Re: [PATCH 2/3] dt-bindings: spi: Add DT schema for Tegra SPIDEV controller

From: Jon Hunter
Date: Tue Mar 25 2025 - 06:37:07 EST


Hi Mark,

On 27/11/2024 17:31, Mark Brown wrote:
On Wed, Nov 27, 2024 at 05:24:01PM +0000, Jon Hunter wrote:
On 27/11/2024 16:09, Mark Brown wrote:

I understand what he's trying to accomplish, it's the same thing as
what everyone who wants to put a raw spidev compatible in their DT is
trying to do. The way to do this would be something like a DT overlay
that describes whatever is actually connected, or just customise the DT
locally.

We could certainly use an overlay, but how do we handle the kernel side? My
understanding is that per patch 3/3 we need to reference a compatible string
the kernel is aware of. I guess we could use an existing one, but feels like
a massive hack. It would be nice if there is something generic we can use
for this like 'linux,spidev'.

I see that ACPI has something and it does print a warning that this should
not be used in production systems.

You can put 'spidev' in as the compatible and get the warning, we don't
require specific compatibles if the Linux device ID is good enough. If
you genuinely just have bare wires you're probably able to cope with the
warning. If something is actually connected you should use the
compatible for whatever that is, if spidev makes sense for it then
that'd be OK to add to spidev.


We finally got back to this. Looks like just having 'spidev' as the compatible does not work. Apparently, it use to work and yes you would get the warning, but that no longer seems to be the case. I see a few others have been doing similar things and hacking their device-trees in different ways [0].

I completely agree that ideally we would have a proper compatible string for this because after all device-tree describes the hardware. One use-case that we use is external loop back for verifying SPI by simply connecting MOSI to the MISO. Would it be acceptable to have a compatible string for external loopback connections?

Thanks
Jon

[0] https://stackoverflow.com/questions/53634892/linux-spidev-why-it-shouldnt-be-directly-in-devicetree

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