netdev - Re: [PATCH net-next] enetc: Workaround for MDIO register access issue

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Date:   Tue, 10 Nov 2020 15:25:12 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Claudiu Manoil <claudiu.manoil@....com>, netdev@...r.kernel.org
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Alex Marginean <alexandru.marginean@....com>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH net-next] enetc: Workaround for MDIO register access issue

On 11/10/20 7:43 AM, Claudiu Manoil wrote:
> From: Alex Marginean <alexandru.marginean@....com>
> 
> Due to a hardware issue, an access to MDIO registers
> that is concurrent with other ENETC register accesses
> may lead to the MDIO access being dropped or corrupted.
> The workaround introduces locking for all register accesses
> to the ENETC register space.  To reduce performance impact,
> a readers-writers locking scheme has been implemented.
> The writer in this case is the MDIO access code (irrelevant
> whether that MDIO access is a register read or write), and
> the reader is any access code to non-MDIO ENETC registers.
> Also, the datapath functions acquire the read lock fewer times
> and use _hot accessors.  All the rest of the code uses the _wa
> accessors which lock every register access.

I suppose this is the best you can do given the nature of the bug, my
only concern is that it is error prone and may not really scale over
time as you start adding more features/bug fixes that require making
register accesses and use incorrectly _hot in a context where the
reader's lock is not acquired.
-- 
Florian