Re: [PATCH] ARM: dts: at91: add serial MFD sub-node for usart

[Codrin.Ciubotariu]

On 02.11.2020 14:55, Codrin.Ciubotariu@xxxxxxxxxxxxx wrote:
> On 02.11.2020 14:29, Lee Jones wrote:
>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>
>> On Mon, 02 Nov 2020, Codrin.Ciubotariu@xxxxxxxxxxxxx wrote:
>>
>>> On 02.11.2020 11:01, Lee Jones wrote:
>>>> On Fri, 30 Oct 2020, Nicolas Ferre wrote:
>>>>
>>>>> On 30/10/2020 at 12:07, Codrin Ciubotariu wrote:
>>>>>> The "atmel,at91sam9260-usart" driver is a MFD driver, so it needs sub-nodes
>>>>>> to match the registered platform device. For this reason, we add a serial
>>>>>> subnode to all the "atmel,at91sam9260-usart" serial compatible nods. This
>>>>>> will also remove the boot warning:
>>>>>> "atmel_usart_serial: Failed to locate of_node [id: -2]"
>>>>>
>>>>> I don't remember this warning was raised previously even if the MFD driver
>>>>> was added a while ago (Sept. 2018).
>>>>>
>>>>> I would say it's due to 466a62d7642f ("mfd: core: Make a best effort attempt
>>>>> to match devices with the correct of_nodes") which was added on mid August
>>>>> and corrected with 22380b65dc70 ("mfd: mfd-core: Ensure disabled devices are
>>>>> ignored without error") but maybe not covering our case.
>>>>>
>>>>> So, well, I don't know what's the best option to this change. Moreover, I
>>>>> would say that all other USART related properties go into the child not if
>>>>> there is a need for one.
>>>>>
>>>>> Lee, I suspect that we're not the only ones experiencing this ugly warning
>>>>> during the boot log: can you point us out how to deal with it for our
>>>>> existing atmel_serial.c users?
>>>>
>>>> You should not be instantiating drivers through Device Tree which are
>>>> not described there.  If the correct representation of the H/W already
>>>> exists in Device Tree i.e. no SPI and UART IP really exists, use the
>>>> MFD core API to register them utilising the platform API instead.
>>>>
>>>> This should do it:
>>>>
>>>> diff --git a/drivers/mfd/at91-usart.c b/drivers/mfd/at91-usart.c
>>>> index 6a8351a4588e2..939bd2332a4f6 100644
>>>> --- a/drivers/mfd/at91-usart.c
>>>> +++ b/drivers/mfd/at91-usart.c
>>>> @@ -17,12 +17,10 @@
>>>>
>>>>     static const struct mfd_cell at91_usart_spi_subdev = {
>>>>            .name = "at91_usart_spi",
>>>> -       .of_compatible = "microchip,at91sam9g45-usart-spi",
>>>>     };
>>>>
>>>>     static const struct mfd_cell at91_usart_serial_subdev = {
>>>>            .name = "atmel_usart_serial",
>>>> -       .of_compatible = "atmel,at91rm9200-usart-serial",
>>>>     };
>>>>
>>>>     static int at91_usart_mode_probe(struct platform_device *pdev)
>>>
>>> [snip]
>>>
>>> Hi Lee, thank you for looking through our usart driver and for sharing
>>> your thoughts. Removing the usage of compatible string means that for
>>> similar serial/SPI IPs we would need to create new platform drivers.
>>
>> Why would you need to do that?
> 
> In the case we will have to support another similar IP, but with a
> different set of features. Not a new platform driver from scratch, but
> at least a new struct platform_driver for each variant.

I guess we could use struct mfd_cell.platform_data to select the 
features for the serial/SPI. This platform data can be per compatible of 
our MFD driver. I will send a patch with the changes you suggested. 
Thank you!

Best regards,
Codrin

> 
>>
>>> This is not ideal, but it's a solution. What I proposed is more
>>> flexible, but, as you pointed out, I am not sure it correctly describes
>>> the HW, because the decision of whether to use this IP as a serial or a
>>> SPI is a configurable one.
>>>
>>> Thanks and best regards,
>>> Codrin
>>
>> --
>> Lee Jones [李琼斯]
>> Senior Technical Lead - Developer Services
>> Linaro.org │ Open source software for Arm SoCs
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>>
>