Re: arm: ERROR: modpost: "__aeabi_uldivmod" [drivers/gpu/drm/sun4i/sun4i-drm-hdmi.ko] undefined!

From: "Arnd Bergmann" <arnd@arndb.de>
To: "Andre Przywara" <andre.przywara@arm.com>
Cc: "Naresh Kamboju" <naresh.kamboju@linaro.org>,
	"open list" <linux-kernel@vger.kernel.org>,
	"Linux ARM" <linux-arm-kernel@lists.infradead.org>,
	linux-sunxi@lists.linux.dev, dri-devel@lists.freedesktop.org,
	lkft-triage@lists.linaro.org,
	"Maxime Ripard" <mripard@kernel.org>,
	"Dave Airlie" <airlied@redhat.com>,
	"Dan Carpenter" <dan.carpenter@linaro.org>,
	"Ard Biesheuvel" <ardb@kernel.org>
Subject: Re: arm: ERROR: modpost: "__aeabi_uldivmod" [drivers/gpu/drm/sun4i/sun4i-drm-hdmi.ko] undefined!
Date: Mon, 04 Mar 2024 13:34:49 +0100	[thread overview]
Message-ID: <badf279a-f1fa-4938-a5d2-492b89d7c27c@app.fastmail.com> (raw)
In-Reply-To: <20240304114546.4e8e1e32@donnerap.manchester.arm.com>

On Mon, Mar 4, 2024, at 12:45, Andre Przywara wrote:
> On Mon, 04 Mar 2024 12:26:46 +0100
> "Arnd Bergmann" <arnd@arndb.de> wrote:
>
>> On Mon, Mar 4, 2024, at 12:24, Andre Przywara wrote:
>> > On Mon, 04 Mar 2024 12:11:36 +0100 "Arnd Bergmann" <arnd@arndb.de> wrote:  
>> >>
>> >> This used to be a 32-bit division. If the rate is never more than
>> >> 4.2GHz, clock could be turned back into 'unsigned long' to avoid
>> >> the expensive div_u64().  
>> >
>> > Wouldn't "div_u64(clock, 200)" solve this problem?  
>> 
>> Yes, that's why I mentioned it as the worse of the two obvious
>> solutions. ;-)
>
> Argh, should have cleaned my glasses first ;-)
>
> I guess I was put somehow put off by the word "expensive". While it's
> admittedly not trivial, I wonder if we care about the (hidden) complexity
> of that function? I mean it's neither core code nor something called
> frequently?

It's not critical if this is called infrequently, and as Maxime
just replied, the 64-bit division is in fact required here.
Since we are dividing by a constant value (200), there is a good
chance that this will be get turned into fairly efficient
multiply/shift code.

> I don't think we have any clock exceeding 3GHz at the moment, but it
> sounds fishy to rely on that.

Right, it's just important to look at each case individually.
The cost of 64-bit division is crazy if it gets called repeatedly,
which is of course the entire reason we don't provide a
__aeabi_uldivmod() function and require developers to think
before adding div_u64().

     Arnd

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