linux-kernel - Re: [PATCH v2 tty-next 1/2] dt-bindings: serial: ni,ni16650: add bindings

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Message-ID: <6f9cfd54-c8cf-7395-e7bd-c350a06c8f16@linaro.org>
Date:   Thu, 13 Apr 2023 09:39:53 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Brenda Streiff <brenda.streiff@...com>
Cc:     ilpo.jarvinen@...ux.intel.com,
        Gratian Crisan <gratian.crisan@...com>,
        Jason Smith <jason.smith@...com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 tty-next 1/2] dt-bindings: serial: ni,ni16650: add
 bindings

On 13/04/2023 00:24, Brenda Streiff wrote:
> On 4/11/23 00:44, Krzysztof Kozlowski wrote:
>> On 10/04/2023 23:11, Brenda Streiff wrote:
>>> +
>>> +  interrupts:
>>> +    maxItems: 1
>>> +
>>> +  clock-frequency: true
>>
>> I missed it last time - why do you need this property? You do not have
>> any clock input, so which clock's frequency is it?
>>
> 
> This is the clock frequency of the UART; on our x86-based platforms this
> comes from the LPC clock, on Zynq-7000 it's derived from a clock in the
> FPGA. This is used to set struct uart_port::uartclk in the serial core,
> as it is for other UARTs.
> 
> This clock frequency can vary based on board design (especially on the
> x86 side, due to different LPC clocks) but for a given design is fixed-
> frequency.

So you must have clock input - clocks property. Once you add this, use
assigned-clocks to get the rate you want.

> 
> Would you prefer this be documented further? I was following 8250.yaml's
> lead here with the simple `true`.

I prefer to drop it. It is not correct and a legacy property. Without
clock inputs how can you even configure some clock?

Best regards,
Krzysztof