Re: [PM-WIP-OPP][PATCH] OPP: Introduces enum for addressing different OPP types

Romit Dasgupta
2010-01-15T10:42:10Z

Nishanth,
I was about to post a re-worked patch. Anyways, please see below:
> Here is a sample commit message I can think of:
> ----
> Using omap_opp * to refer to domain types restricts opp implementation
> into maintaining pointers outside the opp layer. This causes issues such as:
> a) Describing cross domain dependencies (e.g. dsp vs mpu)
> b) Ease of transitioning/supporting to multiple silicon variants and
> families
> c) Choice of varied options in implementing opp layer internals.
>
> Since all we need a identifying a specific domain for query/operational
> purposes, we introduce enum for identifying OPP types instead of using
> opp layer's internal data structure pointer.
>
> Currently, OMAP3 is the only silicon supporting the OPP layer, hence
> mpu_opps, l3_opps and dsp_opps are deprecated and replaced with OPP_MPU,
> OPP_L3 and OPP_DSP respectively.

I like this message. I will include it.

>>>>
>>>>
>>> definition of enum and the implicit usage of enums are in two different
>>> files. there is a distinct possibility of some one modifying the header
>>> without actually knowing that .c depends on the exact values of the enum
>>> definition.
>> As I said before one needs to make changes in the kernel by knowing what they
>> are doing.
>>> pm34xx.c has no right to depend on opp.h definition values -> if it does
>>> it ties it down and a constraint for future flexibility. please change.
>> The right approach should be to take out the loop in pm34xx.c for now and
>> explicitly call the opp_init_list function after passing OPP_MPU, OPP_L3,
>> OPP_DSP in any order. So pm34xx.c needs to change not opp.[ch]. What do you think?
>
> I did dig into this a few mins ago.. and yes I can see similar example
> in drivers/mfd/twl4030-core.c
>
> The intent of my comment is this: when someone else, few months from
> now, is focusing on adding/changing opp logic, they will focus on opp.c
> and .h. we have two choices to handle this:
> a) Ensure that users of opp.h do not know how it works internally ->
> e.g. ordering of opp list for example.
> b) add
> /* WARNING: See file:arch/arm/mach-omap2/pm34xx.c before modifying the
> sequence of these enums */ to opp.h
> and
> /* WARNING: See file:arch/arm/plat-omap/include/plat/opp.h before
> modifying this */ in pm34xx.c
>
> now, I was recommending doing a, till a thought a little more on the
> implementation(array based) and how long that implementation might
> last(we might potentially move opp.c to a list implementation). the
> effort would be to complicate the opp_init,add functions for a very
> short lifetime. This effort maynot be worth it.

I understand your concern. I have made some changes in the code. Please look at
the reposted patch (in few mins from now I shall post them).
>>>>> Enum type and variable have the same name :( mebbe a rename of variable is
>>>>> appropriate
>>>>>
>>>> Not sure why you say this. Did you see the compiler throwing up any warning?
>>>>
>>> The usage later in the code is opp_t -> this is a readability issue not
>>> a compiler warning.
>> What is the readability issue? Why cant we declare something like enum opp_t opp_t?
>
> Let me try to explain this clearly. assume we have a struct opp_t (not
> enum) for the time being.
> void some_func(struct opp_t *opp_t)
> {
> struct opp_t *opp;
>
> ..
> 200 line of code (>one page full)
> ....
> /* point 1 */
> BUG_ON(opp_t.xyz)
> ...
> 200 lines of more code
> ..
> /* point 2 */
> BUG_ON(opp.xyz)
> ...
>
> }
>
> lets say this is compiled by some non follower of this mail chain,
> compiler throws an error for point 1: filex:liney
> so the guy/gal fires up vim and opens the filex, goes to line y
> he/she cannot see the start of the function, knows that there is a
> struct opp_t

If a function is that big then the fault lies there to start with! What do you say?
Nevertheless, your suggestion is cosmetic but I think we should not assume that
developers are so ignorant. For now I will do away with your suggestion. Please
feel free to change the code if you think what you say is the right thing.

Regards,
-Romit