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

[Conor Dooley]

On Mon, Jan 22, 2024 at 03:38:41AM +0000, Dharma.B@xxxxxxxxxxxxx wrote:
> 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.

If both clocks can physically be provided to the IP then both of them
should be in the dt. The hcldc appears to me to be a part of the SoC and
the clock routing to the IP is likely fixed.

> 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.

This is probably a question for the folks on the DRM or media side of
things, but is it not possible to determine based on the endpoint what
protocol is required?
I know that on the media side of things there's an endpoint property
that can be used to specific the bus-type - is there an equivalent
property for DRM stuff?

Cheers,
Conor.

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