Re: [PATCH 00/14] staging: vc04_services: bcm2835-isp support

From: Stefan Wahren
Date: Fri Dec 02 2022 - 07:42:14 EST

Hi Dave,

Am 02.12.22 um 12:23 schrieb Dave Stevenson:
Hi Laurent, Umang, and Stefan.

On Fri, 2 Dec 2022 at 09:17, Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
Hi Umang,

On Fri, Dec 02, 2022 at 11:57:18AM +0800, Umang Jain wrote:
On 12/2/22 6:45 AM, Stefan Wahren wrote:
Am 30.11.22 um 11:58 schrieb Umang Jain:
On 11/27/22 6:56 AM, Stefan Wahren wrote:
Am 26.11.22 um 17:26 schrieb Umang Jain:
On 11/26/22 8:12 PM, Stefan Wahren wrote:
Am 21.11.22 um 22:47 schrieb Umang Jain:
This series aims to upport bcm2835-isp from the RPi kernel [1] and is a
independent subset of earlier series [2] posted to upport CSI-2/CCP2
receiver IP core("Unicam) + the ISP driver found in BCM283x and compatible
SoCs (namely BCM2711). Unicam is still under active development to work
with multistream support to get into mainline. Hence only the ISP driver
will remain the primary area of this series.
thanks for working on this. But honestly i would prefer that vchiq
comes out of staging before adding more features. As Greg said
some time ago staging is not a place to "dump code and run away".
These new files are in the same bad shape as the rest of vc04
before the clean-up here in staging started.
Certainly, I am not here to do that - but I am still learning the ropes.
no problem.

If the staging issue is becoming a blocker for bcm2835-isp going
upstream, I would be happy to help here! Though I must mention that
I still have limited visibility so my aim would be to chart out a
plan of things needed to be done to get vc04_services out of staging!
The vchiq driver is in staging since 2016, so every step forwards is
good. Unfortunately all of the low hanging fruits has been gathered.

For me the most important, but not to tricky steps to get vchiq out
of staging would be:

* Cleanup logging mechanism

* Get rid of custom function return values

There was already an attempt for this [1]

* Get rid of all non essential global structures and create a proper per
device structure

I agree that VCSM is on the TODO list for vchiq, but this driver
is not necessary for making bcm2835-audio & bcm2835-camera leave
staging. It just binds more resources on a new feature.
bcm2835-camera is the legacy camera stack which probably need to be
dropped from hereon...
I don't not know if there any users left, so i would be careful here.
Can bcm2835-isp completely replace bcm2835-camera? Sorry, for this
dumb question but i'm not expert here.
I am careful too here and probably need Input from RaspberryPi in order
to proceed to drop it. But from my perspective - bcm2835-camera is _not_
going out of staging - it'll either sit here (or probably dropped) as
statied from [1]

```
+ * There are two camera drivers in the kernel for BCM283x - this one
+ * and bcm2835-camera (currently in staging).
```

The bcm2835-camera is meant to be replaced by unicam [1] , but the ISP
(bcm2835-isp) is meant to be worked with unicam [1]. In fact, I have
mentioned in my cover the testing of bcm2835-isp happened on top of
unicam patches.
To be accurate, the bcm2835-camera driver supports the VC4
firmware-based camera stack. In that setup, the camera sensors (OV5647
or IMX219), CSI-2 receiver (Unicam) and ISP are all controlled by the
firmware, which provides a high-level interface towards the kernel. This
architecture has been replaced by Linux-side control of the camera
sensors (through existing drivers in drivers/media/i2c/), Unicam
(through the driver from [1]) and ISP (through this driver). Moving
control to the Linux side requires complex processing in userspace,
handled by libcamera.

bcm2835-camera is thus replaced by multiple drivers combined with
libcamera, and that is the camera stack that is shipped by Raspberry Pi
these days. While this may affect some userspace use cases), we will not
work on destaging bcm2835-camera, and as far as I'm aware, nobody else
is planning to do so either. I don't mind much if the driver stays in
staging for some more time, but I'd rather drop it if possible.
Thanks for clarification. Okay, so Unicam + bcm2835-isp are able to handle the old camera (OV5647)?
It would be reasonable to drop it at the point that Libcamera can work
to a similar level with at least the following list of applications:
- FFmpeg
- Gstreamer
- Chromium
- Firefox
- Motion
And that still leaves a huge number of existing V4L2 apps out in the cold.

