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

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Message-ID: <925fe847-68b4-4689-832c-08f8de3dfeb1@nvidia.com>
Date: Wed, 27 Nov 2024 17:24:01 +0000
From: Jon Hunter <jonathanh@...dia.com>
To: Mark Brown <broonie@...nel.org>
Cc: Vishwaroop A <va@...dia.com>, krzk+dt@...nel.org, robh@...nel.org,
 conor+dt@...nel.org, thierry.reding@...il.com, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org,
 linux-spi@...r.kernel.org
Subject: Re: [PATCH 2/3] dt-bindings: spi: Add DT schema for Tegra SPIDEV
 controller


On 27/11/2024 16:09, Mark Brown wrote:
> On Wed, Nov 27, 2024 at 03:54:52PM +0000, Jon Hunter wrote:
> 
>> On the Tegra Jetson boards we have a 40-pin expansion header similar to what
>> is found on boards like Raspberry Pi and allows users to connect various
>> cards to. By having a pseudo device we can interact with different SPI
>> devices.
> 
>> Yes by default nothing is connected and so there is nothing to talk to.
>> However, this does enable us to do SPI loopback testing for example.
> 
>> So I am wondering if it would be acceptable to having some generic dummy
>> device-tree compatible string for this? I guess it does not need to be Tegra
>> specific.
> 
> 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.

Jon

-- 
nvpublic