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

[Maxime Ripard]

On Mon, Feb 21, 2022 at 09:10:27AM +0200, Laurent Pinchart wrote:
> Hi Stefan,
> 
> 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@xxxxxxxxxxxxxxx>
> > >>>> Signed-off-by: Naushir Patuck <naush@xxxxxxxxxxxxxxx>
> > >>>> Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@xxxxxxxxxxxxxxxx>
> > >>>> Reviewed-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> > >>>>
> > >>>> ---
> > >>>> 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@xxxxxxxxxxxxxxx>
> > >>>> +
> > >>>> +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.

The RPi kernel uses the firmware clocks driver for all the platforms, so
it shouldn't be too bad.

Maxime

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