On Tue 2022-11-08 16:33:22, Russell King wrote: > From: Hector Martin <marcan@xxxxxxxxx> > > %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@xxxxxxxxx> > Signed-off-by: Russell King (Oracle) <rmk+kernel@xxxxxxxxxxxxxxx> > --- > Documentation/core-api/printk-formats.rst | 32 +++++++++++++++++++ > lib/test_printf.c | 39 +++++++++++++++++++---- > lib/vsprintf.c | 35 ++++++++++++++++---- > 3 files changed, 93 insertions(+), 13 deletions(-) > > diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst > index dbe1aacc79d0..92a488884cf8 100644 > --- a/Documentation/core-api/printk-formats.rst > +++ b/Documentation/core-api/printk-formats.rst > @@ -625,6 +625,38 @@ Passed by reference. > %p4cc Y10 little-endian (0x20303159) > %p4cc NV12 big-endian (0xb231564e) > > +Generic FourCC code > +------------------- > + > +:: > + %p4c[hrbl] gP00 (0x67503030) > + > +Print a generic FourCC code, as both ASCII characters and its numerical > +value as hexadecimal. > + > +The additional ``h``, ``r``, ``b``, and ``l`` specifiers are used to specify > +host, reversed, big or little endian order data respectively. Host endian > +order means the data is interpreted as a 32-bit integer and the most > +significant byte is printed first; that is, the character code as printed > +matches the byte order stored in memory on big-endian systems, and is reversed > +on little-endian systems. I though a bit more about the semantic and got a bit confused. It might be because I am not familiar with FourCC. Anyway, the description in the commit message provided some more clues. The following documentation looks be more clear to me: <proposal> Generic FourCC code ------------------- :: %p4c[hrbl] gP00 (0x67503030) Print a generic FourCC code, as both ASCII characters and its numerical value as hexadecimal. The generic FourCC code is always printed in the the big-endian format, the most significant byte first. This is the opposite of V4L/DRM FOURCCs. The additional ``h``, ``r``, ``b``, and ``l`` specifiers define what endianes is used to load the stored value as 32-bit integer. The value might be stored as host-endian, reverse-host-endian, big-endian, or little endian. Examples for a little-endian machine, host native load &(u32)0x67503030:: %p4ch gP00 (0x67503030) %p4cr 00Pg (0x30305067) %p4cb 00Pg (0x30305067) %p4cl gP00 (0x67503030) Examples for a big-endian machine, host native load &(u32)0x67503030:: %p4ch gP00 (0x67503030) %p4cr 00Pg (0x30305067) %p4cb gP00 (0x67503030) %p4cl 00Pg (0x30305067) </proposal> Best Regards, Petr
