		val = readl(fep->hwp + FEC_DMA_CFG(i));
> > +		val &= ~IDLE_SLOPE_MASK;
> > +		val |= idle_slope & IDLE_SLOPE_MASK;
> 
> u32_replace_bits() will do it for you.
> 
Sorry, I can't find the definition of u32_replace_bits().

> > +		writel(val, fep->hwp + FEC_DMA_CFG(i));
> > +	}
> > +
> > +	/* Enable Credit-based shaper. */
> > +	val = readl(fep->hwp + FEC_QOS_SCHEME);
> > +	val &= ~FEC_QOS_TX_SHEME_MASK;
> > +	val |= CREDIT_BASED_SCHEME;
> 
> (same)
> 
> > +	writel(val, fep->hwp + FEC_QOS_SCHEME); }
> > +
> > +static int fec_enet_setup_tc_cbs(struct net_device *ndev, void
> > +*type_data) {
> > +	struct fec_enet_private *fep = netdev_priv(ndev);
> > +	struct tc_cbs_qopt_offload *cbs = type_data;
> > +	int queue = cbs->queue;
> > +	int speed = fep->speed;
> > +	int queue2;
> 
> (types)
> 
> > +
> > +	if (!(fep->quirks & FEC_QUIRK_HAS_AVB))
> > +		return -EOPNOTSUPP;
> > +
> > +	/* Queue 1 for Class A, Queue 2 for Class B, so the ENET must
> > +	 * have three queues.
> > +	 */
> > +	if (fep->num_tx_queues != FEC_ENET_MAX_TX_QS)
> > +		return -EOPNOTSUPP;
> > +
> > +	if (!speed) {
> > +		netdev_err(ndev, "Link speed is 0!\n");
> 
> ??? Is this possible? If so, why is it checked only here and why can it be
> possible?
> 
It's possible if the board bootup without cable.

> > +		return -ECANCELED;
> 
> (already mentioned in other review)
> 
> > +	}
> > +
> > +	/* Queue 0 is not AVB capable */
> > +	if (queue <= 0 || queue >= fep->num_tx_queues) {
> 
> Is `< 0` possible? I realize it's `s32`, just curious.
> 
If the wrong parameter is entered and the app does not check the value,
It might be 0. I'm not sure, just in case.

> > +		netdev_err(ndev, "The queue: %d is invalid!\n", queue);
> 
> Maybe less emotions? There's no point in having `!` at the end of every error.
> 
OK, it fine to remove '!'.

> > +		return -EINVAL;
> > +	}
> > +
> > +	if (!cbs->enable) {
> > +		u32 val;
> > +
> > +		val = readl(fep->hwp + FEC_QOS_SCHEME);
> > +		val &= ~FEC_QOS_TX_SHEME_MASK;
> > +		val |= ROUND_ROBIN_SCHEME;
> 
> (u32_replace_bits())
> 
> > +		writel(val, fep->hwp + FEC_QOS_SCHEME);
> > +
> > +		memset(&fep->cbs, 0, sizeof(fep->cbs));
> > +
> > +		return 0;
> > +	}
> > +
> > +	if (cbs->idleslope - cbs->sendslope != speed * 1000 ||
> > +	    cbs->idleslope <= 0 || cbs->sendslope >= 0)
> 
> So you check slopes here, why check them above then?
> 
Because this function will be invoked due to some events in
fec_restart too, so I added these checks here. If you think it
is redundant, I will move these checks to fec_restart. Thanks.


> > +		return -EINVAL;
> > +
> > +	/* Another AVB queue */
> > +	queue2 = (queue == 1) ? 2 : 1;
> 
> Braces are unneeded.
> 
> > +	if (cbs->idleslope + fep->cbs.idleslope[queue2] > speed * 1000) {
> > +		netdev_err(ndev,
> > +			   "The sum of all idle slope can't exceed link speed!\n");
> > +		return -EINVAL;
> > +	}
> [...]
> 
> Thanks,
> Olek

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