Re: [PATCH 1/2] spi: dt-bindings: cdns,xspi: add sdma-io-width

[Krzysztof Kozlowski <krzk@kernel.org>]

On 20/05/2026 15:18, Jisheng Zhang wrote:
> On Wed, May 20, 2026 at 03:22:07PM +0200, Krzysztof Kozlowski wrote:
>> On 20/05/2026 14:30, Mark Brown wrote:
>>> On Wed, May 20, 2026 at 02:16:21PM +0200, Krzysztof Kozlowski wrote:
>>>> On 20/05/2026 13:48, Jisheng Zhang wrote:
>>>
>>>>> If you mean "Why this cannot be deduced from the compatible?", I think
>>>>> the slave dma port is part of the cdns xspi, so its io width belongs
>>>>> to xSPI device property.
>>>>> From another side, we have seen such property in other drivers such as
>>>>> the reg-io-width for the dw spi DW_SPI_DR port io width.
>>>
>>>> So you mean it depends on SPI device? Then why spi-peripheral-props is
>>>> not applicable here?
>>>
>>> That will be controller side, not device side.
>>>
>>>> If this is not bus width, but DMA-something, is not really then SPI
>>>> device dependent, but rather DMA controller limitation, so either
>>>> deducible from compatible or something else is missing here.
>>>
>>> My understanding is that this is a connection between the SPI and DMA
>>> controllers so it's not as obvious as it could be which side of that
>>> link should have the property, eg:
>>>
>>>   https://www.cadence.com/content/dam/cadence-www/global/en_US/documents/tools/silicon-solutions/design-ip/controller-ip-qspi.pdf
>>>
>>> shows a separate direct connection between the DMA controller and the
>>> xSPI controller, the DMA controller isn't interacting with registers on
>>> the CPU visible buses.  The width is probably a design time configurable
>>> option on both sides of the link.
>>
>> Yes and that sounds a lot specific to particular controller, thus should
>> be implied by / deducible from the compatible.
> 
> This is IP feature, so if we couple the IP's feature with platform
> compatible, I would see some unnecessary LoCs. For example,
> Let's assume the IP has 10 users, they all support 4 bytes io width,
> other features are the same. 
> 
> If implied by the compatible string, we need to add 10 compatible
> string support both in code and dt-bindings.

You always need 10 compatible strings in bindings.

But driver would need only one, since devices are compatible as you
described.

> 
> vs
> 
> If supported by "sdma-io-width", nothing is needed after this patch
> 
> IMHO, the 2nd sounds better, what do you think?

We answered this in writing bindings. Properties are not replacement for
specific compatible. I don't know how to write that sentence in bindings
clearer - it's exactly this case.

Best regards,
Krzysztof