Re: [PATCH v3 3/3] dt-bindings: mfd: atmel,hlcdc: Convert to DT schema format

[Dharma.B]

Hi Conor,
On 19/01/24 5:33 pm, Conor Dooley - M52691 wrote:
> On Fri, Jan 19, 2024 at 03:32:49AM +0000, Dharma.B@xxxxxxxxxxxxx wrote:
>> On 18/01/24 9:10 pm, Conor Dooley wrote:
>>> On Thu, Jan 18, 2024 at 02:56:12PM +0530, Dharma Balasubiramani wrote:
>>>> Convert the atmel,hlcdc binding to DT schema format.
>>>>
>>>> Adjust the clock-names property to clarify that the LCD controller expects
>>>> one of these clocks (either sys_clk or lvds_pll_clk to be present but not
>>>> both) along with the slow_clk and periph_clk. This alignment with the actual
>>>> hardware requirements will enable accurate device tree configuration for
>>>> systems using the HLCDC IP.
>>>>
>>>> Signed-off-by: Dharma Balasubiramani<dharma.b@xxxxxxxxxxxxx>
>>>> ---
>>>> changelog
>>>> v2 -> v3
>>>> - Rename hlcdc-display-controller and hlcdc-pwm to generic names.
>>>> - Modify the description by removing the unwanted comments and '|'.
>>>> - Modify clock-names simpler.
>>>> v1 -> v2
>>>> - Remove the explicit copyrights.
>>>> - Modify title (not include words like binding/driver).
>>>> - Modify description actually describing the hardware and not the driver.
>>>> - Add details of lvds_pll addition in commit message.
>>>> - Ref endpoint and not endpoint-base.
>>>> - Fix coding style.
>>>> ...
>>>>    .../devicetree/bindings/mfd/atmel,hlcdc.yaml  | 97 +++++++++++++++++++
>>>>    .../devicetree/bindings/mfd/atmel-hlcdc.txt   | 56 -----------
>>>>    2 files changed, 97 insertions(+), 56 deletions(-)
>>>>    create mode 100644 Documentation/devicetree/bindings/mfd/atmel,hlcdc.yaml
>>>>    delete mode 100644 Documentation/devicetree/bindings/mfd/atmel-hlcdc.txt
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/mfd/atmel,hlcdc.yaml b/Documentation/devicetree/bindings/mfd/atmel,hlcdc.yaml
>>>> new file mode 100644
>>>> index 000000000000..eccc998ac42c
>>>> --- /dev/null
>>>> +++ b/Documentation/devicetree/bindings/mfd/atmel,hlcdc.yaml
>>>> @@ -0,0 +1,97 @@
>>>> +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
>>>> +%YAML 1.2
>>>> +---
>>>> +$id:http://devicetree.org/schemas/mfd/atmel,hlcdc.yaml#
>>>> +$schema:http://devicetree.org/meta-schemas/core.yaml#
>>>> +
>>>> +title: Atmel's HLCD Controller
>>>> +
>>>> +maintainers:
>>>> +  - Nicolas Ferre<nicolas.ferre@xxxxxxxxxxxxx>
>>>> +  - Alexandre Belloni<alexandre.belloni@xxxxxxxxxxx>
>>>> +  - Claudiu Beznea<claudiu.beznea@xxxxxxxxx>
>>>> +
>>>> +description:
>>>> +  The Atmel HLCDC (HLCD Controller) IP available on Atmel SoCs exposes two
>>>> +  subdevices, a PWM chip and a Display Controller.
>>>> +
>>>> +properties:
>>>> +  compatible:
>>>> +    enum:
>>>> +      - atmel,at91sam9n12-hlcdc
>>>> +      - atmel,at91sam9x5-hlcdc
>>>> +      - atmel,sama5d2-hlcdc
>>>> +      - atmel,sama5d3-hlcdc
>>>> +      - atmel,sama5d4-hlcdc
>>>> +      - microchip,sam9x60-hlcdc
>>>> +      - microchip,sam9x75-xlcdc
>>>> +
>>>> +  reg:
>>>> +    maxItems: 1
>>>> +
>>>> +  interrupts:
>>>> +    maxItems: 1
>>>> +
>>>> +  clocks:
>>>> +    maxItems: 3
>>> Hmm, one thing I probably should have said on the previous version, but
>>> I missed somehow: It would be good to add an items list to the clocks
>>> property here to explain what the 3 clocks are/are used for - especially
>>> since there is additional complexity being added here to use either the
>>> sys or lvds clocks.
>> May I inquire if this approach is likely to be effective?
>>
>>     clocks:
>>       items:
>>         - description: peripheral clock
>>         - description: generic clock or lvds pll clock
>>             Once the LVDS PLL is enabled, the pixel clock is used as the
>>             clock for LCDC, so its GCLK is no longer needed.
>>         - description: slow clock
>>       maxItems: 3
> 
> Hmm that sounds very suspect to me. "Once the lvdspll is enabled the
> generic clock is no longer needed" sounds like both clocks can be provided
> to the IP on different pins and their provision is not mutually
> exclusive, just that the IP will only actually use one at a time. If
> that is the case, then this patch is nott correct and the binding should
> allow for 4 clocks, with both the generic clock and the lvds pll being
> present in the DT at the same time.
> 
> I vaguely recall internal discussion about this problem some time back
> but the details all escape me.

Let's delve deeper into the clock configuration for LCDC_PCK.

Considering the flexibility of the design, it appears that both clocks, 
sys_clk (generic clock) and lvds_pll_clk, can indeed be provided to the 
IP simultaneously. The crucial aspect, however, is that the IP will 
utilize only one of these clocks at any given time. This aligns with the 
specific requirements of the application, where the choice of clock 
depends on whether the LVDS interface or MIPI/DSI is in use.

To ensure proper configuration of the pixel clock period, we need to 
distinctly identify which clocks are being utilized. For instance, in 
the LVDS interface scenario, the lvds_pll_clk is essential, resulting in 
LCDC_PCK being set to the source clock. Conversely, in the MIPI/DSI 
case, the LCDC GCLK is required, leading to LCDC_PCK being defined as 
source clock/CLKDIV+2.

Considering the potential coexistence of sys_clk and lvds_pll_clk in the 
Device Tree (DT), we may need to introduce an additional flag in the DT. 
This flag could serve as a clear indicator of whether the LVDS interface 
or MIPI/DSI is being employed. As we discussed to drop this flag and 
just have any one of the clocks I believe that this approach provides a 
sensible and scalable solution, allowing for a comprehensive 
representation of the clocking configuration.
> 
> Thanks,
> Conor.

-- 
With Best Regards,
Dharma B.