Use of enums, why?

[John Whitmore]

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;

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?
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 */
        ...
}


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.

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. 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.

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.

thanks for any thoughts or clarification

John