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SubjectRe: [PATCH 7/9] media: hantro: Add core bits to support H264 decoding
On Thu, 25 Jul 2019 15:31:41 +0200
Rasmus Villemoes <linux@rasmusvillemoes.dk> wrote:

> On 19/06/2019 14.15, Boris Brezillon wrote:
> > From: Hertz Wong <hertz.wong@rock-chips.com>
> >
> > Add helpers and patch hantro_{drv,v4l2}.c to prepare addition of H264
> > decoding support.
> >
> > Signed-off-by: Hertz Wong <hertz.wong@rock-chips.com>
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> > ---
> > +
> > + /*
> > + * Short term pics in descending pic num order, long term ones in
> > + * ascending order.
> > + */
> > + if (!(a->flags & V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM))
> > + return b->frame_num - a->frame_num;
> > +
> > + return a->pic_num - b->pic_num;
> > +}
>
> Pet peeve: This works because ->frame_num and ->pic_num are u16, so
> their difference is guaranteed to fit in an int.
>
> > +static int b0_ref_list_cmp(const void *ptra, const void *ptrb, const void *data)
> > +{
> > + const struct hantro_h264_reflist_builder *builder = data;
> > + const struct v4l2_h264_dpb_entry *a, *b;
> > + s32 poca, pocb;
> > + u8 idxa, idxb;
> > +
> > + idxa = *((u8 *)ptra);
> > + idxb = *((u8 *)ptrb);
> > + a = &builder->dpb[idxa];
> > + b = &builder->dpb[idxb];
> > +
> > + if ((a->flags & V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM) !=
> > + (b->flags & V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM)) {
> > + /* Short term pics firt. */
> > + if (!(a->flags & V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM))
> > + return -1;
> > + else
> > + return 1;
> > + }
> > +
> > + /* Long term pics in ascending pic num order. */
> > + if (a->flags & V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM)
> > + return a->pic_num - b->pic_num;
> > +
> > + poca = builder->pocs[idxa];
> > + pocb = builder->pocs[idxb];
> > +
> > + /*
> > + * Short term pics with POC < cur POC first in POC descending order
> > + * followed by short term pics with POC > cur POC in POC ascending
> > + * order.
> > + */
> > + if ((poca < builder->curpoc) != (pocb < builder->curpoc))
> > + return poca - pocb;
> > + else if (poca < builder->curpoc)
> > + return pocb - poca;
> > +
> > + return poca - pocb;
> > +}
>
> Here, however, poca and pocb are ints. What guarantees that their values
> are not more than 2^31 apart?

Good question. Both should normally be >= 0, which I guess prevents the
s32 overflow. This being said, it's something passed by userspace, and
I don't think we check the value (yet).

> I know absolutely nothing about this code
> or what these numbers represent, so it may be obvious that they are
> smallish.

Well, a safe approach would be to replace those subtraction by a
ternary operator.

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