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

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Date:   Mon, 2 Jan 2023 14:11:00 +0200
From:   Tudor Ambarus <tudor.ambarus@...aro.org>
To:     Michael Walle <michael@...le.cc>
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



On 02.01.2023 13:48, Michael Walle wrote:
> Hi,

Hi,

> 
> Am 2023-01-02 10:37, schrieb Tudor Ambarus:
>> Hi,
>>
>> On 18.11.2022 17:30, Mark Brown wrote:
>>> On Fri, Nov 18, 2022 at 03:14:58PM +0100, Michael Walle wrote:
>>>> From: Tudor Ambarus <tudor.ambarus@...rochip.com>
>>>
>>>>> +  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.

> 
> 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.