netdev - RE: [PATCH v1 net-next 2/4] net: dsa: microchip: add MIB counter reading support

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Message-ID: <BC575D76-0419-4318-B924-81E29858B8E9@gmail.com>
Date:   Tue, 12 Feb 2019 19:51:19 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Tristram.Ha@...rochip.com, andrew@...n.ch
CC:     sergio.paracuellos@...il.com, pavel@....cz,
        UNGLinuxDriver@...rochip.com, netdev@...r.kernel.org
Subject: RE: [PATCH v1 net-next 2/4] net: dsa: microchip: add MIB counter reading support



On February 12, 2019 6:39:49 PM PST, Tristram.Ha@...rochip.com wrote:
>> > +static void ksz9477_freeze_mib(struct ksz_device *dev, int port,
>bool
>> freeze)
>> > +{
>> > +	struct ksz_port *p = &dev->ports[port];
>> > +	u32 val = freeze ? MIB_COUNTER_FLUSH_FREEZE : 0;
>> 
>> Reverse Christmas tree.
>
>There was a checkpatch.pl patch in 2016 that tried to check this, but
>it was never accepted?
> 
>> > +		/* read only dropped counters when link is not up */
>> > +		if (p->link_just_down)
>> > +			p->link_just_down = 0;
>> > +		else if (!p->phydev.link)
>> > +			mib->cnt_ptr = dev->reg_mib_cnt;
>> 
>> This link_just_down stuff is not clear at all. Why can the drop
>> counters not be read when the link is up?
>
>All of the MIB counters, except some that may be marked by driver, do
>not get updated when the link is down, so it is a waste of time to read
>them.

Can you use netif_running() to determine that condition? Maintaining your own set of variables when the PHY state machine should already determine the link state sounds redundant if not error prone.

>
>My intention is the driver eventually reads the MIB counters at least
>every second or faster so that the ethtool API called to show MIB
>counters gets them from memory rather than starting a read operation. 
>For now that API is called from user space with the ethtool utility, so
>it may not be called too often and too fast.  But theoretically that
>API can be called from a program continually.
>
>For simple switches that do not need to do anything special the MIB
>read operation does not cause any issue except CPU load, for more
>complicate switches that need to do some background operations too many
>read operation can affect some critical functions.

Some switches have a MIB autocast feature taking a snapshot which AFAIR is internally implemented as a fast read register with no contention on other registers internally, do you have something similar?
-- 
Florian