Re: [PATCH] media: hevc: fix pictures lists type
From: Benjamin Gaignard
Date: Fri Aug 27 2021 - 07:35:53 EST
Le 27/08/2021 à 12:10, John Cox a écrit :
Le 26/08/2021 à 18:09, Nicolas Dufresne a écrit :
Le lundi 23 août 2021 à 12:35 +0100, John Cox a écrit :
Hi
Le 23/08/2021 à 11:50, John Cox a écrit :
The lists embedded Picture Order Count values which are s32 so their type
most be s32 and not u8.
I'm not convinced that you can't calculate all of those lists from the
info already contained in the DPB array so this is probably redundant
info though I grant that having the list pre-calced might make your life
easier, and the userland side will have calculated the lists to
calculate other required things so it isn't much extra work for it.
Yes the userland have already compute these lists and the number of items
in each of them.
Build them in the kernel would means to also compute the values of NumPocStCurrBefore,
NumPocStCurrAfter, NumPocLtCurr, NumPocStCurrAfter, NumPocStCurrBefore and NumPocLtCurr
and that requires information (NumNegativePics, NumPositivePics...) not provided to the kernel.
Since it have to be done in userland anyway, I'm reluctant to modify the API to redo in the kernel.
Well, fair enough, I'm not going to argue
Even if you do need the lists wouldn't it be a better idea to have them
as indices into the DPB (you can't have a frame in any of those lists
that isn't in the DPB) which already contains POCs then it will still
fit into u8 and be smaller?
Hantro HW works with indexes but I think it is more simple to send PoC rather than indexes.
I'd disagree but as I don't use the info I'm not concerned. Though I
think I should point out that when Hantro converts the POCs to indicies
it compares the now s32 POC in these lists with the u16 POC in the DPB
so you might need to fix that too; by std (8.3.1) no POC diff can be
outside s16 so you can mask & compare or use u16 POCs in the lists or
s32 in the DPB.
Fun fact, my interpretation with the API when I drafted GStreamer support was
that it was DPB indexes:
https://gitlab.freedesktop.org/ndufresne/gst-plugins-bad/-/blob/hevc_wip/sys/v4l2codecs/gstv4l2codech265dec.c#L850
It felt quite natural to be, since this is also how we pass references for l0/l1
(unused by hantro I guess).
Looking at old rkvdec code as a refresher:
for (j = 0; j < run->num_slices; j++) {
sl_params = &run->slices_params[j];
dpb = sl_params->dpb;
hw_ps = &priv_tbl->rps[j];
memset(hw_ps, 0, sizeof(*hw_ps));
for (i = 0; i <= sl_params->num_ref_idx_l0_active_minus1; i++) {
WRITE_RPS(!!(dpb[sl_params->ref_idx_l0[i]].rps == V4L2_HEVC_DPB_ENTRY_RPS_LT_CURR),
REF_PIC_LONG_TERM_L0(i));
WRITE_RPS(sl_params->ref_idx_l0[i], REF_PIC_IDX_L0(i));
}
for (i = 0; i <= sl_params->num_ref_idx_l1_active_minus1; i++) {
WRITE_RPS(!!(dpb[sl_params->ref_idx_l1[i]].rps == V4L2_HEVC_DPB_ENTRY_RPS_LT_CURR),
REF_PIC_LONG_TERM_L1(i));
WRITE_RPS(sl_params->ref_idx_l1[i], REF_PIC_IDX_L1(i));
}
This is code is clearly unsafe, but now I remember that dpb_entry has a flag
"rps". So we know from the DPB in which of the list the reference lives, if any.
In the case of RKVDEC the HW only cares to know if this is long term or not.
So without looking at the spec, is that dpb represention enough to reconstruct
these array ? If we pass these array, shall we keep the rps flag ? I think a
little step back and cleanup will be needed. I doubt there is a single answer,
perhaps list what others do (VA, DXVA, NVDEC, Khronos, etc) and we can
collectively decide were we want V4L2 to sit ?
I have done some tests with Hantro driver and look at the spec, the order of the PoC
in the reference lists matters. You can deducted the order for DPB rps flags.
I would suggest to remove rps flags to avoid information duplication.
I want the DPB rps member for long term reference marking. I don't care
about before / after, but LTR can't be deduced from PoC and if you are
going to keep the member you might as well keep before / after.
Ok so keep like it is.
In this case my patch is enough, right ?
Benjamin
John Cox
Benjamin
Regards
John Cox
Benjamin
Full disclosure: Pi decode doesn't use this info at all so I'm only
arguing from a theoretical point of view - I think it is only relevant
if your h/w is parsing the reference list setups.
Regards
John Cox
Reported-by: John Cox <jc@xxxxxxxxxxxxx>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxxxxx>
---
Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst | 6 +++---
include/media/hevc-ctrls.h | 6 +++---
2 files changed, 6 insertions(+), 6 deletions(-)
diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
index 976d34445a24..db9859ddc8b2 100644
--- a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
+++ b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
@@ -3323,15 +3323,15 @@ enum v4l2_mpeg_video_hevc_size_of_length_field -
* - __u8
- ``num_poc_lt_curr``
- The number of reference pictures in the long-term set.
- * - __u8
+ * - __s32
- ``poc_st_curr_before[V4L2_HEVC_DPB_ENTRIES_NUM_MAX]``
- PocStCurrBefore as described in section 8.3.2 "Decoding process for reference
picture set.
- * - __u8
+ * - __s32
- ``poc_st_curr_after[V4L2_HEVC_DPB_ENTRIES_NUM_MAX]``
- PocStCurrAfter as described in section 8.3.2 "Decoding process for reference
picture set.
- * - __u8
+ * - __s32
- ``poc_lt_curr[V4L2_HEVC_DPB_ENTRIES_NUM_MAX]``
- PocLtCurr as described in section 8.3.2 "Decoding process for reference
picture set.
diff --git a/include/media/hevc-ctrls.h b/include/media/hevc-ctrls.h
index 781371bff2ad..04cd62e77f25 100644
--- a/include/media/hevc-ctrls.h
+++ b/include/media/hevc-ctrls.h
@@ -219,9 +219,9 @@ struct v4l2_ctrl_hevc_decode_params {
__u8 num_poc_st_curr_before;
__u8 num_poc_st_curr_after;
__u8 num_poc_lt_curr;
- __u8 poc_st_curr_before[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
- __u8 poc_st_curr_after[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
- __u8 poc_lt_curr[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
+ __s32 poc_st_curr_before[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
+ __s32 poc_st_curr_after[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
+ __s32 poc_lt_curr[V4L2_HEVC_DPB_ENTRIES_NUM_MAX];
__u64 flags;
};