Use of enums, why?

[John Whitmore]

On Wed, Jul 11, 2018 at 11:40:18AM +0200, Greg KH wrote:
> On Wed, Jul 11, 2018 at 10:30:28AM +0100, John Whitmore wrote:
> > I only learning the ropes, and might have missed the memo on the use of enums
> > so forgive me. I have looked at
> > https://www.kernel.org/doc/html/v4.10/process/coding-style.html#macros-enums-and-rtl
> > but that didn't answer my question.
> > 
> > I'm doing a clean up of staging:rtl8192u: clearing out checkpatch errors and
> > triming out data structures which aren't used in the code. So I came across a
> > typedef of an enumerated type in the file:
> > 
> > drivers/staging/rtl8192u/r8192U.h
> > 
> > typedef enum rf_optype {
> >        RF_OP_By_SW_3wire = 0,
> >        RF_OP_By_FW,
> >        RF_OP_MAX
> > } rf_op_type;
> 
> To fix this up for checkpatch, it should just look like:
> 
> enum rf_optype {
> 	RF_OP_By_SW_3wire = 0,
> 	RF_OP_By_FW,
> 	RF_OP_MAX
> };
> 
> > A quick grep for this type in drivers/staging/rtl8192u directory shows that
> > the type is never used outside that header definition. So I can remove it?
> 
> As you found out, no :)
> 
> > Well no you can't because the values defined in the enum are used.
> > 
> > In drivers/staging/rtl8192u/r8192U_core.c for example:
> >         priv->Rf_Mode = RF_OP_By_SW_3wire;
> > 
> > that element priv->Rf_Mode is defined in the structure
> > 
> > typedef struct r8192_priv {
> >         ...
> > 	u8 Rf_Mode;	/* For Firmware RF -R/W switch */
> 
> Ah, that should be:
> 	enum rf_optype RF_Mode;
> right?
> 
> >         ...
> > }
> > 
> > 
> > So now to the question, as I understand it the compiler will use an int type
> > for the enumerated type. The data structure r8192_priv doesn't use this int type
> > because the programmer knows that a u8 will do to hold the 3 possible values
> > defined by the enumerated type.
> 
> Or someone messed up and didn't realize that is what was holding those
> values :)
> 
> > So you're saving a few bytes in a data structure, which I'm happy about, but
> > the point of enumerated types is, as I understand it, so that the compiler can
> > do some checking to ensure a value is not assigned in error.
> 
> You are correct.
> 
> > Since the enum isn't being used, there is no compiler type checking,
> > so why use an enumerated type?  Might as well have used three #define
> > values.
> 
> That too would work, but you loose the typechecking.
> 
> > The obvious thing to do is leave well enough alone. But I had to ask what is
> > the correct implementation? Use the enum in the data structure, instead of
> > u8? Or just #define a few constants?
> > 
> > #define   RF_OP_By_SW_3wire    1
> > #define   RF_OP_By_FW          2
> > #define   RF_OP_MAX            3
> > 
> > Given the space saving of u8 over int I'd probably go with the #define. Guess
> > it depends on how many of those data structures are being declared.
> 
> I'd stick to the define, UNLESS this structure is coming from/to the
> hardware directly.  Then you need to use u8.  So look and see if that is
> what is happening here or not.   If it's just a "normal" structure in
> memory, then use an enum to keep the type safety.
> 
> hope this helps,
> 
> greg k-h

Yes that clears that up.

Thanks a million

John