netdev - Re: [PATCH 9/9] net: dsa: mt7530: use external PCS driver

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Date:   Sat, 4 Feb 2023 18:13:42 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Daniel Golle <daniel@...rotopia.org>
Cc:     Vladimir Oltean <olteanv@...il.com>, netdev@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Russell King <linux@...linux.org.uk>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Mark Lee <Mark-MC.Lee@...iatek.com>,
        John Crispin <john@...ozen.org>, Felix Fietkau <nbd@....name>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        DENG Qingfang <dqfext@...il.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Sean Wang <sean.wang@...iatek.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jianhui Zhao <zhaojh329@...il.com>,
        Bjørn Mork <bjorn@...k.no>
Subject: Re: [PATCH 9/9] net: dsa: mt7530: use external PCS driver

> The SerDes PCS units are only available for port 5 and 6. The code
> should make sure that the corresponding interface modes are only used
> on these two ports, so a BUG_ON(!mt753x_is_mac_port(port)) would also
> do the trick, I guess. However, as dsa_port_phylink_mac_select_pcs may
> also return ERR_PTR(-EOPNOTSUPP), returning ERR_PTR(-EINVAL) felt like
> the right thing to do in that case.
> Are you suggesting to use BUG_ON() instead or rather return
> ERR_PTR(-EOPNOTSUPP)?

BUG_ON() is considered to mean something which you cannot recover from
has happened, and going further will only cause more file system
corruption, memory corruption, etc. WARN_ON() is better, since it
gives the user a chance to perform a controlled shutdown, so
potentially not loosing files etc.

But even a WARN_ON() seems a bit extreme. If -EINVAL is sufficient to
cause either the whole device, or the port to fail to probe, that
should be enough to get the DT developers attention. Then either a
dev_err() or a dev_dbg() to help narrow down which check failed.

	  Andrew