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[220.235.124.2]) by smtp.gmail.com with ESMTPSA id w66sm8631924pfw.102.2019.12.19.05.12.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Dec 2019 05:13:00 -0800 (PST) Date: Thu, 19 Dec 2019 13:12:49 +0000 From: Kent Gibson To: Bartosz Golaszewski Cc: Andy Shevchenko , Bartosz Golaszewski , Linus Walleij , "open list:GPIO SUBSYSTEM" , Linux Kernel Mailing List Subject: Re: [PATCH v2 10/11] gpiolib: add new ioctl() for monitoring changes in line info Message-ID: <20191219131249.GA12008@firefly> References: <20191204155941.17814-1-brgl@bgdev.pl> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-gpio-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-gpio@vger.kernel.org On Thu, Dec 19, 2019 at 02:05:19PM +0100, Bartosz Golaszewski wrote: > wt., 10 gru 2019 o 18:00 Andy Shevchenko napisał(a): > > > > > On a different note: why would endianness be an issue here? 32-bit > > > variables with 64-bit alignment should still be in the same place in > > > memory, right? > > > > With explicit padding, yes. > > > > > Any reason not to use __packed for this structure and not deal with > > > this whole compat mess? > > > > Have been suggested that explicit padding is better approach. > > (See my answer to Kent) > > > > > I also noticed that my change will only allow user-space to read one > > > event at a time which seems to be a regression with regard to the > > > current implementation. I probably need to address this too. > > > > Yes, but we have to have ABI v2 in place. > > Hi Andy, > > I was playing with some ideas for the new ABI and noticed that on > 64-bit architecture the size of struct gpiochip_info is reported to be > 68 bytes, not 72 as I would expect. Is implicit alignment padding not > applied to a struct if there's a non-64bit-aligned 32-bit field at the > end of it? Is there something I'm missing here? > Struct alignment is based on the size of the largest element. The largest element of struct gpiopchip_info is a __u32, so the struct gets 32-bit alignment, even on 64-bit. The structs with the problems all contain a __u64, and so get padded out to a 64-bit boundary. Cheers, Kent.