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

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Date:   Mon, 14 Feb 2022 12:32:59 +0100
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Maxime Ripard <maxime@...no.tech>
Cc:     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

Hi,

Am 14.02.22 um 10:54 schrieb Laurent Pinchart:
> 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.

okay, as a result we need a DTS patch for bcm2835-rpi.dtsi to enable the
firmware clocks and its driver in this series.

Best regards