Date: Thu, 29 Feb 2024 18:26:09 +0000
From: Conor Dooley <conor@...nel.org>
To: Varshini.Rajendran@...rochip.com
Cc: radu_nicolae.pirea@....ro, richard.genoud@...il.com,
	gregkh@...uxfoundation.org, jirislaby@...nel.org,
	robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
	conor+dt@...nel.org, Nicolas.Ferre@...rochip.com,
	alexandre.belloni@...tlin.com, claudiu.beznea@...on.dev,
	linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org,
	linux-serial@...r.kernel.org, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 12/39] dt-bindings: serial: atmel,at91-usart: add
 compatible for sam9x7.

On Thu, Feb 29, 2024 at 08:55:11AM +0000, Varshini.Rajendran@...rochip.com wrote:
> Hi Conor,
> 
> On 28/02/24 5:19 pm, Conor Dooley wrote:
> > EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> > On Wed, Feb 28, 2024 at 07:03:01AM +0000, Varshini.Rajendran@...rochip.com wrote:
> >> Hi Conor,
> >>
> >> On 25/02/24 1:32 am, Conor Dooley wrote:
> >>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> >>> On Fri, Feb 23, 2024 at 10:55:59PM +0530, Varshini Rajendran wrote:
> >>>> Add sam9x7 compatible to DT bindings documentation.
> >>>>
> >>>> Signed-off-by: Varshini Rajendran <varshini.rajendran@...rochip.com>
> >>>> ---
> >>>> Changes in v4:
> >>>> - Fixed the wrong addition of compatible
> >>>> - Added further compatibles that are possible correct (as per DT)
> >>>> ---
> >>>>  .../devicetree/bindings/serial/atmel,at91-usart.yaml | 12 +++++++++---
> >>>>  1 file changed, 9 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/Documentation/devicetree/bindings/serial/atmel,at91-usart.yaml b/Documentation/devicetree/bindings/serial/atmel,at91-usart.yaml
> >>>> index 65cb2e5c5eee..30af537e8e81 100644
> >>>> --- a/Documentation/devicetree/bindings/serial/atmel,at91-usart.yaml
> >>>> +++ b/Documentation/devicetree/bindings/serial/atmel,at91-usart.yaml
> >>>> @@ -23,11 +23,17 @@ properties:
> >>>>            - const: atmel,at91sam9260-dbgu
> >>>>            - const: atmel,at91sam9260-usart
> >>>>        - items:
> >>>> -          - const: microchip,sam9x60-usart
> >>>> +          - enum:
> >>>> +              - microchip,sam9x60-usart
> >>>> +              - microchip,sam9x7-usart
> >>>>            - const: atmel,at91sam9260-usart
> >>>>        - items:
> >>>> -          - const: microchip,sam9x60-dbgu
> >>>> -          - const: microchip,sam9x60-usart
> >>>> +          - enum:
> >>>> +              - microchip,sam9x60-dbgu
> >>>> +              - microchip,sam9x7-dbgu
> >>>
> >>>> +          - enum:
> >>>> +              - microchip,sam9x60-usart
> >>>> +              - microchip,sam9x7-usart
> >>>
> >>> This doesn't make sense - this enum should be a const.
> >>> I don't really understand the idea behind of the original binding here that
> >>> allowed:
> >>> "microchip,sam9x60-dbgu", "microchip,sam9x60-usart", "atmel,at91sam9260-dbgu", "atmel,at91sam9260-usart"
> >>>
> >>> Specifically, I don't get the purpose of the "microchip,sam9x60-usart".
> >>> Either make it
> >>>       - items:
> >>>           - enum:
> >>>               - microchip,sam9x60-dbgu
> >>>               - microchip,sam9x7-dbgu
> >>>           - const: microchip,sam9x60-usart
> >>>           - const: atmel,at91sam9260-dbgu
> >>>           - const: atmel,at91sam9260-usart
> >>> or add
> >>>       - items:
> >>>           - const: microchip,sam9x60-dbgu
> >>>           - const: atmel,at91sam9260-dbgu
> >>>           - const: atmel,at91sam9260-usart
> >>> or explain exactly why this needs to be
> >>> "chipa-dgbu", "chipa-usart", "chipb-dbgu", "chipb-dbgu"
> >> The compatible has to be "chipa-usart", "chipb-usart", "chipa-dbgu", 
> >> "chipb-dbgu" for the device to work as a debug console over UART
> >> wher the chipa-<periph> is the device specific compatible
> >> and the chipb-<periph> is the fallback compatible that the driver 
> >> actually uses.
> > 
> > This examples why you have "microchip,sam9x60-dbgu", "atmel,at91sam9260-dbgu"
> > and "atmel,at91sam9260-usart".
> > It does not explain "microchip,sam9x60-usart" though, I don't see what
> > purpose that serves. If used as a debug uart, you fall back to the
> > sam9260 debug uart compatible and if not you fall back to the sam9260
> > usart compatible.
> > 
> Here, if it is not used as debug uart it has to fallback to the default 
> usart compatible which in this case should have a device specific 
> compatible too right?
> 
> The common usart compatible looks as follows,
> 
>      compatible = "microchip,sam9x60-usart", "atmel,at91sam9260-usart";
> 
> meaning the 1st one is the device specific usart compatible and the 2nd 
> one is the fallback compatible which the driver actually supports.
> 
> The debug uart looks as follows,
> 
> compatible = "microchip,sam9x60-dbgu", "atmel,at91sam9260-dbgu", 
> "microchip,sam9x60-usart", "atmel,at91sam9260-usart";

This version here makes a lot more sense than what is currently in use
and what is being added in your original patch. I wouldn't object to
this being used.

> In this case, there is a device specific debug uart compatible, a 
> fallback tot he debug uart compatible and as you said if not used as a 
> debug uart it should fallback and work as a normal uart device which has 
> both a device specific compatible and a fallback to work.
> 
> In case the device specific compatible is supported with some other 
> features in the driver in the future, the debug uart also should get its 
> perk. Does this make sense?
> 
> 
> > In addition, the current setup implies that sam9x60 usart supports all
> > the features that the sam9260 debug usart does. I doubt that that is
> > true.

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