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

[Florian Fainelli]

+William,

On 3/17/23 03:02, 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";
>      reg = <0x1800 0x600>, <0x2000 0x10>;
>      reg-names = "nand", "nand-int-base";
> }:
> 
> Above binding is based on the documentation:
> Documentation/devicetree/bindings/mtd/brcm,brcmnand.yaml
> 
> 
> ### Linux drivers
> 
> Linux has separated drivers for few Broadcom's NAND controller bindings:
> 
> 1. drivers/mtd/nand/raw/brcmnand/bcm63138_nand.c for:
> brcm,nand-bcm63138
> 
> 2. drivers/mtd/nand/raw/brcmnand/brcmnand.c for:
> brcm,brcmnand-v2.1
> brcm,brcmnand-v2.2
> brcm,brcmnand-v4.0
> brcm,brcmnand-v5.0
> brcm,brcmnand-v6.0
> brcm,brcmnand-v6.1
> brcm,brcmnand-v6.2
> brcm,brcmnand-v7.0
> brcm,brcmnand-v7.1
> brcm,brcmnand-v7.2
> brcm,brcmnand-v7.3
> 
> 3. drivers/mtd/nand/raw/brcmnand/brcmstb_nand.c for:
> 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.

Why does it fail?

> 
> 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.

Well, the missing piece here is that brcmnand.c is a library driver, 
therefore it needs an entry point, the next one that matches is 
brcmstb_nand.c.

> 
> 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?
> Or should I not claim hw to be "brcm,brcmnand" compatible if it REQUIRES
> SoC-specific handling?
> 
> An extra note: that fallback probing happens even with .probe()
> returning -EPROBE_DEFER. This actually smells fishy for me on the Linux
> core part.
> I'm not an expect but I think core should wait for actual error without
> trying less-specific compatible strings & drivers.
> 
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-- 
Florian