Re: [PATCH v5 03/11] dt-bindings: media: Add bindings for bcm2835-unicam

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Stefan,

On Mon, Feb 21, 2022 at 01:45:10PM +0100, Stefan Wahren wrote:
> Am 21.02.22 um 08:10 schrieb Laurent Pinchart:
> > On Mon, Feb 14, 2022 at 12:32:59PM +0100, Stefan Wahren wrote:
> >> Am 14.02.22 um 10:54 schrieb Laurent Pinchart:
> >>> On Mon, Feb 14, 2022 at 10:39:54AM +0100, Maxime Ripard wrote:
> >>>> On Sun, Feb 13, 2022 at 04:48:45PM +0100, Stefan Wahren wrote:
> >>>>> as someone with a little more insight to the clocks, i like to know your
> >>>>> opinion about the bcm2835-unicam binding.
> >>>>>
> >>>>> Am 08.02.22 um 16:50 schrieb Jean-Michel Hautbois:
> >>>>>> Introduce the dt-bindings documentation for bcm2835 CCP2/CSI2 Unicam
> >>>>>> camera interface.
> >>>>>>
> >>>>>> Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
> >>>>>> Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
> >>>>>> Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
> >>>>>> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> >>>>>>
> >>>>>> ---
> >>>>>> v4:
> >>>>>> - make MAINTAINERS its own patch
> >>>>>> - describe the reg and clocks correctly
> >>>>>> - use a vendor entry for the number of data lanes
> >>>>>> ---
> >>>>>>  .../bindings/media/brcm,bcm2835-unicam.yaml   | 117 ++++++++++++++++++
> >>>>>>  1 file changed, 117 insertions(+)
> >>>>>>  create mode 100644 Documentation/devicetree/bindings/media/brcm,bcm2835-unicam.yaml
> >>>>>>
> >>>>>> diff --git a/Documentation/devicetree/bindings/media/brcm,bcm2835-unicam.yaml b/Documentation/devicetree/bindings/media/brcm,bcm2835-unicam.yaml
> >>>>>> new file mode 100644
> >>>>>> index 000000000000..1938ace23b3d
> >>>>>> --- /dev/null
> >>>>>> +++ b/Documentation/devicetree/bindings/media/brcm,bcm2835-unicam.yaml
> >>>>>> @@ -0,0 +1,117 @@
> >>>>>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> >>>>>> +%YAML 1.2
> >>>>>> +---
> >>>>>> +$id: http://devicetree.org/schemas/media/brcm,bcm2835-unicam.yaml#
> >>>>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> >>>>>> +
> >>>>>> +title: Broadcom BCM283x Camera Interface (Unicam)
> >>>>>> +
> >>>>>> +maintainers:
> >>>>>> +  - Raspberry Pi Kernel Maintenance <kernel-list@raspberrypi.com>
> >>>>>> +
> >>>>>> +description: |-
> >>>>>> +  The Unicam block on BCM283x SoCs is the receiver for either
> >>>>>> +  CSI-2 or CCP2 data from image sensors or similar devices.
> >>>>>> +
> >>>>>> +  The main platform using this SoC is the Raspberry Pi family of boards.  On
> >>>>>> +  the Pi the VideoCore firmware can also control this hardware block, and
> >>>>>> +  driving it from two different processors will cause issues.  To avoid this,
> >>>>>> +  the firmware checks the device tree configuration during boot. If it finds
> >>>>>> +  device tree nodes whose name starts with 'csi' then it will stop the firmware
> >>>>>> +  accessing the block, and it can then safely be used via the device tree
> >>>>>> +  binding.
> >>>>>> +
> >>>>>> +properties:
> >>>>>> +  compatible:
> >>>>>> +    const: brcm,bcm2835-unicam
> >>>>>> +
> >>>>>> +  reg:
> >>>>>> +    items:
> >>>>>> +      - description: Unicam block.
> >>>>>> +      - description: Clock Manager Image (CMI) block.
> >>>>>> +
> >>>>>> +  reg-names:
> >>>>>> +    items:
> >>>>>> +      - const: unicam
> >>>>>> +      - const: cmi
> >>>>>> +
> >>>>>> +  interrupts:
> >>>>>> +    maxItems: 1
> >>>>>> +
> >>>>>> +  clocks:
> >>>>>> +    items:
> >>>>>> +      - description: Clock to drive the LP state machine of Unicam.
> >>>>>> +      - description: Clock for the VPU (core clock).
> >>>>>> +
> >>>>>> +  clock-names:
> >>>>>> +    items:
> >>>>>> +      - const: lp
> >>>>>> +      - const: vpu
> >>>>>> +
> >>>>>
> >>>>> according to this patch [1], the unicam driver only needs the VPU clock
> >>>>> reference just to enforce a minimum of 250 MHz. The firmware clock
> >>>>> binding and its driver is specific to the bcm2711, but the Unicam IP
> >>>>> exists since bcm2835.
> >>>>>
> >>>>> So do you think the clock part is correct or should be the VPU clock
> >>>>> optional?
> >>>>
> >>>> I think we should keep it mandatory. Indeed, that clock is shared with
> >>>> the HVS that will change its rate on a regular basis, so even just
> >>>> enforcing that 250MHz while it's on without a clock handle will be
> >>>> fairly hard.
> >>>>
> >>>> Also, those are the constraints we have now, but having the clock handle
> >>>> all the time will allow us to add any constraint we might need in the
> >>>> future.
> >>>>
> >>>> And BCM2711 or not, the clock has probably always been there.
> >>>
> >>> Furthermore, regardless of what the driver needs, Unicam operates with
> >>> the VPU clock, so I think it makes sense to reference it in the device
> >>> tree.
> >>
> >> okay, as a result we need a DTS patch for bcm2835-rpi.dtsi to enable the
> >> firmware clocks and its driver in this series.
> >
> > Can't we do that on top, enabling Unicam support for bcm2711 only first
> > ? I have no idea how to deal with firmware clocks on bcm2825, and I'm
> > not sure Jean-Michel even has a hardware platform to test it.
> 
> sorry as being a bcm2835 maintainer so long, i'm not always aware of the
> ambiguous meaning of bcm2835. The bcm2835-rpi.dtsi is used by all
> Raspberry Pi generations. So it's sufficient to test it with a Raspberry
> Pi 4, but we would gain support for all generations.
> 
> So my request is to move the changes from bcm2711 specific dtsi to the
> general dtsi. There is no need to touch any driver, please have a look
> at this patch [1] to see what i had in my mind. Just compile tested.
> 
> I hope you understand, i want to have it for all generations in one step.
> 
> [1] -
> https://github.com/lategoodbye/rpi-zero/commit/67897cc22c03204e6464d00ff4ddac6bf5dc65dc

Got it. I thought changes would be needed in the firmware clocks driver,
if it's just about moving the node to a different file, it's pretty
easy. Thank you for taking the time to cook up a patch to educate me :-)

> > If you want to send a patch series to enable firmware clocks on bcm2835,
> > we'll be happy to rebase on top.

-- 
Regards,

Laurent Pinchart