linux-kernel - Re: [PATCH 07/15] dt-bindings: clock: Add Ambarella clock bindings

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Date:   Wed, 25 Jan 2023 21:40:19 +0800
From:   Li Chen <me@...ux.beauty>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Li Chen <lchen@...arella.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        "moderated list:ARM/Ambarella SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:COMMON CLK FRAMEWORK" <linux-clk@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 07/15] dt-bindings: clock: Add Ambarella clock bindings

On Wed, 25 Jan 2023 20:14:16 +0800,

Hi Krzysztof,

Krzysztof Kozlowski wrote:
>
> On 25/01/2023 13:06, Li Chen wrote:
> >>> Feel free to correct me if you think this
> >>> is not a good idea.
> >>
> >> This is bad idea. Compatibles should be specific. Devices should not use
> >> syscons to poke other registers, unless strictly necessary, but have
> >> strictly defined MMIO address space and use it.
> >
> > Ok, I will convert syscon-based regmaps to SoC-specific compatibles and of_device_id->data.
> >
> > But I have three questions:
> >
> > 0. why syscon + offsets is a bad idea copared to specific compatibles?
>
> Specific compatibles are a requirement. They are needed to match device
> in exact way, not some generic and unspecific. The same with every other
> interface, it must be specific to allow only correct usage.
>
> It's of course different with generic fallbacks, but we do not talk
> about them here...
>
> > 1. when would it be a good idea to use syscon in device tree?
>
> When your device needs to poke one or few registers from some
> system-controller block.
>
> > 2. syscon VS reg, which is preferred in device tree?
>
> There is no such choice. Your DTS *must* describe the hardware. The
> hardware description is for example clock controller which has its own
> address space. If you now do not add clock controller's address space to
> the clock controller, it is not a proper hardware description. The same
> with every other property. If your device has interrupts, but you do not
> add them, it is not correct description.

Got it. But Ambarella hardware design is kind of strange. I want to add mroe
expalaination about why Ambarella's downstream kernel
use so much syscon in device trees:

For most SoCs from other vendors, they have seperate address space regions
for different peripherals, like
axi address space A: ENET
axi address space B: PCIe
axi address space B: USB
...

Ambarella is somewhat **different**, its SoCs have two system controllers regions:
RCT and scratchpad, take RCT for example:
"The S6LM system software
interacts with PLLs, PHYs and several other low-level hardware blocks using APB reset clock and test (RCT)
registers with a system-layer application programming interface (API).
This includes the setting of clock frequencies."

There are so many peripherals registers located inside RCT and scratchpad
(like usb/phy, gpio, sd, dac, enet, rng), and some peripherals even have no their
own modules for register definitions.

So most time(for a peripheral driver), the only differences between different
Ambarella SoCs are just the syscon(rct or scratchpad) offsets get changed.

I don't think such lazy hardware design is common in vendors other than ambarella.

If I switch to SoC-specific compatibles, and remove these syscon from device tree,
of_device_id->data may only contain system controller(rct or scratchpad) offset for many Ambarella drivers,
and ioremap/devm_ioremap carefully.

The question is: can upstream kernel accept such codes?

If yes, I will switch to SoC-specific compatibles and remove syscon without hesitation.

Regards,
Li