linux-kernel - Re: [PATCH 4.19 60/87] usb: charger: assign specific number for enum value

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Date:   Wed, 4 Mar 2020 18:39:20 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Pavel Machek <pavel@...x.de>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Peter Chen <peter.chen@....com>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 60/87] usb: charger: assign specific number for enum
 value

On Wed, Mar 04, 2020 at 06:27:36PM +0100, Pavel Machek wrote:
> On Tue 2020-03-03 18:43:51, Greg Kroah-Hartman wrote:
> > From: Peter Chen <peter.chen@....com>
> > 
> > commit ca4b43c14cd88d28cfc6467d2fa075aad6818f1d upstream.
> > 
> > To work properly on every architectures and compilers, the enum value
> > needs to be specific numbers.
> 
> All compilers are expected to handle this in the same way, as this is
> in C standard. This patch is not neccessary, and should not be in mainline,
> either.
> 
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf
> 
> 6.7.2.2 Enumeration specifiers
> Syntax
> ...
> 3 The identifiers in an enumerator list are declared as constants that have type int and
> may appear wherever such are permitted.107) An enumerator with = defines its
> enumeration constant as the value of the constant expression. If the first enumerator has
> no =, the value of its enumeration constant is 0. Each subsequent enumerator with no =
> defines its enumeration constant as the value of the constant expression obtained by
> adding 1 to the value of the previous enumeration constant. (The use of enumerators with
> = may produce enumeration constants with values that duplicate other values in the same
> enumeration.) The enumerators of an enumeration are also known as its members.
> 
> Best regards,
> 								Pavel
> 
> >  enum usb_charger_type {
> > -	UNKNOWN_TYPE,
> > -	SDP_TYPE,
> > -	DCP_TYPE,
> > -	CDP_TYPE,
> > -	ACA_TYPE,
> > +	UNKNOWN_TYPE = 0,
> > +	SDP_TYPE = 1,
> > +	DCP_TYPE = 2,
> > +	CDP_TYPE = 3,
> > +	ACA_TYPE = 4,
> >  };

It specified that we need to do this by the in-kernel documentation
about how to write a solid api (which I can't find at the moment to
point you at, sorry...)  Also, you pointed at a draft C standard, is
that really implemented by older compilers?

thanks,

greg k-h