On Fri, Oct 05, 2012 at 09:20:56AM +0100, Mans Rullgard wrote: > On 5 October 2012 08:12, Russell King - ARM Linux > <li...@arm.linux.org.uk> wrote: > > On Fri, Oct 05, 2012 at 03:25:16AM +0100, Mans Rullgard wrote: > >> On 5 October 2012 02:56, Rob Herring <robherri...@gmail.com> wrote: > >> > This struct is the IP header, so a struct ptr is just set to the > >> > beginning of the received data. Since ethernet headers are 14 bytes, > >> > often the IP header is not aligned unless the NIC can place the frame at > >> > a 2 byte offset (which is something I need to investigate). So this > >> > function cannot make any assumptions about the alignment. Does the ABI > >> > define structs have some minimum alignment? Does the struct need to be > >> > declared as packed or something? > >> > >> The ABI defines the alignment of structs as the maximum alignment of its > >> members. Since this struct contains 32-bit members, the alignment for the > >> whole struct becomes 32 bits as well. Declaring it as packed tells gcc it > >> might be unaligned (in addition to removing any holes within). > > > > This has come up before in the past. > > > > The Linux network folk will _not_ allow - in any shape or form - for > > this struct to be marked packed (it's the struct which needs to be > > marked packed) because by doing so, it causes GCC to issue byte loads/ > > stores on architectures where there isn't a problem, and that decreases > > the performance of the Linux IP stack unnecessarily. > > Which architectures? I have never seen anything like that.
Does it matter? I'm just relaying the argument against adding __packed which was used before we were forced (by the networking folk) to implement the alignment fault handler. _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain
