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

[Nathan Chancellor <natechancellor@gmail.com>]

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):

        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.

> Either way, I suspect without the change to
> drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c, that this fails to
> compile?

Correct so this is targeted at -next, rather than mainline, probably
should get in the habit of adding '-next' to my patch subjects...

> -- 
> Thanks,
> ~Nick Desaulniers

Thank you for the review!
Nathan