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

Laurent Pinchart
2022-02-14T10:21:16Z

On Mon, Feb 14, 2022 at 10:39:54AM +0100, Maxime Ripard wrote:
> Hi,
>
> 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.

--
Regards,

Laurent Pinchart