> On 22 Feb 2025, at 2:37 PM, Aditya Garg <[email protected]> wrote: > >> >> 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 > > 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?
And for justification why driver was not working, with appletbdrm_msg_information not packed is because sizeof(struct appletbdrm_msg_information) is being used in kzalloc in the driver. Similar is the case for most other __packed structs. Maybe the author wanted to keep this value consistent across various compiler options? I don’t think CPU architecture really matters here though since this driver is exclusively for x86_64 Intel Macs.
