linux-kernel - Re: [PATCH v2 2/3] lib/vsprintf: Add support for generic FOURCCs by extending %p4cc

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Message-ID:
 <PN3PR01MB9597DDF5CD7965128C2075DBB8C02@PN3PR01MB9597.INDPRD01.PROD.OUTLOOK.COM>
Date: Mon, 24 Feb 2025 10:43:54 +0000
From: Aditya Garg <gargaditya08@...e.com>
To: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>
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Subject: Re: [PATCH v2 2/3] lib/vsprintf: Add support for generic FOURCCs by
 extending %p4cc



> On 24 Feb 2025, at 4:11 PM, andriy.shevchenko@...ux.intel.com wrote:
> 
> On Mon, Feb 24, 2025 at 10:32:27AM +0000, Aditya Garg wrote:
>>>> On 24 Feb 2025, at 3:54 PM, andriy.shevchenko@...ux.intel.com wrote:
>>> On Mon, Feb 24, 2025 at 10:18:48AM +0000, Aditya Garg wrote:
>>>>>> On 24 Feb 2025, at 3:28 PM, andriy.shevchenko@...ux.intel.com wrote:
>>>>> On Sat, Feb 22, 2025 at 03:46:03PM +0000, Aditya Garg wrote:
>>>>>>>> On 20 Feb 2025, at 10:09 PM, Aditya Garg <gargaditya08@...e.com> wrote:
> 
> ...
> 
>>>>>>> %p4cc is designed for DRM/V4L2 FOURCCs with their specific quirks, but
>>>>>>> it's useful to be able to print generic 4-character codes formatted as
>>>>>>> an integer. Extend it to add format specifiers for printing generic
>>>>>>> 32-bit FOURCCs with various endian semantics:
>>>>>>> 
>>>>>>> %p4ch   Host-endian
>>>>>>> %p4cl Little-endian
>>>>>>> %p4cb Big-endian
>>>>>>> %p4cr Reverse-endian
>>>>>>> 
>>>>>>> The endianness determines how bytes are interpreted as a u32, and the
>>>>>>> FOURCC is then always printed MSByte-first (this is the opposite of
>>>>>>> V4L/DRM FOURCCs). This covers most practical cases, e.g. %p4cr would
>>>>>>> allow printing LSByte-first FOURCCs stored in host endian order
>>>>>>> (other than the hex form being in character order, not the integer
>>>>>>> value).
>>>>> 
>>>>> ...
>>>>> 
>>>>>> BTW, after looking at the comments by Martin [1], its actually better to use
>>>>>> existing specifiers for the appletbdrm driver.  The driver needs the host
>>>>>> endian as proposed by this patch, so instead of that, we can use %.4s
>>>>> 
>>>>> Do you mean this patch will not be needed? If this a case, that would be the
>>>>> best solution.
>>>> 
>>>> I tested with %4pE, and the results are different from expected. So this
>>>> would be preferred. Kindly see my latest email with a proposed workaround for
>>>> the sparse warnings.
>>> 
>>> %.4s sounded okay, but %4pE is always about escaping and the result may occupy
>>> %4x memory (octal escaping of non-printable characters). Of course, you may vary
>>> the escaping classes, but IIRC the octal or hex escaping is unconditional.
>> 
>> %.4s is used for unsigned int iirc, here it's __le32.
> 
> No, it's used to 'char *'. in case one may guarantee that it at least is
> four characters long.

Still the argument here is __le32. %p4h is showing reverse values than what %4pE as well as %.4s shows.
> 
>>>>>> [1]: https://lore.kernel.org/asahi/E753B391-D2CB-4213-AF82-678ADD5A7644@cutebit.org/
>>>>>> 
>>>>>> Alternatively we could add a host endian only. Other endians are not really
>>>>>> used by any driver AFAIK. The host endian is being used by appletbdrm and
>>>>>> Asahi Linux’ SMC driver only.
> 
> --
> With Best Regards,
> Andy Shevchenko
> 
>