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

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Message-ID: <Z7ySdcYWZjCVd-7v@smile.fi.intel.com>
Date: Mon, 24 Feb 2025 17:38:29 +0200
From: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>
To: Aditya Garg <gargaditya08@...e.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 Mon, Feb 24, 2025 at 03:32:56PM +0000, Aditya Garg wrote:
> > On 24 Feb 2025, at 8:50 PM, Aditya Garg <gargaditya08@...e.com> wrote:
> >> 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.
> 
> And now that I oberved again, %p4cc is actually printing these CLRD, REDY etc
> in reverse order, probably the reason %p4ch was chosen. And I am unable to
> find what macro upstream can be used.

%.4s should work as it technically not DRM 4cc, but specifics of the protocol
(that reminds me about ACPI that uses 4cc a lot).

-- 
With Best Regards,
Andy Shevchenko