Hi Laurent,

Thanks for the comments.

On Fri, Apr 03, 2020 at 01:24:49PM +0300, Laurent Pinchart wrote:

Hi Sakari,

Thank you for the patch.

On Fri, Apr 03, 2020 at 12:11:56PM +0300, Sakari Ailus wrote:

...

Add a printk modifier %ppf (for pixel format) for printing V4L2 and DRM pixel formats denoted by 4ccs. The 4cc encoding is the same for both so the same implementation can be used.

Suggested-by: Mauro Carvalho Chehab mchehab@kernel.org Signed-off-by: Sakari Ailus sakari.ailus@linux.intel.com

---

since v1:

• Improve documentation (add -BE suffix, refer to "FourCC".

• Use '%p4cc' conversion specifier instead of '%ppf'.

• Fix 31st bit handling in printing FourCC codes.

• Use string() correctly, to allow e.g. proper field width handling.

• Remove loop, use put_unaligned_le32() instead.

Documentation/core-api/printk-formats.rst | 12 +++++++++++ lib/vsprintf.c | 25 +++++++++++++++++++++++ 2 files changed, 37 insertions(+)

diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst index 8ebe46b1af39..550568520ab6 100644 --- a/Documentation/core-api/printk-formats.rst +++ b/Documentation/core-api/printk-formats.rst @@ -545,6 +545,18 @@ For printing netdev_features_t.

Passed by reference.

+V4L2 and DRM FourCC code (pixel format) +---------------------------------------

+::

• %p4cc

+Print a FourCC code used by V4L2 or DRM. The "-BE" suffix is added on big endian +formats.

+Passed by reference.

Thanks

diff --git a/lib/vsprintf.c b/lib/vsprintf.c index 7c488a1ce318..93eea6a320da 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -1721,6 +1721,28 @@ char *netdev_bits(char *buf, char *end, const void *addr, return special_hex_number(buf, end, num, size); }

+static noinline_for_stack +char *fourcc_string(char *buf, char *end, const u32 *fourcc,

•     struct printf_spec spec, const char *fmt)
  

+{ +#define FOURCC_STRING_BE "-BE"

• char s[sizeof(*fourcc) + sizeof(FOURCC_STRING_BE)] = { 0 };
• if (check_pointer(&buf, end, fourcc, spec))

• return buf;
  

• if (fmt[1] != 'c' || fmt[2] != 'c')

• return error_string(buf, end, "(%p4?)", spec);
  

• put_unaligned_le32(*fourcc & ~BIT(31), s);
• if (*fourcc & BIT(31))

• strscpy(s + sizeof(*fourcc), FOURCC_STRING_BE,
  

• 	sizeof(FOURCC_STRING_BE));
  

• return string(buf, end, s, spec);

Taking V4L2_PIX_FMT_Y16_BE as an example, this will print 'Y16 -BE' (without quotes). There are other 4CCs that contain spaces and would suffer from a similar issue. Even in little-endian format, it would result in additional spaces in the output string. Is this what we want ? Should the caller always enclose the 4CC in quotes or brackets for clarity ? Or should still be done here ?

Good question. Space is indeed a valid character in a 4cc code.

If I omit one or more spaces, it will no longer be a 4cc, but a 3cc or even a 2cc. Jokes aside, there are probably fair arguments both ways.

I presume there's no 4cc code where the location of a space would make a difference but all of the spaces are trailing spaces.

It's also worth noting that the formats printed are mostly for debugging purpose and thus even getting a hypothetical case wrong is not a grave issue. This would also support just printing them as-is though.

I'm leaning slightly towards omitting any spaces if the code has them. This is something that couldn't be done by using a macro...