From mboxrd@z Thu Jan 1 00:00:00 1970 From: zongbox@gmail.com (Zong Li) Date: Sat, 27 Oct 2018 00:19:38 +0800 Subject: [GIT PULL] RISC-V Patches for the 4.20 Merge Window, Part 1 In-Reply-To: <41b34e47-57de-bcc1-c86e-8b98f663367a@sifive.com> References: <41b34e47-57de-bcc1-c86e-8b98f663367a@sifive.com> Message-ID: To: linux-riscv@lists.infradead.org List-Id: linux-riscv.lists.infradead.org Paul Walmsley ? 2018?10?26? ?? ??9:35??? > > + Zong Li > > Zong, > > On 10/25/18 6:12 PM, Linus Torvalds wrote: > > On Wed, Oct 24, 2018 at 1:42 PM Palmer Dabbelt wrote: > >> I'm pretty sure this is our largest patch set since the original kernel > >> contribution, and it's certainly the one with the most contributors. > >> While I don't have anything else I know I'm going to submit for the > >> merge window, I would be somewhat surprised if I didn't screw anything > >> up. > > So I'm not entirely sure this is a screw-up, but it does have some > > signs of being one: > > > >> lib: Add umoddi3 and udivmoddi4 of GCC library routines > > We have *actively* tried to avoid needing umoddi3 and friends, and > > have so far succceeded in doing that across many different > > architectures, and many many years. > > > > And when we _have_ added it, we've added it as architecture-specific > > optimized routines when people really need it. > > > > The reason to avoid a full 64-by-64 divide is that > > > > (a) it is really expensive > > > > (b) code that needs it is almost always actually wrong > > > > Almost always, the 64-bit divide turns out to be entirely pointless, > > and the code can be written as a (much cheaper) 64-by-32 divide > > instead using the functions in instead. > > > > Does RISC-V _really_ need that disgusting full 64-bit divide, or was > > the addition of that simply a "we didn't know better, and didn't > > realize just how expensive and wrong it is"? > > > I believe the request for those routines came from you: > > > https://lore.kernel.org/lkml/779b0b25cc96101a525402f38053db5e2f360d6f.1538535528.git.zongbox at gmail.com/ > > > Do you in fact need a full 64-by-64 bit divide in your DMA driver, or > can you > > get away with using div_u64() et al.? > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/math64.h#n123 > Thanks for the information. The DMA driver could use the 64-by-32 divides in fact of the situation. I would submit a patch to drop this. 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