linux-kernel - Re: [PATCH 6/7] dsa: marvell: Correct value of max_frame

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Date:   Fri, 10 Mar 2023 12:06:15 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Lukasz Majewski <lukma@...x.de>
Cc:     Andrew Lunn <andrew@...n.ch>, Vladimir Oltean <olteanv@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/7] dsa: marvell: Correct value of max_frame_size
 variable after validation

On Fri, Mar 10, 2023 at 12:53:46PM +0100, Lukasz Majewski wrote:
> Hi Andrew,
> 
> > > > If I understand this correctly, in patch 4, you add a call to the
> > > > 6250 family to call mv88e6185_g1_set_max_frame_size(), which sets
> > > > a bit called MV88E6185_G1_CTL1_MAX_FRAME_1632 if the frame size
> > > > is larger than 1518.  
> > > 
> > > Yes, correct.
> > >   
> > > > 
> > > > However, you're saying that 6250 has a frame size of 2048. That's
> > > > fine, but it makes MV88E6185_G1_CTL1_MAX_FRAME_1632 rather
> > > > misleading as a definition. While the bit may increase the frame
> > > > size, I think if we're going to do this, then this definition
> > > > ought to be renamed. 
> > > 
> > > I thought about rename, but then I've double checked; register
> > > offset and exact bit definition is the same as for 6185, so to avoid
> > > unnecessary code duplication - I've reused the existing function.
> > > 
> > > Maybe comment would be just enough?  
> > 
> > The driver takes care with its namespace in order to add per switch
> > family defines. So you can add MV88E6250_G1_CTL1_MAX_FRAME_2048. It
> > does not matter if it is the same bit. You can also add a
> > mv88e6250_g1_set_max_frame_size() and it also does not matter if it is
> > in effect the same as mv88e6185_g1_set_max_frame_size().
> > 
> > We should always make the driver understandably first, compact and
> > without redundancy second. We are then less likely to get into
> > situations like this again where it is not clear what MTU a device
> > actually supports because the code is cryptic.
> 
> Ok, I will add new function.
> 
> Thanks for hints.

It may be worth doing:

static int mv88e6xxx_g1_modify(struct mv88e6xxx_chip *chip, int reg,
			       u16 mask, u16 val)
{
	int addr = chip->info->global1_addr;
	int err;
	u16 v;

	err = mv88e6xxx_read(chip, addr, reg, &v);
	if (err < 0)
		return err;

	v = (v & ~mask) | val;

	return mv88e6xxx_write(chip, addr, reg, v);
}

Then, mv88e6185_g1_set_max_frame_size() becomes:

int mv88e6185_g1_set_max_frame_size(struct mv88e6xxx_chip *chip, int mtu)
{
	u16 val = 0;

	if (mtu + ETH_HLEN + ETH_FCS_LEN > 1518)
		val = MV88E6185_G1_CTL1_MAX_FRAME_1632;

	return mv88e6xxx_g1_modify(chip, MV88E6XXX_G1_CTL1,
				   MV88E6185_G1_CTL1_MAX_FRAME_1632, val);
}

The 6250 variant becomes similar.

We can also think about converting all those other read-modify-writes
to use mv88e6xxx_g1_modify().

The strange thing is... we already have mv88e6xxx_g1_ctl2_mask() which
is an implementation of mv88e6xxx_g1_modify() specifically for
MV88E6XXX_G1_CTL2 register, although it uses (val & mask) rather than
just val. That wouldn't be necessary if the bitfield macros (e.g.
FIELD_PREP() were used rather than explicit __bf_shf().

-- 
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