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

[Jisheng Zhang <jszhang@kernel.org>]

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.

vs

If supported by "sdma-io-width", nothing is needed after this patch

IMHO, the 2nd sounds better, what do you think?

Thanks