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fix(linux): recover the DMA-BUF plane size the producer leaves at zero - #319
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The DMA-BUF import path #299 added has never run: mutter and sway both offer shared memory, so params[0] wins and the memfd branch is taken. Forcing the dmabuf object (new OPENSCREEN_PIPEWIRE_FORCE_DMABUF, see below) shows the import fails on the very first buffer. wlroots hands out planes with maxsize = 0 — legitimate, since the size of a dmabuf belongs to the exporting buffer and not to the SPA descriptor. osc_map_dmabuf refused on that and returned NULL *before calling mmap*, and the failure was reported as "this driver does not allow CPU mapping of the capture buffer". The GPU was blamed for a refusal it was never asked for. Measured on AMD Mendocino / amdgpu, sway 1.9 via xdg-desktop-portal-wlr: [osc-dmabuf] refused before mmap: fd=23 len=0 lseek_size=3686400 [osc-dmabuf] maxsize=0, recovered 3686400 bytes via lseek [osc-dmabuf] mmap fd=23 len=3686400 ok 3686400 is exactly 1280x720x4. The size is recoverable by seeking the fd, which is the documented way, and the mmap then succeeds. Also here: - The resolved length is stored in dmabuf_maps[].len. It used to store the producer's 0, leaving every later bounds check and the munmap on an empty mapping. - The failure reason is carried up instead of assumed, so "no fd", "size unknown" and "driver refused" no longer read as the same thing. - OPENSCREEN_PIPEWIRE_DEBUG traces the negotiated modifier and each mapping. Without it there is no way to tell whether a capture used this path at all. - OPENSCREEN_PIPEWIRE_FORCE_DMABUF drops the shm object so a compositor that offers both is forced onto DMA-BUF. Test-only, and the only way to exercise any of this without the compositor from #287. - Corrects the claim that every Smithay/wlroots compositor lacks a memfd path. sway 1.9 has one and takes it. 56 crate tests pass, enum_format_survives_a_dmabuf_only_producer included. Not verified: whether niri reports maxsize = 0 as wlroots does, so this may not be the whole of #287. And no frames are encoded under nested sway — an empty desktop generates almost no damage — so osc_read_frame's dmabuf arm is still unexercised beyond the mapping itself.
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Follow-up to #299, whose import half turns out never to have run.
How this was found
#299 fixed the negotiation half — advertising
SPA_FORMAT_VIDEO_modifier— and that half is well covered offline byenum_format_survives_a_dmabuf_only_producer. The import half had no coverage, so I tried to exercise it locally.It cannot be reached by any compositor available here:
uses_dmabuf=0,dataType: MemFd— even after #299uses_dmabuf=0,dataType: MemFdThat is correct behaviour, not a bug: the helper publishes shm first and DMA-BUF second, and
pw_streamtreats the array as a preference list, so any producer able to do shm winsparams[0]. It also means the DMA-BUF branch is unreachable outside the compositor of #287.The bug
Forcing the dmabuf object (
OPENSCREEN_PIPEWIRE_FORCE_DMABUF, added here) gets DMA-BUF negotiated — and the import fails on the first buffer:wlroots reports
maxsize = 0. That is legitimate — the size of a dmabuf is a property of the exporting buffer, not of the SPA descriptor — butosc_map_dmabufrefused on it and returned NULL before calling mmap. The user then saw:The driver was blamed for a refusal it was never asked for. That message is what a bug report from a niri user would have contained, pointing whoever picked it up at the GPU.
The fix, measured
3686400is exactly 1280×720×4 — the size is right there on the fd. Seeking it and retrying:AMD Mendocino / amdgpu, sway 1.9, xdg-desktop-portal-wlr 0.7.1. So CPU mapping does work on this driver; the size was the whole obstacle.
Also in here
dmabuf_maps[].lenused to take the producer's0, leaving every later bounds check and themunmappointed at an empty mapping.OPENSCREEN_PIPEWIRE_DEBUGtraces the negotiated modifier and every mapping. Without it a capture reports identical success whichever path it took, which is why this went unnoticed.OPENSCREEN_PIPEWIRE_FORCE_DMABUFdrops the shm object. Test-only, and the only way to exercise any of this without niri — worth keeping for the same reason it was needed today.What this does not settle
maxsize = 0as wlroots does. If it does not, this is not the whole of [Bug]: Screen recording fails on Niri/Wayland with PipeWire (no more input formats) #287.osc_read_frame's dmabuf arm beyond the mapping: no frames are encoded under nested sway, an empty desktop producing almost no damage. The mapping is proven; the read is not.56 crate tests pass.
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