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

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Message-ID: <YgomyazKaV2QnfYQ@pendragon.ideasonboard.com>
Date:   Mon, 14 Feb 2022 11:54:17 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Stefan Wahren <stefan.wahren@...e.com>,
        Jean-Michel Hautbois <jeanmichel.hautbois@...asonboard.com>,
        dave.stevenson@...pberrypi.com, devicetree@...r.kernel.org,
        kernel-list@...pberrypi.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linux-rpi-kernel@...ts.infradead.org, lukasz@...y.st,
        mchehab@...nel.org, naush@...pberrypi.com, robh@...nel.org,
        tomi.valkeinen@...asonboard.com,
        bcm-kernel-feedback-list@...adcom.com,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: [PATCH v5 03/11] dt-bindings: media: Add bindings for
 bcm2835-unicam

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@...pberrypi.com>
> > > Signed-off-by: Naushir Patuck <naush@...pberrypi.com>
> > > Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@...asonboard.com>
> > > Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.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@...pberrypi.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