netdev - Re: [RFC PATCH v2 linux-next 03/14] net: dsa: sja1105: the 0x1F0000 SGMII "base address" is actually MDIO_MMD

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Message-ID: <20210526154218.GI30436@shell.armlinux.org.uk>
Date:   Wed, 26 May 2021 16:42:18 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>
Subject: Re: [RFC PATCH v2 linux-next 03/14] net: dsa: sja1105: the 0x1F0000
 SGMII "base address" is actually MDIO_MMD_VEND2

On Wed, May 26, 2021 at 06:34:47PM +0300, Vladimir Oltean wrote:
> Hi Russell,
> 
> On Wed, May 26, 2021 at 04:24:54PM +0100, Russell King (Oracle) wrote:
> > On Wed, May 26, 2021 at 04:55:24PM +0300, Vladimir Oltean wrote:
> > > -	const struct sja1105_regs *regs = priv->info->regs;
> > > +	u64 addr = (mmd << 16) | pcs_reg;
> > 
> > What is the reason for using "u64" here. pcs_reg is 16-bits, and mmd is
> > five bits, which is well below 32 bits. So, why not u32?
> 
> The "addr" variable holds a SPI address, and in the sja1105 driver, the
> SPI addresses are universally held in u64 variables, mainly because of
> the packing() API (Documentation/core-api/packing.rst).

As you are passing it into a function, the argument of which is a u64,
the compiler will promote the u32 to a u64 by itself. I guess it
doesn't actually matter, but the current code just looks really weird.

-- 
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