Re: [RFC PATCH 0/3] media: rockchip: VEPU510 H.264 encoder for RK3576

Hi Nicolas, Paul, Detlev,

Thanks both for laying out the trade-offs so clearly.

Decision from my side: I'll converge on the DRM-ish / Vulkan Video
direction and sync with Detlev, rather than push the stateful V4L2
driver further. Nicolas' point about UAPI lock-in is the deciding one
for me -- I don't want to expose an encoder UAPI that then has to be
supported forever -- and if RK3576 is just a minor delta on Detlev's
RK3588 base, that's exactly where my hardware work is worth the most.

Paul -- thank you for the generous offer. I read both your Media Summit
decks and your posted series ([PATCH 00/14], the generic in-kernel
h264-enc core + rbsp + rate control on VC8000E); it's clearly further
along than I'd assumed. I'm not closing the door on a V4L2 version
later, but I'd rather not commit to maintaining two drivers up front, so
I'll treat that as a possible follow-up, not the primary path.

Where I think I can be useful right now is the hardware, since that part
is shared with RK3588 regardless of interface:

> I know in his case he was trying to avoid reconstructed frame
> compression, and it only started working when he enable that
> compression. Though, he had mmu faults prior to that.

Useful data point, but let me be precise about where I already am, so I
don't send Detlev chasing something I've done. This driver already runs
with reconstruction compression enabled (enc_pic.rec_fbc_dis = 0) -- the
state Detlev needed -- and the first inter frame still hangs. Disabling
it (rec_fbc_dis = 1) was one of my experiments and also hung, so FBC
state alone doesn't explain my stall.

The part that does line up is the mmu fault. I still have a residual
rk_iommu write fault on this path; I traced it from a boot-varying
garbage IOVA down to a benign IOVA 0, and I'd concluded it was a
separate AXI transaction from the recon write, independent of the
P-frame hang. Detlev's report -- that his mmu faults and his missing
P-frames cleared together -- is a direct reason to distrust that
"independent" conclusion and re-check whether the fault and the stall
share a root cause on RK3576 too.

Detlev -- when you have a moment: on RK3588, were the mmu fault and the
P-frame failure the same underlying problem? And did enabling
reconstruction compression clear the fault on its own, or did the
reference-read mapping (how the previous frame's reconstruction is
mapped for the encoder to fetch) need a separate change as well?
Whatever you can share, I'm happy to test on RK3576 and feed back a
Tested-by.

One point on the current code, since it came up:

> A lot of the code is hex tables generated from inspecting a running
> driver.

Fair, and I won't pretend otherwise -- the PARAM/SQI classes are
currently shipped as fixed tables taken from a captured encode rather
than computed. The one thing I'd add is that those particular values are
mpp's public default-tuning tables (the RDO lambda/cost and
subjective-quality tables in Rockchip's open-source mpp HAL, Apache-2.0),
so they're constants I can cite to mpp source rather than opaque state.
But I fully agree the driver needs significant cleanup before it's
upstream-worthy, and in the DRM-ish model most of that tuning moves to
userspace anyway, so a lot of it won't survive the transition in its
current form.

So: please point me at Detlev's shared branch / early code whenever it's
in a shape to build against, and I'll start porting the RK3576 hardware
bring-up onto it.

Thanks,
Jiaxing

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