On Sat 05 Oct 19, 23:21, Tomasz Figa wrote: > On Sat, Oct 5, 2019 at 11:12 PM Paul Kocialkowski > <paul.kocialkowski@xxxxxxxxxxx> wrote: > > > > On Sat 05 Oct 19, 22:54, Tomasz Figa wrote: > > > On Sat, Oct 5, 2019 at 10:39 PM Paul Kocialkowski > > > <paul.kocialkowski@xxxxxxxxxxx> wrote: > > > > > > > > Hi, > > > > > > > > On Sat 05 Oct 19, 17:22, Tomasz Figa wrote: > > > > > On Fri, Oct 4, 2019 at 6:12 AM Paul Kocialkowski > > > > > <paul.kocialkowski@xxxxxxxxxxx> wrote: > > > > > > > > > > > > Hi, > > > > > > > > > > > > On Thu 05 Sep 19, 13:42, Philipp Zabel wrote: > > > > > > > To explain why num_ref_idx_active_override_flag is not part of the API, > > > > > > > describe how the num_ref_idx_l[01]_active_minus1 fields and the > > > > > > > num_ref_idx_l[01]_default_active_minus1 fields are used, depending on > > > > > > > whether the decoder parses slice headers itself or not. > > > > > > > > > > > > Is there any particular reason why this is preferable to exposing the flag? > > > > > > It feels like having the flag around sticks closer to the bitstream, > > > > > > so it's more straightforward for everyone. > > > > > > > > > > > > In case there's only one set of fields exposed by the hardware (and it doesn't > > > > > > do slice parsing itself), we could always check the flag in the driver and use > > > > > > either the default PPS values or the slice-specific values there. > > > > > > > > > > > > What do you think? > > > > > > > > > > IMHO it would only add further logic to the drivers, because they > > > > > would need to have a conditional block that selects default or > > > > > per-slice value based on the flag. The goal was to be able for the > > > > > driver to just passively write num_ref_idx_l[01]_active_minus1 (or the > > > > > default one for slice-parsing hardware) to corresponding hardware > > > > > registers. > > > > > > > > Well, the Allwinner block has both set of fields and a field for the flag, > > > > so in this case I find that it is cleaner to just copy the values and flag > > > > rather than always setting the flag even when it's the default value used. > > > > > > > > More generally, the two sets + flag are closer to bitstream which feels less > > > > confusing than re-purposing these fields from the slice header to fit the > > > > result of flag ? per-slice : per-picture. > > > > > > > > > We're talking about kernel drivers here and for security reasons any > > > > > logic should be reduced to the strictly necessary minimum. > > > > > > > > I definitely agree that bitstream parsing in the kernel is to be avoided for > > > > security reasons (among other things), but don't see the harm in considering > > > > the flags in-driver if needed. We do it for a number of other flags already > > > > (and strongly need to). > > > > > > If the fields are well documented, does it really matter? I'd suggest > > > just keeping it as is, rather than overpolishing things and preventing > > > the interface from stabilizing. > > > > I just don't see the benefit of changing the purpose of a bitstream element. > > Even with documentation, it adds some unnecessary confusion and I find this to > > be a complicated enough topic without it ;) > > > > Especially for the case of hardware that does slice header parsing itself, it > > feels particularly unsettling to have to take the default PPS values for the > > fields from the slice header control rather than PPS. > > num_ref_idx_l[01]_default_active_minus1 are members of v4l2_ctrl_h264_pps. Oh right, they are currently still here at the moment. So I'm just advocating for adding the flag then. I was under the impression that only the set from the slice header was still around. Thanks! > > > > The bottomline is that we have use cases for each of the two set of fields > > independently, so I feel like this is reason enough to avoid mixing them > > together. > > What do you mean by mixing together? Hardware parsing the slices > always uses num_ref_idx_l[01]_default_active_minus1 from the PPS. > Hardware not parsing the slices always sets override to 1 and uses > num_ref_idx_l[01]_active_minus1 from the slice header struct. Well, just specifying that the per-slice set takes values from the PPS set if the flag is not there in the original bitstream is mixing up both fields. What I mean is that the per-slice value is not specified to take the PPS value as a fallback in bitstream syntax: that's why there are two distinct sets of elements. Adding the flag avoids mixing them up and sticks closer to bitstream. > > We are still in the process of polishing the API anyway, so now feels like a > > good time to change these things. > > Hmm, it seemed to me like things already calmed down and we were just > polishing the documentation. I was convinced we were actually about to > destage things. Are you aware of some other changes coming? I believe the next step is to go through some bitstream coverage testing before we can have a clearer idea of whether the current controls are ready to be stabilized or not. I also feel like I haven't looking into existing and possible H.264 features enough to have a clear idea of whether we're missing something or not. I'm also under the impression that there wasn't strong feedback about the control fields either so I feel like it would be best to be careful here. What do you think? Cheers, Paul -- Paul Kocialkowski, Bootlin Embedded Linux and kernel engineering https://bootlin.com
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