From mboxrd@z Thu Jan 1 00:00:00 1970 From: zongbox@gmail.com (Zong Li) Date: Sat, 27 Oct 2018 00:13:05 +0800 Subject: [GIT PULL] RISC-V Patches for the 4.20 Merge Window, Part 1 In-Reply-To: References: <41b34e47-57de-bcc1-c86e-8b98f663367a@sifive.com> Message-ID: To: linux-riscv@lists.infradead.org List-Id: linux-riscv.lists.infradead.org Linus Torvalds ? 2018?10?26? ?? ??9:51??? > > On 10/25/18 6:12 PM, Linus Torvalds wrote: > > > > 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. > > The other common situation for the kernel is that something was > written as "xyz / abc", when it turns out that abc is really just a > power-of-two. > > So it's not necessarily always that the 64-bit divide can be > re-written as a 64-by-32 divide, sometimes it can be rewritten as > simply a shift. Which is even cheaper. > > That's the reason I try to actively discourage full 64-by-64 divides. > We have had situations where people end up using a divide that can > take hundres of cycles, where alarmingly often it can be rewritten as > one that takes tens of cycles or even just _single_ cycles. > > People don't sometimes appreciate just how expensive 64-by-64 divides > are when you don't have direct hardware support for them, and how > often there are much simpler and faster ways to avoid them. > Our driver guys implement the DMA driver which has a full 64-bit divide in its initialization function. I had checked the code, it could use the 64-by-32 divides in fact of the situation. I was not conscious of the mistake and naively thought that umoddi3 is useful for the other drivers which need the 64-by-64 divide. I would submit a patch to get rid of these routines. 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charset="utf-8" Content-Transfer-Encoding: base64 Sender: "linux-riscv" Errors-To: linux-riscv-bounces+infradead-linux-riscv=archiver.kernel.org@lists.infradead.org Message-ID: <20181026161305.BWfsagC_m33x7FD0OxB2WBO1NWQUwIHwKVE_zFpftxY@z> TGludXMgVG9ydmFsZHMgPHRvcnZhbGRzQGxpbnV4LWZvdW5kYXRpb24ub3JnPiDmlrwgMjAxOOW5 tDEw5pyIMjbml6Ug6YCx5LqUIOS4iuWNiDk6NTHlr6vpgZPvvJoKPgo+IE9uIDEwLzI1LzE4IDY6 MTIgUE0sIExpbnVzIFRvcnZhbGRzIHdyb3RlOgo+ID4KPiA+IEFsbW9zdCBhbHdheXMsIHRoZSA2 NC1iaXQgZGl2aWRlIHR1cm5zIG91dCB0byBiZSBlbnRpcmVseSBwb2ludGxlc3MsCj4gPiBhbmQg dGhlIGNvZGUgY2FuIGJlIHdyaXR0ZW4gYXMgYSAobXVjaCBjaGVhcGVyKSA2NC1ieS0zMiBkaXZp ZGUKPiA+IGluc3RlYWQgdXNpbmcgdGhlIGZ1bmN0aW9ucyBpbiA8YXNtL2Rpdi5oPiBpbnN0ZWFk Lgo+Cj4gVGhlIG90aGVyIGNvbW1vbiBzaXR1YXRpb24gZm9yIHRoZSBrZXJuZWwgaXMgdGhhdCBz b21ldGhpbmcgd2FzCj4gd3JpdHRlbiBhcyAieHl6IC8gYWJjIiwgd2hlbiBpdCB0dXJucyBvdXQg dGhhdCBhYmMgaXMgcmVhbGx5IGp1c3QgYQo+IHBvd2VyLW9mLXR3by4KPgo+IFNvIGl0J3Mgbm90 IG5lY2Vzc2FyaWx5IGFsd2F5cyB0aGF0IHRoZSA2NC1iaXQgZGl2aWRlIGNhbiBiZQo+IHJlLXdy aXR0ZW4gYXMgYSA2NC1ieS0zMiBkaXZpZGUsIHNvbWV0aW1lcyBpdCBjYW4gYmUgcmV3cml0dGVu IGFzCj4gc2ltcGx5IGEgc2hpZnQuIFdoaWNoIGlzIGV2ZW4gY2hlYXBlci4KPgo+IFRoYXQncyB0 aGUgcmVhc29uIEkgdHJ5IHRvIGFjdGl2ZWx5IGRpc2NvdXJhZ2UgZnVsbCA2NC1ieS02NCBkaXZp ZGVzLgo+IFdlIGhhdmUgaGFkIHNpdHVhdGlvbnMgd2hlcmUgcGVvcGxlIGVuZCB1cCB1c2luZyBh IGRpdmlkZSB0aGF0IGNhbgo+IHRha2UgaHVuZHJlcyBvZiBjeWNsZXMsIHdoZXJlIGFsYXJtaW5n bHkgb2Z0ZW4gaXQgY2FuIGJlIHJld3JpdHRlbiBhcwo+IG9uZSB0aGF0IHRha2VzIHRlbnMgb2Yg Y3ljbGVzIG9yIGV2ZW4ganVzdCBfc2luZ2xlXyBjeWNsZXMuCj4KPiBQZW9wbGUgZG9uJ3Qgc29t ZXRpbWVzIGFwcHJlY2lhdGUganVzdCBob3cgZXhwZW5zaXZlIDY0LWJ5LTY0IGRpdmlkZXMKPiBh cmUgd2hlbiB5b3UgZG9uJ3QgaGF2ZSBkaXJlY3QgaGFyZHdhcmUgc3VwcG9ydCBmb3IgdGhlbSwg YW5kIGhvdwo+IG9mdGVuIHRoZXJlIGFyZSBtdWNoIHNpbXBsZXIgYW5kIGZhc3RlciB3YXlzIHRv IGF2b2lkIHRoZW0uCj4KCk91ciBkcml2ZXIgZ3V5cyBpbXBsZW1lbnQgdGhlIERNQSBkcml2ZXIg d2hpY2ggaGFzIGEgZnVsbCA2NC1iaXQKZGl2aWRlIGluIGl0cyBpbml0aWFsaXphdGlvbiBmdW5j dGlvbi4gSSBoYWQgY2hlY2tlZCB0aGUgY29kZSwgaXQKY291bGQgdXNlIHRoZSA2NC1ieS0zMiBk aXZpZGVzIGluIGZhY3Qgb2YgdGhlIHNpdHVhdGlvbi4gSSB3YXMgbm90CmNvbnNjaW91cyBvZiB0 aGUgbWlzdGFrZSBhbmQgbmFpdmVseSB0aG91Z2h0IHRoYXQgdW1vZGRpMyBpcyB1c2VmdWwKZm9y IHRoZSBvdGhlciBkcml2ZXJzIHdoaWNoIG5lZWQgdGhlIDY0LWJ5LTY0IGRpdmlkZS4gSSB3b3Vs ZCBzdWJtaXQgYQpwYXRjaCB0byBnZXQgcmlkIG9mIHRoZXNlIHJvdXRpbmVzLgoKX19fX19fX19f X19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX18KbGludXgtcmlzY3YgbWFpbGlu ZyBsaXN0CmxpbnV4LXJpc2N2QGxpc3RzLmluZnJhZGVhZC5vcmcKaHR0cDovL2xpc3RzLmluZnJh ZGVhZC5vcmcvbWFpbG1hbi9saXN0aW5mby9saW51eC1yaXNjdgo=