netdev - Re: [PATCH v3 net-next 09/10] net: mscc: ocelot: initialize watermarks to sane defaults

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Date:   Fri, 8 Jan 2021 19:59:36 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>, netdev@...r.kernel.org
Cc:     alexandre.belloni@...tlin.com, andrew@...n.ch,
        vivien.didelot@...il.com, alexandru.marginean@....com,
        claudiu.manoil@....com, xiaoliang.yang_1@....com,
        hongbo.wang@....com, kuba@...nel.org, jiri@...nulli.us,
        idosch@...sch.org, UNGLinuxDriver@...rochip.com
Subject: Re: [PATCH v3 net-next 09/10] net: mscc: ocelot: initialize
 watermarks to sane defaults



On 1/8/2021 9:59 AM, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> This is meant to be a gentle introduction into the world of watermarks
> on ocelot. The code is placed in ocelot_devlink.c because it will be
> integrated with devlink, even if it isn't right now.
> 
> My first step was intended to be to replicate the default configuration
> of the congestion watermarks programatically, since they are now going
> to be tuned by the user.
> 
> But after studying and understanding through trial and error how they
> work, I now believe that the configuration used out of reset does not do
> justice to the word "reservation", since the sum of all reservations
> exceeds the total amount of resources (otherwise said, all reservations
> cannot be fulfilled at the same time, which means that, contrary to the
> reference manual, they don't guarantee anything).
> 
> As an example, here's a dump of the reservation watermarks for frame
> buffers, for port 0 (for brevity, the ports 1-6 were omitted, but they
> have the same configuration):
> 
> BUF_Q_RSRV_I(port 0, prio 0) = max 3000 bytes
> BUF_Q_RSRV_I(port 0, prio 1) = max 3000 bytes
> BUF_Q_RSRV_I(port 0, prio 2) = max 3000 bytes
> BUF_Q_RSRV_I(port 0, prio 3) = max 3000 bytes
> BUF_Q_RSRV_I(port 0, prio 4) = max 3000 bytes
> BUF_Q_RSRV_I(port 0, prio 5) = max 3000 bytes
> BUF_Q_RSRV_I(port 0, prio 6) = max 3000 bytes
> BUF_Q_RSRV_I(port 0, prio 7) = max 3000 bytes
> 
> Otherwise said, every port-tc has an ingress reservation of 3000 bytes,
> and there are 7 ports in VSC9959 Felix (6 user ports and 1 CPU port).
> Concentrating only on the ingress reservations, there are, in total,
> 8 [traffic classes] x 7 [ports] x 3000 [bytes] = 168,000 bytes of memory
> reserved on ingress.
> But, surprise, Felix only has 128 KB of packet buffer in total...
> A similar thing happens with Seville, which has a larger packet buffer,
> but also more ports, and the default configuration is also overcommitted.
> 
> This patch disables the (apparently) bogus reservations and moves all
> resources to the shared area. This way, real reservations can be set up
> by the user, using devlink-sb.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Reviewed-by: Florian Fainelli <f.fainelli@...il.com>
-- 
Florian