From mboxrd@z Thu Jan 1 00:00:00 1970 From: James Bottomley Subject: Re: [PATCH] unaligned: delete 1-byte accessors Date: Mon, 22 Jul 2019 14:48:46 +0900 Message-ID: <1563774526.3223.2.camel@HansenPartnership.com> References: <20190721215253.GA18177@avx2> <1563750513.2898.4.camel@HansenPartnership.com> <20190722052244.GA4235@avx2> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20190722052244.GA4235@avx2> Sender: linux-kernel-owner@vger.kernel.org To: Alexey Dobriyan Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, axboe@kernel.dk, kvalo@codeaurora.org, john.johansen@canonical.com, linux-arch@vger.kernel.org List-Id: linux-arch.vger.kernel.org On Mon, 2019-07-22 at 08:22 +0300, Alexey Dobriyan wrote: > On Mon, Jul 22, 2019 at 08:08:33AM +0900, James Bottomley wrote: > > On Mon, 2019-07-22 at 00:52 +0300, Alexey Dobriyan wrote: > > > Each and every 1-byte access is aligned! > > > > The design idea of this is for parsing descriptors. We simply > > chunk up the describing structure using get_unaligned for > > everything. The reason is because a lot of these structures come > > with reserved areas which we may make use of later. If we're using > > get_unaligned for everything we can simply change a u8 to a u16 in > > the structure absorbing the reserved padding. With your change now > > I'd have to chase down every byte access and replace it with > > get_unaligned instead of simply changing the structure. > > > > What's the significant advantage of this change that compensates > > for the problems the above causes? > > HW descriptors have fixed endianness, you're supposed to use > get_unaligned_be32() and friends. Not if this is an internal descriptor format, which is what this is mostly used for. > For that matter, drivers/scsi/ has exactly 2 get_unaligned() calls > one of which can be changed to get_unaligned_be32(). You haven't answered the "what is the benefit of this change" question. I mean sure we can do it, but it won't make anything more efficient and it does help with the descriptor format to treat every structure field the same. James From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from bedivere.hansenpartnership.com ([66.63.167.143]:34170 "EHLO bedivere.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726534AbfGVFs7 (ORCPT ); Mon, 22 Jul 2019 01:48:59 -0400 Message-ID: <1563774526.3223.2.camel@HansenPartnership.com> Subject: Re: [PATCH] unaligned: delete 1-byte accessors From: James Bottomley Date: Mon, 22 Jul 2019 14:48:46 +0900 In-Reply-To: <20190722052244.GA4235@avx2> References: <20190721215253.GA18177@avx2> <1563750513.2898.4.camel@HansenPartnership.com> <20190722052244.GA4235@avx2> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Alexey Dobriyan Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, axboe@kernel.dk, kvalo@codeaurora.org, john.johansen@canonical.com, linux-arch@vger.kernel.org Message-ID: <20190722054846.PoRCI1cji2LGyghE4ezXy5UrRhyLA2Sj0KwuMVzagu4@z> On Mon, 2019-07-22 at 08:22 +0300, Alexey Dobriyan wrote: > On Mon, Jul 22, 2019 at 08:08:33AM +0900, James Bottomley wrote: > > On Mon, 2019-07-22 at 00:52 +0300, Alexey Dobriyan wrote: > > > Each and every 1-byte access is aligned! > > > > The design idea of this is for parsing descriptors. We simply > > chunk up the describing structure using get_unaligned for > > everything. The reason is because a lot of these structures come > > with reserved areas which we may make use of later. If we're using > > get_unaligned for everything we can simply change a u8 to a u16 in > > the structure absorbing the reserved padding. With your change now > > I'd have to chase down every byte access and replace it with > > get_unaligned instead of simply changing the structure. > > > > What's the significant advantage of this change that compensates > > for the problems the above causes? > > HW descriptors have fixed endianness, you're supposed to use > get_unaligned_be32() and friends. Not if this is an internal descriptor format, which is what this is mostly used for. > For that matter, drivers/scsi/ has exactly 2 get_unaligned() calls > one of which can be changed to get_unaligned_be32(). You haven't answered the "what is the benefit of this change" question. I mean sure we can do it, but it won't make anything more efficient and it does help with the descriptor format to treat every structure field the same. James