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Date:   Wed, 3 Mar 2021 17:56:13 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Krzysztof Kozlowski <krzk@kernel.org>
Cc:     Arnd Bergmann <arnd@arndb.de>, 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>
Subject: Re: [PATCH] usb: dwc3: make USB_DWC3_EXYNOS independent
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On Wed, Mar 03, 2021 at 05:49:01PM +0100, Krzysztof Kozlowski wrote:
> 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...

I doubt that will happen, and it's not something that I expect.  It's ok
to have platform-specific "sub-drivers" for common IP blocks, we do it
all the time.  But making it separate is good, much like has been done
for xhci as well.

> > 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.

Totally agreed, that's not ok.  But a different issue than what is
happening here.

thanks,

greg k-h