netdev - Re: [PATCH] net: dsa: microchip: Fix gigabit set and get function for KSZ87xx

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Date:   Wed, 22 Feb 2023 23:25:26 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Marek Vasut <marex@...x.de>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>,
        Arun Ramadoss <arun.ramadoss@...rochip.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, UNGLinuxDriver@...rochip.com,
        Woojung Huh <woojung.huh@...rochip.com>, stable@...r.kernel.org
Subject: Re: [PATCH] net: dsa: microchip: Fix gigabit set and get function
 for KSZ87xx

On Wed, Feb 22, 2023 at 12:50:07PM +0000, Russell King (Oracle) wrote:
> Looking at this driver, I have to say that it looks utterly vile
> from the point of view of being sure that it is correct, and I
> think this patch illustrates why.
...
> Now, what about other KSZ devices - I've analysed this for the KSZ8795,
> but what about any of the others which use this register table? It
> looks to me like those that use ksz8795_regs[] all use ksz8_dev_ops
> and the same masks and bitvals, so they should be the same.
> 
> That is a hell of a lot of work to prove that setting both
> P_XMII_CTRL_0 and P_XMII_CTRL_1 to point at the same register is
> in fact safe. Given the number of registers, the masks, and bitval
> arrays, doing this to prove every combination and then analysing
> the code is utterly impractical - and thus why I label this driver
> as "vile". Is there really no better option to these register
> arrays, bitval arrays and mask arrays - something that makes it
> easier to review and prove correctness?

Only my 2 cents. What is utterly vile is the decision of hardware design
to break software compatibility in such a deliberate and gratuitous way
across switch generations. A driver can only do so much when fed with
such hardware as input.

The ksz driver could use struct reg_field from regmap to mitigate that
to a certain extent (like the ocelot driver does), but certain quirks
will still remain present in the ksz driver. For example, the "bitval"
array. The value "1" written to the P_GMII_1GBIT reg_field indicates
gigabit for ksz8795, but !gigabit on ksz9477. I am not aware of any
abstraction to mask that away in common code other than the bitvals.

Even with struct reg_field, it would still not address the fundamental
problem which is simply that the register fields responsible for a
certain function have hopped so much from generation to generation,
that getting all offsets and bits right for each generation is a
challenge in itself.