linux-kernel - Re: [PATCH net-next 3/5] net: mvpp2: use a data size of 10kB for Tx FIFO on port 0

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Message-ID: <20180304102518.1963cb8a@windsurf.home>
Date:   Sun, 4 Mar 2018 10:25:18 +0100
From:   Thomas Petazzoni <thomas.petazzoni@...tlin.com>
To:     Stefan Chulski <stefanc@...vell.com>
Cc:     Antoine Tenart <antoine.tenart@...tlin.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Yan Markman <ymarkman@...vell.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "maxime.chevallier@...tlin.com" <maxime.chevallier@...tlin.com>,
        "gregory.clement@...tlin.com" <gregory.clement@...tlin.com>,
        "miquel.raynal@...tlin.com" <miquel.raynal@...tlin.com>,
        Nadav Haklai <nadavh@...vell.com>,
        "mw@...ihalf.com" <mw@...ihalf.com>
Subject: Re: [PATCH net-next 3/5] net: mvpp2: use a data size of 10kB for Tx
 FIFO on port 0

Hello,

On Sun, 4 Mar 2018 06:29:59 +0000, Stefan Chulski wrote:

> > Is there a reason to hardcode 10KB for port 0, and 3KB for the other ports ?
> > Would there be use cases where the user may want different configurations
> > ?
> 
> Design requirement are 10KB TX FIFO for the 10Gb/sec and 2.5KB for the 2.5Gb/sec.

What is a "design requirement" ? Is it a HW design limitation ?

> Since only port 0 support 10Gb/sec and ports 1&2 support up to 2.5Gb/sec.
> I don't see any reason to change this configurations.
> Also TX FIFO size could be set only during probe.
> 
> > It's just that it feels very "hardcoded" to enforce specifically those numbers.
> > 
> > Also, does it make sense to mention the CP110 here ? Is this 19 KB limitation
> > a limit of the PPv2.2 IP, or of the CP110 ?  
> 
> PPv2.2 IP is part of 110 communication processor.

Thanks, I know this :-)

> Next communication processor will has different Packet processor or next generation of PPv2.x
> Limit is PPv2.2 TX FIFO.

So, the limitation has nothing to do with CP110 really, it's just a
limitation of PPv2.2, and mentioning CP110 in the comment doesn't make
much sense, correct ?

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com