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

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Message-ID: <20240530132822.xv23at32wj73hzfj@skbuf>
Date: Thu, 30 May 2024 16:28:22 +0300
From: Vladimir Oltean <olteanv@...il.com>
To: Xiaolei Wang <xiaolei.wang@...driver.com>, Andrew Lunn <andrew@...n.ch>
Cc: linux@...linux.org.uk, alexandre.torgue@...s.st.com,
	joabreu@...opsys.com, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, mcoquelin.stm32@...il.com,
	netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [net v2 PATCH] net: stmmac: Update CBS parameters when speed
 changes after linking up

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 <xiaolei.wang@...driver.com>
> ---
> 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.