RE: [PATCH net-next] net: fec: add CBS offload support

[Wei Fang]

> -----Original Message-----
> From: Andrew Lunn <andrew@xxxxxxx>
> Sent: 2023年2月10日 4:38
> To: Wei Fang <wei.fang@xxxxxxx>
> Cc: Shenwei Wang <shenwei.wang@xxxxxxx>; Clark Wang
> <xiaoning.wang@xxxxxxx>; davem@xxxxxxxxxxxxx; edumazet@xxxxxxxxxx;
> kuba@xxxxxxxxxx; pabeni@xxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; dl-linux-imx
> <linux-imx@xxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH net-next] net: fec: add CBS offload support
> 
> > +	/* cbs->idleslope is in kilobits per second. speed is the port rate
> > +	 * in megabits per second. So bandwidth ratio bw = (idleslope /
> > +	 * (speed * 1000)) * 100, the unit is percentage.
> > +	 */
> > +	bw = cbs->idleslope / (speed * 10UL);
> 
> This appears to be a / 0 when the link is not up yet?  Also, if the link goes
> does, fep->speed keeps the old value, so if it comes up again at a different
> speed, your calculations are all wrong. So i think you need
> fec_enet_adjust_link() involved in this.
> 
Yes, speed = 0 is indeed a problem, we should check the value first.
For speed change, I'll think about how to handle this situation.

> 
> > +	/* bw% can not >= 100% */
> > +	if (bw >= 100)
> > +		return -EINVAL;
> 
> Well > 100% could happen when the link goes from 1G to 10Half, or even
> 100Full. You should probably document the policy of what you do then. Do
> you dedicate all the available bandwidth to the high priority queue, or do you
> go back to best effort? 
Actually, the FEC has always used the credit-based shaper by default. So I think
it's better to fall back the default setting and return error if the bw > 100%.

>Is it possible to indicate in the tc show command that
> the configuration is no longer possible?
> 
Sorry, I have no knowledge about the tc show command.

> Presumably other drivers have already addressed all these issues, so you just
> need to find out what they do.
> 
>     Andrew