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

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Message-ID: <6CB20172-906F-4D13-B5E4-100A9CF74F02@live.com>
Date: Sat, 22 Feb 2025 15:46:03 +0000
From: 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 20 Feb 2025, at 10:09 PM, Aditya Garg <gargaditya08@...e.com> wrote:
> 
> From: Hector Martin <marcan@...can.st>
> 
> %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).
> 
> Signed-off-by: Hector Martin <marcan@...can.st>
> Signed-off-by: Aditya Garg <gargaditya08@...e.com>

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

[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.
> 
> 
> ---
> - *p++ = ' ';
> - strcpy(p, orig & BIT(31) ? "big-endian" : "little-endian");
> - p += strlen(p);
> + if (pixel_fmt) {
> + *p++ = ' ';
> + strcpy(p, orig & BIT(31) ? "big-endian" : "little-endian");
> + p += strlen(p);
> + }