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

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Message-ID: <9C31901A-2C5C-478B-BB1C-D4705939EE4D@live.com>
Date: Fri, 21 Feb 2025 19:35:53 +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 21 Feb 2025, at 8:57 PM, andriy.shevchenko@...ux.intel.com wrote:
> 
> 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.

Does this look good now? Made orig a union.

char *fourcc_string(char *buf, char *end, const u32 *fourcc, const char *fmt, struct printf_spec spec) 
{
char output[sizeof("0123 little-endian (0x01234567)")];
char *p = output;
unsigned int i;
bool pixel_fmt = false;
u32 val;

union {
u32 raw;
__le32 le;
__be32 be;
} orig;

if (fmt[1] != 'c')
return error_string(buf, end, "(%p4?)", spec);

if (check_pointer(&buf, end, fourcc, spec))
return buf;

orig.raw = get_unaligned(fourcc);

switch (fmt[2]) {
case 'h':
val = orig.raw;
break;
case 'r':
val = swab32(orig.raw);
break;
case 'l':
val = le32_to_cpu(orig.le);
break;
case 'b':
val = be32_to_cpu(orig.be);
break;
case 'c':
val = swab32(orig.raw & ~BIT(31));
pixel_fmt = true;
break;
default:
return error_string(buf, end, "(%p4?)", spec);
}

for (i = 0; i < sizeof(u32); i++) {
unsigned char c = val >> ((3 - i) * 8);
*p++ = isascii(c) && isprint(c) ? c : '.';
}

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

*p++ = ' ';
*p++ = '(';
p += sprintf(p, "0x%08x", orig.raw);
*p++ = ')';
*p = '\0';

return string_nocheck(buf, end, output, spec);
}