<div dir="ltr"><div class="gmail_quote"><div dir="ltr">On Tue, Dec 11, 2018 at 2:24 PM Nathan Chancellor <<a href="mailto:natechancellor@gmail.com">natechancellor@gmail.com</a>> wrote: </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">On Tue, Dec 11, 2018 at 02:07:31PM -0800, Nick Desaulniers wrote: > On Tue, Dec 11, 2018 at 1:42 PM Nathan Chancellor > <<a href="mailto:natechancellor@gmail.com" target="_blank">natechancellor@gmail.com</a>> 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 > > > <<a href="mailto:natechancellor@gmail.com" target="_blank">natechancellor@gmail.com</a>> 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 <<a href="mailto:natechancellor@gmail.com" target="_blank">natechancellor@gmail.com</a>> > > > > --- > > > > 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? > <a href="https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/gpu/drm/amd/display/modules/freesync/freesync.c#n675" rel="noreferrer" target="_blank">https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/gpu/drm/amd/display/modules/freesync/freesync.c#n675</a> > Unfortunately, no. GCC does not warn like Clang does when an expression that equals zero is passed to a function requiring a pointer (it just silently treats it as NULL, whereas Clang audibly complains). </blockquote><div> </div><div>That C allows this is troubling to me. I may need to go seek therapy to deal with my anguish this has caused me...</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> They both do when the expression is something other than zero. <a href="https://godbolt.org/z/goo91G" rel="noreferrer" target="_blank">https://godbolt.org/z/goo91G</a> Nathan > > > > 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 <<a href="mailto:ndesaulniers@google.com" target="_blank">ndesaulniers@google.com</a>> > > -- > Thanks, > ~Nick Desaulniers </blockquote></div> <div> </div>-- <div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr">Thanks,<div>~Nick Desaulniers</div></div></div></div>