From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from xavier.telenet-ops.be (xavier.telenet-ops.be [195.130.132.52]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E684AB666 for ; Mon, 5 May 2025 07:37:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=195.130.132.52 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1746430662; cv=none; b=D61fAV91nxKH4L4FID4UfVrJn2QbmEa4HP2wdH5Z7GErr9ER3yS3H6cxmbYlkQ9y5urxQgc4/YBfdJPOJL97IZl3u6OyyT0Fye30V1sfcg7ZxuGvYC5myYfxe/u2ZlGU2JEVApTTpcjLT9E0f9iW/9zOE3nj586utfudbteImtQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1746430662; c=relaxed/simple; bh=h1YW50sLoUMWSUmuaI9GruHDftw5IyamJf0qvphiN1w=; h=Date:From:To:cc:Subject:In-Reply-To:Message-ID:References: MIME-Version:Content-Type; b=uHkhrl3GTE4AcRlgl2yGwA41SN1e6R3qfFhOQUcfA8OL2ysR97mPPqkIvKvmrQ+Fl1lmLHU0pdb51mA+KqdZDURbiCWr7n8k/iwJyJXA+7h0CWta7vq51lTM9MjTXKpkytYWYlUj6rCqkCb1gLWdPRhBWPgO44VoqeL83Ch4bj8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=linux-m68k.org; spf=none smtp.mailfrom=linux-m68k.org; arc=none smtp.client-ip=195.130.132.52 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=linux-m68k.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=linux-m68k.org Received: from ramsan.of.borg ([IPv6:2a02:1810:ac12:ed80:d7e4:b7bb:4b2:82d6]) by xavier.telenet-ops.be with cmsmtp id lKdX2E00M1ymvt901KdXWs; Mon, 05 May 2025 09:37:32 +0200 Received: from geert (helo=localhost) by ramsan.of.borg with local-esmtp (Exim 4.97) (envelope-from ) id 1uBqOJ-00000000jmg-1kdV; Mon, 05 May 2025 09:37:31 +0200 Date: Mon, 5 May 2025 09:37:31 +0200 (CEST) From: Geert Uytterhoeven To: "Russell King (Oracle)" cc: Christian Schrefl , Miguel Ojeda , Jocelyn Falempe , Arnd Bergmann , rust-for-linux , Linux ARM , dri-devel , Linus Walleij Subject: Re: `u64` by `u64` div/mod in DRM QR for arm32 In-Reply-To: Message-ID: <5bcf120e-72a4-fd2e-c70a-3cd34d04fc88@linux-m68k.org> References: <38867e79-c0e3-4bcd-bdf9-3fb5b571d51e@gmail.com> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Hi Russell, On Mon, 14 Apr 2025, Russell King (Oracle) wrote: > On Mon, Apr 14, 2025 at 09:21:42PM +0200, Christian Schrefl wrote: >> Hi Miguel, >> >> On 14.04.25 8:14 PM, Miguel Ojeda wrote: >>> Hi Jocelyn, Christian, >>> >>> I started build-testing arm 32-bit within my other usual routine >>> tests, and I hit: >>> >>> ld.lld: error: undefined symbol: __aeabi_uldivmod >>> >>> referenced by drm_panic_qr.rs:417 (drivers/gpu/drm/drm_panic_qr.rs:417) >>> >>> drivers/gpu/drm/drm_panic_qr.o:(>> as core::iter::traits::iterator::Iterator>::next) in archive vmlinux.a >>> >>> which comes from both these `u64` by `u64`: >>> >>> let out = (self.carry / pow) as u16; >>> self.carry = self.carry % pow; >>> >>> Christian: I guess we can offer a set of `div64` functions using the C >>> ones, at least for the time being, and eventually wire the actual >>> operator with some support from upstream Rust. Or do you have >>> something else in mind? (i.e. I think you have been discussing >>> intrinsics lately) >> >> I think using the C implementations is fine. Not sure how much the >> FFI is going to matter for performance, but it should be rare enough >> that is shouldn't matter (and hopefully we will get cross lang LTO >> or something similar at some point). >> >> We could also just implement the intrinsic(s) ourselves, but then >> the u64 divisions would be implicit which is probably undesired. >> We could also rename the intrinsics so they are only usable from >> specific crates. >> >> I think we need the opinion of the some arm people here. >> >> CC Russell King and Linus Walleij. > > The kernel has had the general position that u64 by u64 division is > silly and isn't supported. Several 32-bit architectures including s/isn't supported/isn't supported implicitly/ > 32-bit ARM don't support it. It is supported when called explicitly through div64_u64() https://elixir.bootlin.com/linux/v6.14.5/source/include/linux/math64.h#L60 But you better think twice before using it, especially in performance-critical code. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds