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Subject: Re: [PATCH] usb: dwc3: make USB_DWC3_EXYNOS independent
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arnd Bergmann <arnd@arndb.de>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        taehyun cho <taehyun.cho@samsung.com>,
        Felipe Balbi <balbi@kernel.org>,
        USB list <linux-usb@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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From:   Krzysztof Kozlowski <krzk@kernel.org>
Message-ID: <a6ac3887-38d0-48f4-e06f-81b45484a54a@kernel.org>
Date:   Wed, 3 Mar 2021 17:49:01 +0100
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On 03/03/2021 17:43, Greg Kroah-Hartman wrote:
>>>>> I don't think that will work in practice. Many ARCH_ symbols for various
>>>>> architectures contradict with each other. Almost all watchdog drivers
>>>>> only _build_ for specific platforms/architectures.
>>>>
>>>> Great, that's horrible to hear, so much for a "generic arm64 kernel
>>>> binary" which I _thought_ was the goal.
>>>>
>>>> ugh, you would have thought we would have learned our lesson with
>>>> arm32...
>>
>> I have no idea what you are talking about here. arm64 kernels have
>> always been generic, but you still need drivers for each piece of
>> hardware, we unfortunately can't stop SoC vendors from reinventing
>> the wheel with each new platform and then having to add yet another
>> driver for each subsystems.
> 
> That's fine, drivers are easy, but when I see comments like "ARCH_
> symbols contradict" that means that we can not make a generic kernel
> image.  Otherwise there's no contradiction :)

No, they don't contradict.

> 
> And "new drivers" are almost always not really "new" as everyone uses
> much the same IP blocks.  As proof of this patch where the DWC3 IP block
> is being used by multiple SoC vendors.  To handle that, you split out
> the SoC-specific portions into sub-drivers, so that you can build a
> single image of the driver that works on multiple platforms.  Nothing
> new, we've been doing this for years, it's just that out-of-mainline SoC
> trees that think they can touch "core IP block code" break this all the
> time, which is what I am pushing back on.

I am perfectly fine with (and like it!) putting dwc3 exynos back into 
base/main dwc3  and getting rid of USB_DWC3_EXYNOS entirely. But this 
was not part of this patch...

> 
> Anyway, this is just me as a driver subsystem maintainer being grumpy to
> see ARCH_ dependancies on tiny little things like SoC-portions for
> generic IP drivers.  Or on individual drivers (i.e. Samsung serial port
> driver), where they don't belong at all.

At least with Samsung serial driver we see adding new SoC - Apple M1.

Here, the guys in Samsung want to tweak several kernel parts to work 
with their out-of-tree code without contributing this code back. It's 
not a community-friendly approach. The upstream kernel should be tweaked 
to the out-of-tree unknown, hidden and uncontrollable code.

Instead I expect from Samsung to contribute the basic Exynos9 support to 
the upstream.


Best regards,
Krzysztof