linux-kernel - Re: [PATCH v3 3/3] drm/tiny: add driver for Apple Touch Bars in x86 Macs

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Message-ID: <2500F60C-DEB0-49C8-B67B-49351C802646@live.com>
Date: Sat, 22 Feb 2025 12:22:36 +0000
From: Aditya Garg <gargaditya08@...e.com>
To: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>
CC: "pmladek@...e.com" <pmladek@...e.com>, Steven Rostedt
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	<j@...nau.net>
Subject: Re: [PATCH v3 3/3] drm/tiny: add driver for Apple Touch Bars in x86
 Macs



> On 22 Feb 2025, at 2:37 PM, Aditya Garg <gargaditya08@...e.com> 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.