Do you wish to make any predictions as to when that would be
achievable? Or even when a v1.0 release of libcamera is going to
happen?
Dropping anything prior to those points would be rather premature in my book.


The TODOs on bcm2835-camera are:
1) Zero copy. That comes almost for free as bcm2835-isp already does
this, but it does rely on vcsm-cma.
The main reason I haven't pushed it is that it then requires
reasonable amounts of CMA heap for all the buffers, which until
recently haven't been present in the default configurations. With the
vc4 DRM driver now being default (at least for the vendor kernel) and
also requiring CMA, making the change makes more sense.
AFAIK there is no easy way to have one driver choosing between using
vb2_vmalloc_memops and vb2_dma_contig_memops at runtime, but I may be
wrong.
Actually bcm2835_defconfig appears to only allocate a 32MB CMA heap,
so perhaps we don't get very far.
CMA configuration should actually happen in device tree or kernel cmdline. The bcm2835_defconfig is limited to make it work even with the original Pi.

2) This isn't workable within the current V4L2 frameworks. The
multi-planar V4L2 pixel formats are currently allocated as independent
buffers for each plane, whereas the firmware needs a single buffer
with (currently) specific offsets for the chroma planes. The
V4L2/videobuf2 core changes required to implement that are going to be
significant, and have minimal gain.
The specific stride handling is already dealt with (set bytesperline
appropriately), it's the padding of the height to a multiple of 16
before the chroma planes on YUV420 and NV12 formats that require the
firmware to do a small amount of repacking. The performance hit is
actually minimal anyway.

If bcm2835-camera is the only thing holding back vc04_services, then I
can have a look at it.

No, it's the vchiq interface which needs the work.

Thanks Stefan


Dave

[1]: https://lore.kernel.org/linux-media/20220208155027.891055-5-jeanmichel.hautbois@xxxxxxxxxxxxxxxx/

I see two TODO files in vc04_services:
./bcm2835-camera/TODO
./interface/TODO

One of the bcm2835-camera TODO points to the vc-sm-cma driver
itself. So that's address in the series. The other remaining one -
I will need to take a deeper look before commenting on it.

The main chunk of TODO are in vc04_services/interfaces/TODO. Doing
a cursory reading of them suggests that these apply to *all*
vc04_services components? Am I right?
Actually these applies just for the interfaces directory. Some of
them could apply to the services, but this is no priority.
By no priority, you mean this doesn't affect the criteria required to
ful-fill to get these out of staging?
Correct

Are these are the specific bits of cleanup you are referring to in
your comment?
You mean about bcm2835-isp? There were too many changes to vchiq
that i don't remember them all. The first that come to my mind was
those fancy comment sections which is not kernel coding style. It
has been removed.
No, I don't mean the bcm2835-isp changes (those are upcoming /
out-of-tree still so...). I mean what are the specific bits / points
that needs to be addressed to get vc04_services out of the staging.
These were the points which i mentioned in my last email. They came
from interface/TODO.

You have mentioned it above now, so I'll follow up on those.
That would be great :)

The many vchiq changes you referred to above comment (that you don't
remember) are from [1] as well or some other series ?
Sorry, for the confusing. The many changes i refer were the dozens of
clean up patches for vc04_interfaces in mainline staging since the
last years. [1] was just a single patch which has been accepted yet.
Ah I see. There are many others that I've to dig out then. Thanks for
clarifying!

[1] -
https://lore.kernel.org/linux-staging/20220712181928.17547-1-jslebodn@xxxxxxxxxx/

Unfortuntately i hadn't much time to work on vchiq by myself.

Just my two cents
Stefan
--
Regards,

Laurent Pinchart
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