linux-kernel - Re: [PATCH v4 2/2] drm/tiny: add driver for Apple Touch Bars in x86 Macs

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Message-ID:
 <PN3PR01MB9597D4FD9FD8B8A8FB0F2B3CB8C02@PN3PR01MB9597.INDPRD01.PROD.OUTLOOK.COM>
Date: Mon, 24 Feb 2025 15:20:13 +0000
From: Aditya Garg <gargaditya08@...e.com>
To: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>
CC: "maarten.lankhorst@...ux.intel.com" <maarten.lankhorst@...ux.intel.com>,
	"mripard@...nel.org" <mripard@...nel.org>, "tzimmermann@...e.de"
	<tzimmermann@...e.de>, "airlied@...il.com" <airlied@...il.com>,
	"simona@...ll.ch" <simona@...ll.ch>, Kerem Karabay <kekrby@...il.com>,
	Atharva Tiwari <evepolonium@...il.com>, Aun-Ali Zaidi <admin@...eit.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v4 2/2] drm/tiny: add driver for Apple Touch Bars in x86
 Macs



> On 24 Feb 2025, at 8:41 PM, andriy.shevchenko@...ux.intel.com wrote:
> 
> On Mon, Feb 24, 2025 at 03:03:40PM +0000, Aditya Garg wrote:
>>>> On 24 Feb 2025, at 8:27 PM, andriy.shevchenko@...ux.intel.com wrote:
>>> On Mon, Feb 24, 2025 at 02:32:37PM +0000, Aditya Garg wrote:
>>>>> On 24 Feb 2025, at 7:30 PM, andriy.shevchenko@...ux.intel.com wrote:
>>>>> On Mon, Feb 24, 2025 at 01:40:20PM +0000, Aditya Garg wrote:
> 
> ...
> 
>>>>>> +#define __APPLETBDRM_MSG_STR4(str4) ((__le32 __force)((str4[0] << 24) | (str4[1] << 16) | (str4[2] << 8) | str4[3]))
>>>>> 
>>>>> As commented previously this is quite strange what's going on with endianess in
>>>>> this driver. Especially the above weirdness when get_unaligned_be32() is being
>>>>> open coded and force-cast to __le32.
>>>> 
>>>> I would assume it was also mimicked from the Windows driver, though I haven't
>>>> really tried exploring this there.
>>>> 
>>>> I’d rather be happy if you give me code change suggestions and let me review
>>>> and test them
>>> 
>>> For the starter I would do the following for all related constants and
>>> drop that weird and ugly macros at the top (it also has an issue with
>>> the str4 length as it is 5 bytes long, not 4, btw):
>>> 
>>> #define APPLETBDRM_MSG_CLEAR_DISPLAY cpu_to_le32(0x434c5244) /* CLRD */
>> 
>> Lemme test this.
> 
> Just in case it won't work, reverse bytes in the integer. Because I was lost in
> this conversion.

It works. What I understand is that you used the macro to get the final hex and converted it into little endian, which on the x86 macs would technically remain the same.
> 
>>> (assuming we stick with __leXX for now). This will be much less confusing.
> 
> ...
> 
>>>> Alright. For some reason (a mistake on my part), some dev_err_probe were also
>>>> still left in this version.
>>> 
>>> But those are seems to me in the correct locations, no? How do we even know
>>> the DRM device before its creation? So, dev_err_probe() calls in ->probe()
>>> seem logical to me. Somebody from DRM should clarify this.
>> 
>> Thomas asked me to do this change. Maybe you didn’t see his reply.
> 
> I saw, maybe I took it wrong, but I really don't understand how on earth
> drm_err()  or whatever can be used in real ->probe() of the physical device.
> 
> Imagine the hypotetical case
> 
> probe(strict device *dev)
> {
>    mydrm;
>    foo;
>    ...
>    foo = devm_gpiod_get(dev, ...);
>    if (IS_ERR(foo))
>        return dev_err_probe(dev, ...); // how?!
>    ...
>    mydrm = ...DRM alloc...;
>    ...
> }
> 
> I don't even believe it will be possible to create drm_err_probe() as it most
> likely will require to have an allocation to be always the first op (that may
> fail) in the ->probe() which might be not the case for some device drivers.
> 
>>>>>> +  */
> 
> --
> With Best Regards,
> Andy Shevchenko
> 
>