Re: [PATCH][next] drm/nouveau/outp: Avoid -Wflex-array-member-not-at-end warning

[Gustavo A. R. Silva]

On 28/03/25 09:05, Danilo Krummrich wrote:
> On Fri, Mar 28, 2025 at 08:45:32AM -0600, Gustavo A. R. Silva wrote:
> > -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> > getting ready to enable it, globally.
> >
> > Use the `DEFINE_RAW_FLEX()` helper for an on-stack definition of
> > a flexible structure where the size of the flexible-array member
> > is known at compile-time, and refactor the rest of the code,
> > accordingly.
> >
> > So, with these changes, fix the following warning:
> >
> > drivers/gpu/drm/nouveau/nvif/outp.c:199:45: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> >
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/nouveau/nvif/outp.c | 16 +++++++---------
> >   1 file changed, 7 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/nouveau/nvif/outp.c b/drivers/gpu/drm/nouveau/nvif/outp.c
> > index 6daeb7f0b09b..403cf16d5e84 100644
> > --- a/drivers/gpu/drm/nouveau/nvif/outp.c
> > +++ b/drivers/gpu/drm/nouveau/nvif/outp.c
> > @@ -195,20 +195,18 @@ nvif_outp_dp_aux_pwr(struct nvif_outp *outp, bool enable)
> >   int
> >   nvif_outp_hda_eld(struct nvif_outp *outp, int head, void *data, u32 size)
> >   {
> > -	struct {
> > -		struct nvif_outp_hda_eld_v0 mthd;
> > -		u8 data[128];
> > -	} args;
> > +	DEFINE_RAW_FLEX(struct nvif_outp_hda_eld_v0, mthd, data, 128);
> >   	int ret;
> >   
> > -	if (WARN_ON(size > ARRAY_SIZE(args.data)))
> > +	if (WARN_ON(size > 128))
>
> Seems a bit unfortunate that the size is duplicated here.

For now, I think we can do something like this instead:

-       if (WARN_ON(size > ARRAY_SIZE(args.data)))
+       if (WARN_ON(size > (__struct_size(mthd) - sizeof(*mthd))))

Thanks
-Gustavo

> Can we have an accessor macro to derive the size for us?
>
> 	union {
> 		u8 bytes[struct_size_t(type, member, count)];
> 		type obj;
> 	} name##_u initializer;
>
> Maybe a macro that returns the size difference between bytes and obj?
>
> >   		return -EINVAL;
> >   
> > -	args.mthd.version = 0;
> > -	args.mthd.head = head;
> > +	mthd->version = 0;
> > +	mthd->head = head;
> >   
> > -	memcpy(args.data, data, size);
> > -	ret = nvif_mthd(&outp->object, NVIF_OUTP_V0_HDA_ELD, &args, sizeof(args.mthd) + size);
> > +	memcpy(mthd->data, data, size);
> > +	ret = nvif_mthd(&outp->object, NVIF_OUTP_V0_HDA_ELD, mthd,
> > +			__struct_size(mthd) + size);
> >   	NVIF_ERRON(ret, &outp->object, "[HDA_ELD head:%d size:%d]", head, size);
> >   	return ret;
> >   }
> > --
> > 2.43.0