linux-kernel - Re: [RFC PATCH net-next 2/2] net: dsa: mt7530: introduce MMIO driver for MT7988 SoC

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Date:   Tue, 28 Mar 2023 18:17:16 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Daniel Golle <daniel@...rotopia.org>
Cc:     Andrew Lunn <andrew@...n.ch>, netdev@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        DENG Qingfang <dqfext@...il.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Sam Shih <Sam.Shih@...iatek.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        John Crispin <john@...ozen.org>, Felix Fietkau <nbd@....name>
Subject: Re: [RFC PATCH net-next 2/2] net: dsa: mt7530: introduce MMIO driver
 for MT7988 SoC

On Tue, Mar 28, 2023 at 04:00:30PM +0100, Daniel Golle wrote:
> On Tue, Mar 28, 2023 at 05:16:28PM +0300, Vladimir Oltean wrote:
> > On Tue, Mar 28, 2023 at 02:08:18PM +0100, Daniel Golle wrote:
> > > I agree that using regmap would be better and I have evaluated that
> > > approach as well. As regmap doesn't allow lock-skipping and mt7530.c is
> > > much more complex than xrs700x in the way indirect access to its MDIO bus
> > > and interrupts work, using regmap accessors for everything would not be
> > > trivial.
> > > 
> > > So here we can of course use regmap_read_poll_timeout and a bunch of
> > > readmap_write operations. However, each of them will individually acquire
> > > and release the mdio bus mutex while the current code acquires the lock
> > > at the top of the function and then uses unlocked operations.
> > > regmap currently doesn't offer any way to skip the locking and/or perform
> > > locking manually. regmap_read, regmap_write, regmap_update_bits, ... always
> > > acquire and release the lock on each operation.
> > 
> > What does struct regmap_config :: disable_locking do?
> 
> I thought I can't use that on a per-operation base because the
> instance of struct regmap_config itself isn't protected by any lock
> and hence setting disable_locking=false before calling one of the
> accessor functions may affect also other congruent calls to the
> accessors which will then ignore locking and screw things up.
> Please correct me if I'm wrong there.

It's not supposed to be used like that. You set disable_locking = true
once, and take care of locking from the mt7530 driver. Not before every
operation.

> Yet another way I thought about now could also be to have two regmap
> instances, one for locked and one for unlocked accessed to the same
> regmap_bus.

That would also be theoretically possible, but the above would be
equivalent and would require a single regmap.