Subject: Re: [net v2 PATCH] net: stmmac: Update CBS parameters when speed changes after linking up

From: "Russell King (Oracle)" <[email-masked]>
Date: Thu, 30 May 2024 14:40:30 +0100

On Thu, May 30, 2024 at 04:28:22PM +0300, Vladimir Oltean wrote:
> On Thu, May 30, 2024 at 02:50:52PM +0200, Xiaolei Wang wrote:
> > When the port is relinked, if the speed changes, the CBS parameters
> > should be updated, so saving the user transmission parameters so
> > that idle_slope and send_slope can be recalculated after the speed
> > changes after linking up can help reconfigure CBS after the speed
> > changes.
> > 
> > Fixes: 1f705bc61aee ("net: stmmac: Add support for CBS QDISC")
> > Signed-off-by: Xiaolei Wang <[email-masked]>
> > ---
> > v1 -> v2
> >  - Update CBS parameters when speed changes
> 
> May I ask what is the point of this patch? The bandwidth fraction, as
> IEEE 802.1Q defines it, it a function of idleSlope / portTransmitRate,
> the latter of which is a runtime variant. If the link speed changes at
> runtime, which is entirely possible, I see no alternative than to let
> user space figure out that this happened, and decide what to do. This is
> a consequence of the fact that the tc-cbs UAPI takes the raw idleSlope
> as direct input, rather than something more high level like the desired
> bandwidth for the stream itself, which could be dynamically computed by
> the kernel.

So what should be the behaviour here? Refuse setting CBS parameters if
the link is down, and clear the hardware configuration of the CBS
parameters each and every time there is a link-down event? Isn't that
going to make the driver's in-use settings inconsistent with what the
kernel thinks have been set? AFAIK, tc qdisc's don't vanish from the
kernel just because the link went down.

I think what you're proposing leads to the hardware being effectively
"de-programmed" for CBS while "tc qdisc show" will probably report
that CBS is active on the interface - which clearly would be absurd.

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