Probing devices by their less-specific "compatible" bindings (here: brcmnand)

[Krzysztof Kozlowski]

On 17/03/2023 11:02, Rafał Miłecki wrote:
> Hi, I just spent few hours debugging hidden hw lockup and I need to
> consult driver core code behaviour.
> 
> I have a BCM4908 SoC based board with a NAND controller on it.
> 
> 
> ### Hardware binding
> 
> Hardware details:
> arch/arm64/boot/dts/broadcom/bcmbca/bcm4908.dtsi
> 
> Relevant part:
> nand-controller at 1800 {
> 	compatible = "brcm,nand-bcm63138", "brcm,brcmnand-v7.1", "brcm,brcmnand";

(...)

> ### Problem
> 
> As first Linux probes my hardware using the "brcm,nand-bcm63138"
> compatibility string driver bcm63138_nand.c. That's good.
> 
> It that fails however (.probe() returns an error) then Linux core starts
> probing using drivers for less specific bindings.
> 
> In my case probing with the "brcm,brcmnand" string driver brcmstb_nand.c
> results in ignoring SoC specific bits and causes a hardware lockup. Hw
> isn't initialized properly and writel_relaxed(0x00000009, base + 0x04)
> just make it hang.
> 
> That obviously isn't an acceptable behavior for me. So I'm wondering
> what's going on wrong here.
> 
> Should Linux avoid probing with less-specific compatible strings?

Why? If less-specific compatible is there, it means device is compatible
with it and it should work.

> Or should I not claim hw to be "brcm,brcmnand" compatible if it REQUIRES
> SoC-specific handling?

As you pointed this compatible does not work for your device, so they
are not compatible.


Best regards,
Krzysztof