On Mon, Feb 24, 2025 at 09:41:43AM +0100, Thomas Zimmermann wrote: > Am 22.02.25 um 10:07 schrieb Aditya Garg: ... > > > What padding, please? Why TCP UAPI headers do not have these attributes? > > > Think about it, and think about what actually __packed does and how it affects > > > (badly) the code generation. Otherwise it looks like a cargo cult. > > > > > > > I tried removing __packed btw and driver no longer works. > > > So, you need to find a justification why. But definitely not due to padding in > > > many of them. They can go without __packed as they are naturally aligned. > > Alright, I did some debugging, basically printk sizeof(struct). Did it for both packed and unpacked with the following results: > > > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_msg_request_header is 16 > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_msg_request_header_unpacked is 16 > > > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_msg_response_header is 20 > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_msg_response_header_unpacked is 20 > > > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_msg_simple_request is 32 > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_msg_simple_request_unpacked is 32 > > > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_msg_information is 65 > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_msg_information_unpacked is 68 > > In the unpacked version, there is a 3-byte gap after the 'bits_per_pixel' to > align the next field. Using __packed removes those gaps at the expense of > runtime overhead. > > > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_frame is 12 > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_frame_unpacked is 12 > > > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_fb_request_footer is 80 > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_fb_request_footer_unpacked is 80 > > > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_fb_request is 48 > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_fb_request_unpacked is 48 > > > > Feb 22 13:02:03 MacBook kernel: size of struct appletbdrm_fb_request_response is 40 > > Feb 22 13:02:04 MacBook kernel: size of struct appletbdrm_fb_request_response_unpacked is 40 > > > > So, the difference in sizeof in unpacked and packed is only in appletbdrm_msg_information. So, I kept this packed, and removed it from others. The Touch Bar still works. > > > > So maybe keep just this packed? > > The fields in the TCP header are aligned by design. > Unfortunately, this hardware's protocol is not. And there's no way of fixing > this now. Just keep all of them packed if you want. It would be nice to see the difference in the code generation for the all __packed vs. only those that require it. > At least it's clear then > what happens. And if your hardware requires this, you can't do much anyway. One aspect (member level alignment) is clear but the other is not (object level alignment). I dunno if it makes sense to be pedantic about this, but would like to see the binary outcome asked for. -- With Best Regards, Andy Shevchenko
