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

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Message-ID: <Z7ibRHb2FS1cTx0O@smile.fi.intel.com>
Date: Fri, 21 Feb 2025 17:27:00 +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 Thu, Feb 20, 2025 at 04:39:23PM +0000, Aditya Garg 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).

...

>  	orig = get_unaligned(fourcc);
> -	val = orig & ~BIT(31);
> +	switch (fmt[2]) {
> +	case 'h':
> +		val = orig;
> +		break;
> +	case 'r':
> +		orig = swab32(orig);
> +		val = orig;
> +		break;
> +	case 'l':

> +		orig = le32_to_cpu(orig);
> +		val = orig;
> +		break;
> +	case 'b':
> +		orig = be32_to_cpu(orig);

I do not see that orig is a union of different types. Have you run sparse?
It will definitely complain on this code.

> +		val = orig;
> +		break;
> +	case 'c':
> +		/* Pixel formats are printed LSB-first */
> +		val = swab32(orig & ~BIT(31));
> +		pixel_fmt = true;
> +		break;
> +	default:
> +		return error_string(buf, end, "(%p4?)", spec);
> +	}

...

> -	*p++ = ' ';
> -	strcpy(p, orig & BIT(31) ? "big-endian" : "little-endian");
> -	p += strlen(p);

> +	if (pixel_fmt) {

Technically we can avoid a boolean by checking fmt[2] again here, but I'm okay
with a temporary holder.

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

-- 
With Best Regards,
Andy Shevchenko