Message-ID: <6f7b0886-5f31-4ba9-b82e-e9d3614b504f@suse.de>
Date: Mon, 24 Feb 2025 09:41:43 +0100
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Aditya Garg <gargaditya08@...e.com>,
 "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>
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Subject: Re: [PATCH v3 3/3] drm/tiny: add driver for Apple Touch Bars in x86
 Macs

Hi

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. At least it's clear then what 
happens. And if your hardware requires this, you can't do much anyway.

Best regards
Thomas


>>
>>
>> ...
>>
>>>>> + if (response->msg == APPLETBDRM_MSG_SIGNAL_READINESS) {
>>>>> + if (!readiness_signal_received) {
>>>>> + readiness_signal_received = true;
>>>>> + goto retry;
>>>>> + }
>>>>> +
>>>>> + drm_err(drm, "Encountered unexpected readiness signal\n");
>>>>> + return -EIO;
>>>>> + }
>>>>> +
>>>>> + if (actual_size != size) {
>>>>> + drm_err(drm, "Actual size (%d) doesn't match expected size (%lu)\n",
>>>>> + actual_size, size);
>>>>> + return -EIO;
>>>>> + }
>>>>> +
>>>>> + if (response->msg != expected_response) {
>>>>> + drm_err(drm, "Unexpected response from device (expected %p4ch found %p4ch)\n",
>>>>> + &expected_response, &response->msg);
>>>>> + return -EIO;
>>>> For three different cases the same error code, can it be adjusted more to the
>>>> situation?
>>> All these are I/O errors, you got any suggestion?
>> Your email client mangled the code so badly that it's hard to read. But I would
>> suggest to use -EINTR in the first case, and -EBADMSG. But also you may consider
>> -EPROTO.
> Thanks
>>>>> + }
>> ...
>>
>>>>> + if (ret)
>>>>> + return ret;
>>>>> + else if (!new_plane_state->visible)
>>>> Why 'else'? It's redundant.
>>> I’ve just followed what other drm drivers are doing here:
>>>
>>> https://elixir.bootlin.com/linux/v6.13.3/source/drivers/gpu/drm/tiny/bochs.c#L436
>>> https://elixir.bootlin.com/linux/v6.13.3/source/drivers/gpu/drm/tiny/cirrus.c#L363
>>>
>>> And plenty more
>> A bad example is still a bad example. 'else' is simply redundant in this
>> case and add a noisy to the code.
>>
>>> I won’t mind removing else. You want that?
>> Sure.
>>
>> ...
>>
>>>>> + request_size = ALIGN(sizeof(struct appletbdrm_fb_request) +
>>>>> +        frames_size +
>>>>> +        sizeof(struct appletbdrm_fb_request_footer), 16);
>>>> Missing header for ALIGN().
>>>>
>>>> But have you checked overflow.h for the possibility of using some helper macros
>>>> from there? This is what should be usually done for k*alloc() in the kernel.
>>> I don’t really think we need a macro here.
>> Hmm... is frames_size known to be in a guaranteed range to make sure no
>> potential overflow happens?
> I don’t really find any cause of potential overflow.
>
>
>>>>> + appletbdrm_state->request = kzalloc(request_size, GFP_KERNEL);
>>>>> +
>>>>> + if (!appletbdrm_state->request)
>>>>> + return -ENOMEM;
>> ...
>>
>>>>> + request->msg_id = timestamp & 0xff;
>>>> Why ' & 0xff'?
>>> https://github.com/imbushuo/DFRDisplayKm/blob/master/src/DFRDisplayKm/DfrDisplay.c#L147
>> This is not an answer.
>> Why do you need this here? Isn't the type of msg_id enough?
> Hmm, I double checked this. msg_id is u8 in the Linux port so would anyways never exceed 0xff. I’ll remove this.
> Its different in the Windows driver.
>> ...
>>
>>>>> + adev->mode = (struct drm_display_mode) {
>>>> Why do you need a compound literal here? Perhaps you want to have that to be
>>>> done directly in DRM_MODE_INIT()?
>>> I really don’t find this as an issue. You want me to declare another structure, basically this?:
>> Nope, I'm asking if the DRM_MODE_INIT() is done in a way that it only can be
>> used for the static data. Seems like the case. Have you tried to convert
>> DRM_MODE_INIT() to be always a compound literal? Does it break things?
> Seems to be breaking things.
>>> struct drm_display_mode mode = {
>>> DRM_MODE_INIT(60, adev->height, adev->width,
>>> DRM_MODE_RES_MM(adev->height, 218),
>>> DRM_MODE_RES_MM(adev->width, 218))
>>> };
>>> adev->mode = mode;
>>>
>>>>> + DRM_MODE_INIT(60, adev->height, adev->width,
>>>>> +       DRM_MODE_RES_MM(adev->height, 218),
>>>>> +       DRM_MODE_RES_MM(adev->width, 218))
>>>>> + };
>> -- 
>> With Best Regards,
>> Andy Shevchenko
>>
>>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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