linux-kernel - Re: [PATCH 1/8] spi: dt-bindings: Introduce spi-cs-setup-ns property

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Date:   Mon, 02 Jan 2023 14:21:59 +0100
From:   Michael Walle <michael@...le.cc>
To:     Tudor Ambarus <tudor.ambarus@...aro.org>
Cc:     Mark Brown <broonie@...nel.org>, tudor.ambarus@...rochip.com,
        alexandre.belloni@...tlin.com, claudiu.beznea@...rochip.com,
        devicetree@...r.kernel.org, krzysztof.kozlowski+dt@...aro.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mtd@...ts.infradead.org, linux-spi@...r.kernel.org,
        nicolas.ferre@...rochip.com, robh+dt@...nel.org
Subject: Re: [PATCH 1/8] spi: dt-bindings: Introduce spi-cs-setup-ns property

>>>>>> +  spi-cs-setup-ns:
>>>>>> +    description:
>>>>>> +      Delay in nanosecods to be introduced by the controller 
>>>>>> after CS is
>>>>>> +      asserted.
>>>> 
>>>>> Does this need a type as the spi-cs-setup-ns is apparently just 
>>>>> 16bit? At
>>>>> least the driver uses it that way.
>>>> 
>>>>> But IMHO this should just be a normal uint32 value to be consistent 
>>>>> with
>>>>> all the other properties. Also the max value with 16bit will be 
>>>>> 'just'
>>>>> 65us.
>>>> 
>>>> Making it 32 bit does seem safer.  I've applied the series
>>> 
>>> Thanks. There are few implications to consider before making this 
>>> prop a
>>> u32, and I'd like to check them with you.
>>> 
>>> struct spi_delay will have to be updated to have a u32 value, now 
>>> it's a
>>> u16. This means that we'll have to update spi_delay_to_ns() to either
>>> return a s64 or to add a u64 *delay parameter to the function so that 
>>> we
>>> can still handle the conversions from usecs and the error codes in 
>>> the
>>> SPI_DELAY_UNIT_SCK case. Then all its callers have to be updated to
>>> consider the u64 delay.
>> 
>> I was talking about the device tree property. Even if the driver 
>> continue
>> to use just 16bit, the DT property could be 32bit IMHO.
> 
> but then you'll have an implicit cast to u16 at:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/spi/spi.c#n2314
> which will make the u32 dt prop misleading.

Nothing will prevent you from checking for a valid range and return an
error :)
But I agree, that converting the u16 to u32 in the driver is probably
the better way.

>> At the moment, the schema says its 32bit (if I'm not mistaken, because
>> it doesn't have a type), but the driver will parse the property as
>> 16bit and your device tree also has this /bits/ thingy. So regardless
>> if the driver is using 16bit or 32bit for the value, there seems to be
>> a discrepancy between the schema and the devicetree (and driver).
> 
> okay, thanks for pointing it out. Let's decide how we fix this.
> 
>> 
>> All other properties are just the regular 32bit values, thus I was
>> suggesting to change the DT property to 32bit.
> 
> If we want to change the dt prop to 32bit I think we should also handle
> the parsed value as u32, not as u16.

Strictly speaking, your device tree is wrong, because the schema
already says it's 32bit.

-michael