[PATCH 1/2] mtd: nand: force NAND_CMD_READID onto 8-bit bus

[Gupta, Pekon]

Hi Ezequiel,

>> >+static inline int nand_opcode_8bits(unsigned int command)
>> >+{
>> >+	return command == NAND_CMD_READID;
>> >+}
>> >+
>> >
>> Can 'nand_opcode_8bits, made a macro instead of inline function ?
>> #define IS_8BIT_CMD(cmd)  (unlikely(cmd == NAND_CMD_READID))
>>
>> Because 'nand_opcode_8bits' is used in nand_command() and nand_command_lp()
>> which is performance critical code (chip->cmd is called multiple times for fetching
>> page data and OOB). Though we should expect compiler to treat this inline function
>> same as macro here, But just to be doubly sure for future changes also.
>>
>
>I'm not a compiler guru, but I'd be very surprised if the compiler would make
>a difference here, given the function is static, inline and small. Besides,
>an inline function is more readable and type-aware. I'd say it's the Right
>Thing to do.
>
>Nevertheless, I did a small check on my platform (handbuilt ARM GCC 4.7.2) and
>both macro and inline is compiled into a "cmp" instruction. The unlikely
>doesn't seem to have any effect.
>
Please check the JMP instruction just after CMP instruction..
In-case of (unlikely(x == y)) JMP/JE will be used to branch when x == y.
where as in-case of (likely(x == y)) JMP/JNE will be used to branch x != y.

As in most cases 'command != NAND_CMD_READID' so with unlikely() 
Compiler should lay down the instructions in such an order that program
executes _without_ branching.  Thus JMP will point to the code when
command == NAND_CMD_READID.

(this is my understanding, however may be for small branches this has
 no effect because both paths can be fetched simultaneously into pipeline).


with regards, pekon