[PATCH v11 01/19] libcamera: property: Add MinimumRequests property

Mon May 5 14:23:13 CEST 2025

Hi Jacopo,

On 5/2/25 18:28, Jacopo Mondi wrote:
> Hi Sevn,
>    thanks for resurecting this!
>
> Can I ask if this solves any actual problem and on which platform ?

The problem I'm facing is that in a more complex setup with the rkisp1 
we need to keep multiple dmabufs in the userspace. But the hardcoded 
value of 4 buffers in the rkisp1 driver seems to expect that the 
application only hold around one buffer and have the other 3 buffers 
queued for a smooth frame capture. But due to holding more dmabufs, I'm 
running into massive frame drops due to usually only having one request 
queued and waiting for it to finish.

Therefore I want to allocate more dmabufs for this problematic use case 
and avoid patching libcamera to increase the hardcoded buffer count of 
rkisp1 (which would also apply globally).

Kieran then pointed me towards this patchset to allow the application to 
control the amount of dmabufs allocated.

> On Mon, Apr 28, 2025 at 11:02:26AM +0200, Sven Püschel wrote:
>> From: Nícolas F. R. A. Prado <nfraprado at collabora.com>
>>
>> The MinimumRequests property reports the minimum number of capture
>> requests that the camera pipeline requires to have queued in order to
>> sustain capture operations without frame drops. At this quantity,
>> there's no guarantee that manual per-frame controls will apply
>> correctly. The quantity needed for that may be added as a separate
>> property in the future.
>>
>> The mali-c55 driver defines min_queued_buffers = 1 [1]. Therefore set
>> set the minimum requests to 2 to account one request in the userspace.
>>
>> The dcmipp and j721e drivers both defines min_queued_buffers = 1
>> in the kernel under
>> drivers/media/platform/st/stm32/stm32-dcmipp/dcmipp-bytecap.c
>> and drivers/media/platform/ti/j721e-csi2rx/j721e-csi2rx.c .
>> Therefore use these values, as they are added 1 more.
>>
>> For the intel-ipu6, mtk-seninf simple devices and the virtual pipeline
>> use a conservative value of 3 minimumBuffers, as no further requirements
>> are known about them.
>>
>> [1] https://lore.kernel.org/linux-media/20240131174355.GB20792@pendragon.ideasonboard.com/T/#t
>>
>> Signed-off-by: Nícolas F. R. A. Prado <nfraprado at collabora.com>
>> Signed-off-by: Paul Elder <paul.elder at ideasonboard.com>
>> Acked-by: Umang Jain <umang.jain at ideasonboard.com>
>> Signed-off-by: Sven Püschel <s.pueschel at pengutronix.de>
>>
>> ---
>> Changes in v11
>> - Add mali-c55, dcmipp, virtual, j721e, intel-ipu6 and mtk-seninf
>> - Hold only the minimum buffers instead of the whole deviceInfo pointer in SimpleCameraData
>> - Adjusted min_buffers_needed property to min_reqbufs_allocation in docs
>> - Relax property description from no frame drops -> only minimal frame
>>    drops, based on the comment from Naush [10]
>> - Changed property type from int32_t -> uint32_t to match all of the
>>    indirect casts to an unsigned value throughout the existing patchset.
>> - Removed Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
>> - Removed Reviewed-by: Paul Elder <paul.elder at ideasonboard.com>
>>
>> [10] https://lists.libcamera.org/pipermail/libcamera-devel/2022-December/035872.html
>>
>> Changes in v10:
>> - ipu3: add a constant to populate MinimumRequests, as we'll also use it
>>    elsewhere
>> - update pipeline handler guide to set vivid' MinimumRequests to 2
>> - expand the MinimumRequests property description to include a line
>>    about startup
>> - add isi
>>
>> Changes in v9:
>> - rebased
>>
>> Changes in v8:
>> - Changed the MinimumRequests property meaning to require that frames aren't
>>    dropped
>> - Set MinimumRequests on SimplePipeline depending on device and usage of
>>    converter
>> - Undid definition of default MinimumRequests on CameraSensor constructor
>> - Updated pipeline-handler guides with new MinimumRequests property
>>
>> Changes in v7:
>> - Renamed property from MinNumRequests to MinimumRequests
>> - Changed MinimumRequests property's description
>>
>> Changes in v6:
>> - Removed comment from Raspberrypi MinNumRequests setting
>> - Removed an extra blank line
>> ---
>>   Documentation/guides/pipeline-handler.rst     | 20 ++++++---
>>   src/libcamera/pipeline/imx8-isi/imx8-isi.cpp  |  2 +
>>   src/libcamera/pipeline/ipu3/ipu3.cpp          |  4 ++
>>   src/libcamera/pipeline/mali-c55/mali-c55.cpp  |  1 +
>>   src/libcamera/pipeline/rkisp1/rkisp1.cpp      |  1 +
>>   .../pipeline/rpi/common/pipeline_base.cpp     |  2 +
>>   src/libcamera/pipeline/simple/simple.cpp      | 41 +++++++++++++++----
>>   src/libcamera/pipeline/uvcvideo/uvcvideo.cpp  |  2 +
>>   src/libcamera/pipeline/vimc/vimc.cpp          |  2 +
>>   src/libcamera/pipeline/virtual/virtual.cpp    |  1 +
>>   src/libcamera/property_ids_core.yaml          | 22 ++++++++++
>>   11 files changed, 85 insertions(+), 13 deletions(-)
>>
> [snip]
>
>> index 834454a4..cc28d677 100644
>> --- a/src/libcamera/property_ids_core.yaml
>> +++ b/src/libcamera/property_ids_core.yaml
>> @@ -701,4 +701,26 @@ controls:
>>
>>           Different cameras may report identical devices.
>>
>> +  - MinimumRequests:
> I'm still not at ease with the definition of this property, I'm
> afraid.
>
> It seems to be mixing two different things (maybe 3 :)
>> +      type: uint32_t
>> +      description: |
>> +        The minimum number of capture requests that the camera pipeline requires
>> +        to have queued in order to sustain capture operations with only minimal
>
> The reference to "minimal frame drops" is a bit too loosely
> specified ?

