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. I don't think there's going to be a satisfactory answer to this issue. I think we're going to be stuck between GCC people saying that the kernel is buggy, and the network people refusing to fix this as they have done in the past. _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain
