netdev - Re: [PATCH RFC net-next 3/7] net: dsa: use fwnode_get_phy

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Date:   Thu, 23 Mar 2023 14:49:01 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Daniel Scally <djrscally@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Jakub Kicinski <kuba@...nel.org>, linux-acpi@...r.kernel.org,
        netdev@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Vladimir Oltean <olteanv@...il.com>
Subject: Re: [PATCH RFC net-next 3/7] net: dsa: use fwnode_get_phy_mode() to
 get phy interface mode

On Thu, Mar 23, 2023 at 04:38:29PM +0200, Andy Shevchenko wrote:
> On Thu, Mar 23, 2023 at 02:31:04PM +0000, Russell King (Oracle) wrote:
> > On Thu, Mar 23, 2023 at 04:03:05PM +0200, Andy Shevchenko wrote:
> > > On Wed, Mar 22, 2023 at 12:00:06PM +0000, Russell King (Oracle) wrote:
> 
> ...
> 
> > > > +	struct fwnode_handle *fwnode;
> > > 
> > > > +	fwnode = of_fwnode_handle(dp->dn);
> > > 
> > > 	const struct fwnode_handle *fwnode = of_fwnode_handle(dp->dn);
> > > 
> > > ?
> > 
> > Why const?
> 
> Do you modify its content on the fly?

Do you want to litter code with casts to get rid of the const?

> For fwnode as a basic object type we want to reduce the scope of the possible
> modifications. If you don't modify and APIs you call do not require non-const
> object, use const for fwnode.

Let's start here. We pass this fwnode to fwnode_get_phy_mode():

include/linux/property.h:int fwnode_get_phy_mode(struct fwnode_handle *fwnode);

Does fwnode_get_phy_mode() alter the contents of the fwnode? Probably
not, but it doesn't take a const pointer. Therefore, to declare my
fwnode as const, I'd need to cast the const-ness away before calling
this.

Then there's phylink_create(). Same problem.

So NAK to this const - until such time that we have a concerted effort
to making functions we call which do not modify the "fwnode" argument
constify that argument. Otherwise it's just rediculously crazy to
declare a variable const only to then litter the code with casts to get
rid of it at every call site.

Please do a bit of research before making suggestions. Thanks.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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