Re: [PATCH 01/12] spi: dt-bindings: introduce the ``fifo-depth`` property

[Tudor Ambarus <tudor.ambarus@linaro.org>]

Hi, Geert, Krzysztof,

On 2/11/24 13:49, Krzysztof Kozlowski wrote:> On 09/02/2024 18:13, Geert
Uytterhoeven wrote:
>> Hi Tudor,
>>
>> On Thu, Feb 8, 2024 at 2:51 PM Tudor Ambarus <tudor.ambarus@linaro.org> wrote:
>>> There are instances of the same IP that are configured by the integrator
>>> with different FIFO depths. Introduce the fifo-depth property to allow
>>> such nodes to specify their FIFO depth.
>>>
>>> We haven't seen SPI IPs with different FIFO depths for RX and TX, thus
>>> introduce a single property.
>>
>> Ha...
>>
>> Current documentation for the Clock-Synchronized Serial Interface with
>> FIFO (MSIOF) on e.g. R-Car Gen2 and later states:
>>
>>     FIFO capacity: 32 bits × 64 stages for transmission and 32 bits ×
>> 256 stages for reception
>>
>> Initially (many years ago), there was some doubt about the validity
>> of these values (older variants on SH supported 64/64), hence
>> drivers/spi/spi-sh-msiof.c still has
>>
>>     .tx_fifo_size = 64,
>>     .rx_fifo_size = 64,
>>
>> Probably we should test and revisit this...
>>
>>> --- a/Documentation/devicetree/bindings/spi/spi-controller.yaml
>>> +++ b/Documentation/devicetree/bindings/spi/spi-controller.yaml
>>> @@ -69,6 +69,11 @@ properties:
>>>           Should be generally avoided and be replaced by
>>>           spi-cs-high + ACTIVE_HIGH.
>>>
>>> +  fifo-depth:
>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>> +    description:
>>> +      Size of the data FIFO in bytes.
>>
>> I think it is prudent to consider the asymmetric case, too.
>> Whether that should be just two properties ("rx-fifo-depth" and
>> "tx-fifo-depth"), or also a third "fifo-depth", I defer to the DT
>> maintainers...

Thanks, Geert for the insight!
> 
> Since most of the cases FIFO depth tx=rx, we could go with three
> properties and:
> 
> allOf:
>  - not:
>      required:
>        - fifo-depth
>        - tx-fifo-depth
>  - not:
>      required:
>        - fifo-depth
>        - rx-fifo-depth
> 
> and probably dependencies between rx and tx (see example-schema).
> 
Great. Thanks, Krzysztof! I'll give it a try.
Cheers,
ta