netdev - Re: [PATCH net-next 6/7] net: phy: smsc: Cache interrupt mask

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Message-ID: <Ymkzno2IbyNbFrEL@lunn.ch>
Date:   Wed, 27 Apr 2022 14:14:22 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Steve Glendinning <steve.glendinning@...well.net>,
        UNGLinuxDriver@...rochip.com, Oliver Neukum <oneukum@...e.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-usb@...r.kernel.org, Andre Edich <andre.edich@...rochip.com>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        Martyn Welch <martyn.welch@...labora.com>,
        Gabriel Hojda <ghojda@...urs.ro>,
        Christoph Fritz <chf.fritz@...glemail.com>,
        Lino Sanfilippo <LinoSanfilippo@....de>,
        Philipp Rosenberger <p.rosenberger@...bus.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH net-next 6/7] net: phy: smsc: Cache interrupt mask

On Wed, Apr 27, 2022 at 07:48:06AM +0200, Lukas Wunner wrote:
> Cache the interrupt mask to avoid re-reading it from the PHY upon every
> interrupt.  The PHY may be located on a USB device, so the additional
> read may unnecessarily increase interrupt overhead and latency.

I don't think your justification is valid. The MDIO bus is clocked at
2.5MHz. So even if you are using USB 1.1 at 12MHz, the USB overheads
are not particularly large. At 480Mbps they are pretty insignificant.

In general, we consider PHYs as slow devices, they take over 1 second
to negotiate a link and declare it up. So we don't do this sort of
micro optimization.

What i think is relevant here is that you could have an interrupt
storm going on because you don't mask interrupts? It is not a true
storm, due to the way USB works, more of a light shower. Do you have
any statistics to show this code actually reduces the amount of rain
in a significant way?

	Andrew