Re: [PATCH] drm/amd/display: Pass app_tf by value rather than by reference

["Wentland, Harry" <Harry.Wentland@amd.com>]

On 2018-12-11 5:07 p.m., Nick Desaulniers wrote:
> On Tue, Dec 11, 2018 at 1:42 PM Nathan Chancellor
> <natechancellor@gmail.com> wrote:
>>
>> On Tue, Dec 11, 2018 at 01:25:00PM -0800, Nick Desaulniers wrote:
>>> On Mon, Dec 10, 2018 at 3:42 PM Nathan Chancellor
>>> <natechancellor@gmail.com> wrote:
>>>>
>>>> Clang warns when an expression that equals zero is used as a null
>>>> pointer constant (in lieu of NULL):
>>>>
>>>> drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:4435:3:
>>>> warning: expression which evaluates to zero treated as a null pointer
>>>> constant of type 'const enum color_transfer_func *'
>>>> [-Wnon-literal-null-conversion]
>>>>                 TRANSFER_FUNC_UNKNOWN,
>>>>                 ^~~~~~~~~~~~~~~~~~~~~
>>>> 1 warning generated.
>>>>
>>>> This warning is caused by commit bb47de736661 ("drm/amdgpu: Set FreeSync
>>>> state using drm VRR properties") and it could be solved by using NULL
>>>> instead of TRANSFER_FUNC_UNKNOWN or casting TRANSFER_FUNC_UNKNOWN as a
>>>> pointer. However, after looking into it, there doesn't appear to be a
>>>> good reason to pass app_tf by reference as it is never mutated along the
>>>> way. This is the only code path in which app_tf is used:
>>>>
>>>> mod_freesync_build_vrr_infopacket ->
>>>>     build_vrr_infopacket_v2 ->
>>>>         build_vrr_infopacket_fs2_data
>>>>
>>>> Neither mod_freesync_build_vrr_infopacket or build_vrr_infopacket_v2
>>>> modify app_tf's value and build_vrr_infopacket_fs2_data expects just
>>>> the value so we can avoid dereferencing anything by just passing in
>>>> app_tf's value to mod_freesync_build_vrr_infopacket and
>>>> build_vrr_infopacket_v2.
>>>>
>>>> There is no functional change because build_vrr_infopacket_fs2_data
>>>> doesn't do anything if TRANSFER_FUNC_UNKNOWN is passed to it, the same
>>>> as not calling build_vrr_infopacket_fs2_data at all like before this
>>>> change when NULL was used for app_tf.
>>>
>>> Nathan,
>>> Thanks for sending this patch.  I was hoping to provide review sooner,
>>> but have been quite busy lately.
>>>
>>
>> Late review is better than no review, I appeciate you taking the time to
>> do this!
>>
>>> Yeah, especially for LP64 targets, the pointer to enum is larger than
>>> just the enum, and if it's not being updated ("in/out paramter")
>>> there's no need to pass by pointer.
>>>
>>
>> Thanks for confirming!
>>
>>>>
>>>> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
>>>> ---
>>>>  drivers/gpu/drm/amd/display/modules/freesync/freesync.c | 7 +++----
>>>>  drivers/gpu/drm/amd/display/modules/inc/mod_freesync.h  | 2 +-
>>>>  2 files changed, 4 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/amd/display/modules/freesync/freesync.c b/drivers/gpu/drm/amd/display/modules/freesync/freesync.c
>>>> index 620a171620ee..520665a9d81a 100644
>>>> --- a/drivers/gpu/drm/amd/display/modules/freesync/freesync.c
>>>> +++ b/drivers/gpu/drm/amd/display/modules/freesync/freesync.c
>>>> @@ -656,7 +656,7 @@ static void build_vrr_infopacket_v1(enum signal_type signal,
>>>>
>>>>  static void build_vrr_infopacket_v2(enum signal_type signal,
>>>>                 const struct mod_vrr_params *vrr,
>>>> -               const enum color_transfer_func *app_tf,
>>>> +               enum color_transfer_func app_tf,
>>>>                 struct dc_info_packet *infopacket)
>>>>  {
>>>>         unsigned int payload_size = 0;
>>>> @@ -664,8 +664,7 @@ static void build_vrr_infopacket_v2(enum signal_type signal,
>>>>         build_vrr_infopacket_header_v2(signal, infopacket, &payload_size);
>>>>         build_vrr_infopacket_data(vrr, infopacket);
>>>>
>>>> -       if (app_tf != NULL)
