netdev - RE: [EXT] Re: [PATCH v12 net-next 12/15] net: mvpp2: add BM protection underrun feature support

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Message-ID: <CO6PR18MB3873E319FA08ADBC3B682828B0899@CO6PR18MB3873.namprd18.prod.outlook.com>
Date:   Sun, 14 Feb 2021 09:20:43 +0000
From:   Stefan Chulski <stefanc@...vell.com>
To:     Andrew Lunn <andrew@...n.ch>, Marcin Wojtas <mw@...ihalf.com>
CC:     David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "thomas.petazzoni@...tlin.com" <thomas.petazzoni@...tlin.com>,
        Nadav Haklai <nadavh@...vell.com>,
        "Yan Markman" <ymarkman@...vell.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "rmk+kernel@...linux.org.uk" <rmk+kernel@...linux.org.uk>,
        "atenart@...nel.org" <atenart@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "sebastian.hesselbarth@...il.com" <sebastian.hesselbarth@...il.com>,
        "gregory.clement@...tlin.com" <gregory.clement@...tlin.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: RE: [EXT] Re: [PATCH v12 net-next 12/15] net: mvpp2: add BM
 protection underrun feature support


> > > Or we have also found out, that pushing back on parameters like
> > > this, the developers goes back and looks at the code, and sometimes
> > > figures out a way to automatically do the right thing, removing the
> > > configuration knob, and just making it all simpler for the user to
> > > use.
> >
> > I think of 2 alternatives:
> > * `ethtool --set-priv-flags` - in such case there is a question if
> > switching this particular feature in runtime is a good idea.
> > * New DT/ACPI property - it is a hardware feature after all, so maybe
> > let the user decide whether to enable it on the platform description
> > level.
> 
> Does this even need to be configurable? What is the cost of turning it on?
> How does having less pools affect the system? Does average latency go up?
> When would i consider an underrun actually a good thing?
> 
> Maybe it should just be hard coded on? Or we should try to detect when
> underruns are happening a lot, and dynamically turn it on for a while?
> 
> 	  Andrew

The cost of this change is that the number of pools reduced from 16 to 8.
The current driver uses only 4pools, but some future features like QoS can use over 4 pools. 

Regards,
Stefan.