Re: [PATCH v2 4/4] arm64: dts: qcom: x1-dell-thena: bump linux,cma to 256 MiB

[Rob Clark]

On Wed, May 20, 2026 at 8:55 PM Val Packett <val@xxxxxxxxxxxx> wrote:
>
>
> On 5/20/26 10:09 PM, Michael Scott wrote:
> > The 128 MiB linux,cma reserved-memory pool on dell-thena is too small
> > to support the camera pipeline in parallel with the normal Linux
> > desktop. On a freshly-booted system with GNOME running, the typical
> > runtime consumers — msm DRM framebuffers (Wayland triple buffering on
> > the eDP panel), qcom_iris video codec buffers, qcom_camss VFE
> > pre-allocated buffers — already occupy ~100 MiB of the pool, leaving
> > only ~25 MiB free.
>
> Huh, I'm surprised that drm framebuffers use CMA… IIRC, msm drm can work
> fine without a cma node present at all.
>
> Indeed, with a desktop on a 4K monitor I'm seeing..
>
> CmaTotal:         131072 kB
> CmaFree:            1704 kB

Is something in userspace allocating from dma-heap and importing into
drm/msm?  We shouldn't otherwise be allocating from CMA, at least not
intentionally.

(I also dislike specifying CMA in dtb, since that seems more like
describing use-case than describing hw..)

BR,
-R

> > The libcamera "simple" pipeline handler used by /dev/media0 on
> > dell-thena allocates four ABGR8888 frames at 1920×1088 = 32 MiB total.
> > That request fails on the fourth frame:
> >
> >      ERROR DmaBufAllocator: dma-heap allocation failure for frame-3
> >      ERROR Allocator: Stream is not part of /base/.../camera@10 active configuration
> >      Can't allocate buffers
> >      Failed to start camera session
> >
> > resulting in gnome-snapshot's "Could not play camera stream" and any
> > other libcamera-mediated app being unable to actually stream.
>
> ..however I couldn't reproduce any failures, Snapshot started up just
> fine, lowering CmaFree to 300 kB.
>
> I have even launched both Snapshot and ffplay with a 4K AV1 video
> through av1_v4l2m2m, CmaFree went all the way down to zero but there
> were no errors whatsoever, both worked simultaneously just fine. o_0
>
> I think drm buffers might just get evicted from that area or something?
>
> > Bumping linux,cma to 256 MiB (a 0.9% reservation on these laptops'
> > typical 27 GiB RAM) leaves ~150 MiB free at runtime — sufficient for
> > the libcamera buffer set plus headroom for video playback or other
> > CMA-hungry workloads in parallel.
> >
> > Tested on Dell Latitude 7455: with the 256 MiB pool, CmaFree at
> > GNOME-desktop idle is ~150 MiB, gnome-snapshot streams the OV02E10
> > camera cleanly, and `cam -c 1 --capture=2` succeeds.
> >
> > The companion board files dell-inspiron-14-plus-7441 and the upstream
> > .dts variants inherit from x1-dell-thena.dtsi, so this changes the
> > pool size for every dell-thena-based laptop in one place.
>
>
> In any case, that's not an objection of course, just wondering why it's
> working fine for me and not for you..
>
> Acked-by: Val Packett <val@xxxxxxxxxxxx>
>
> ~val
>
>