>>>> -               build_vrr_infopacket_fs2_data(*app_tf, infopacket);
>>>> +       build_vrr_infopacket_fs2_data(app_tf, infopacket);
>>>>
>>>>         build_vrr_infopacket_checksum(&payload_size, infopacket);
>>>>
>>>> @@ -676,7 +675,7 @@ void mod_freesync_build_vrr_infopacket(struct mod_freesync *mod_freesync,
>>>>                 const struct dc_stream_state *stream,
>>>>                 const struct mod_vrr_params *vrr,
>>>>                 enum vrr_packet_type packet_type,
>>>> -               const enum color_transfer_func *app_tf,
>>>> +               enum color_transfer_func app_tf,
>>>>                 struct dc_info_packet *infopacket)
>>>>  {
>>>>         /* SPD info packet for FreeSync */
>>>> diff --git a/drivers/gpu/drm/amd/display/modules/inc/mod_freesync.h b/drivers/gpu/drm/amd/display/modules/inc/mod_freesync.h
>>>> index 949a8b62aa98..063af6258fd9 100644
>>>> --- a/drivers/gpu/drm/amd/display/modules/inc/mod_freesync.h
>>>> +++ b/drivers/gpu/drm/amd/display/modules/inc/mod_freesync.h
>>>> @@ -145,7 +145,7 @@ void mod_freesync_build_vrr_infopacket(struct mod_freesync *mod_freesync,
>>>>                 const struct dc_stream_state *stream,
>>>>                 const struct mod_vrr_params *vrr,
>>>>                 enum vrr_packet_type packet_type,
>>>> -               const enum color_transfer_func *app_tf,
>>>> +               enum color_transfer_func app_tf,
>>>
>>> Don't you need to update the callsite of `mod_freesync_build_vrr_infopacket` in
>>>
>>> drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c#4949:
>>>
>>> -                                           NULL,
>>> +                                          transfer_func_unknown,
>>>
>>
>> That change in commit bb47de736661 ("drm/amdgpu: Set FreeSync state
>> using drm VRR properties") in -next is what prompted this patch (the
>> warning in the commit message is not present in mainline):
> 
> Ah! Sorry, I was looking at mainline.  I should have noticed
> bb47de736661 wasn't there.
> 
> Shouldn't that change fail to compile, as transfer_func_unknown is an
> `enum color_transfer_func` value, but
> mod_freesync_build_vrr_infopacket takes a *pointer* to an `enum
> color_transfer_func` value?
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/gpu/drm/amd/display/modules/freesync/freesync.c#n675
> 
>>
>>         mod_freesync_build_vrr_infopacket(
>>                 dm->freesync_module,
>>                 new_stream,
>>                 &vrr,
>>                 packet_type_vrr,
>>                 transfer_func_unknown,
>>                 &vrr_infopacket);
>>
>>> Maybe at that point the `if (app_tf != NULL)` could be replaced with
>>> `if (app_tf != transfer_func_unknown)` hoisted from
>>> `build_vrr_infopacket_fs2_data`? (There's only one caller of
>>> `build_vrr_infopacket_fs2_data` today, maybe fine to leave the
>>> unconditional call and check).
>>>
>>
>> Hmmm that's not unreasonable I suppose. I guess it depends on if
>> build_vrr_infopacket_fs2_data could ever be called from outside of
>> build_vrr_infopacket_v2; if it can, it makes sense to leave the
>> conditional check for 'app_tf != transfer_func_unknown' in
>> build_vrr_infopacket_fs2_data and leaving the unconditional call
>> to it in build_vrr_infopacket_v2 (since app_tf is no longer a pointer,
>> no need to check against NULL).
>>
>> I'm happy to do a v2 if the maintainers feel strongly about it, thank
>> you for bringing that up.
> 
> Don't worry about it, I think it's fine.
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> 

Sorry for being late to the game. FWIW, code is also
Reviewed-by: Harry Wentland <harry.wentland@amd.com>

We can change the comparison separately later.

Not sure why this was ever passed as a pointer.

Harry