I've changed it due to Naushir's claim that there is no practical number 
to gurantee no frame drops [1]

[1] 
https://lists.libcamera.org/pipermail/libcamera-devel/2022-December/035872.html

>> +        frame drops. At this quantity, there's no guarantee that manual per
>> +        frame controls will apply correctly. This is also the minimum number of
> The per-frame control pipeline depth does depend on other factors
> such as the sensor delays, specifically for manual AE/AWB control

and that is the reason it isn't or cannot be part of this property? Or 
what's your point?

>> +        requests that must be queued before capture starts.
> The number of buffers required to start streaming comes directly from
> the min_queued_buffers variable defined by drivers (unfortunatetly not
> available to users through any uAPI). One thing I would like to see
> happening is drivers stop setting min_queued_buffers at all, or if not
> possible maybe expose it through an api (a RO v4l2 control ?).
>
> Drivers should allocate internal buffers and use them if the
> application doesn't keep, and handle both the runtime underruns and
> start streaming conditions using scratch buffers.
>
> Even if this won't happen in drivers, the requirement should not be
> exposed to libcamera users. In the "great scheme of things" the idea
> is to decouple application's Requests from the frames pipeline, and
> providing enough buffers to sustain the frame rate should be a
> pipeline/IPA job.
>
> True, for platforms where drivers already require min_queued_buffers
> applications right now have to queue enough requests before
> start receiving frames back, and we goofly report it through
> StreamConfiguration::bufferCount.
>
> Unless there's an actual use case or any angle I'm grossly
> missing I don't think this property is a good idea..

I understand your problem with this property. But I don't know if we can 
get away without providing the application a hint how many dmabufs it 
should allocate at least.

To recall my problem, the rkisp1 driver seems to already use scratch 
buffers when it doesn't have enough buffers queued. So while queuing one 
buffer at a time worked, it wasn't desired due to the frame drops from 
the scratch buffers.

But the application developer cannot really know how many buffers are 
really required. From my perspective the current operation mode is that 
libcamera allocates a sensible fixed amount of dmabufs and application 
developers just queue all unused buffers to libcamera [2]. So when the 
allocation is now put into the hands of the application developer, it 
may just always allocate 2 buffers (to have still one queued when 
handling a completed buffer) and therefore only queue a maximum of 2 
buffers. The driver fills the lack of buffers with scratch buffers, 
which cause unintended frame drops.

One potential solution may be to pass the desired amount of buffers held 
in the application to the allocator. Then the pipeline could allocate a 
sensible amount of buffers (e.g. 1 requested for the application + 3 
queued => 4 buffers) and the application would queue all unused buffers 
as usual.

Other than that I don't see how libcamera could abstract this property 
away, as applications control which buffers are used for a given request.

[2] 
https://www.libcamera.org/guides/application-developer.html#request-queueing 

>> +        This property is based on the minimum number of memory buffers
>> +        needed to fill the capture pipeline composed of hardware processing
>> +        blocks plus as many buffers as needed to take into account propagation
>> +        delays and avoid dropping frames.
>> +
>> +        \todo Should this be a per-stream property?
>> +
>> +        \todo Extend this property to expose the different minimum buffer and
>> +        request counts (the minimum number of buffers to be able to capture at
>> +        all, the minimum number of buffers to sustain capture without frame
>> +        drop, and the minimum number of requests to ensure proper operation of
>> +        per-frame controls).
>> +
>>   ...
>> --
>> 2.49.0
>>