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

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Message-ID: <Z7xMt2Kp1pFuMar2@smile.fi.intel.com>
Date: Mon, 24 Feb 2025 12:40:55 +0200
From: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>
To: Aditya Garg <gargaditya08@...e.com>
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Subject: Re: [PATCH v2 2/3] lib/vsprintf: Add support for generic FOURCCs by
 extending %p4cc

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.

> >>>> [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