Re: [PATCH v2 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

[Laurent Pinchart]

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@xxxxxxxxxx>
> Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
> ---
> 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 ?

> +}
> +
>  static noinline_for_stack
>  char *address_val(char *buf, char *end, const void *addr,
>  		  struct printf_spec spec, const char *fmt)
> @@ -2131,6 +2153,7 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
>   *       correctness of the format string and va_list arguments.
>   * - 'K' For a kernel pointer that should be hidden from unprivileged users
>   * - 'NF' For a netdev_features_t
> + * - '4cc' V4L2 or DRM FourCC code, with "-BE" suffix on big endian formats.
>   * - 'h[CDN]' For a variable-length buffer, it prints it as a hex string with
>   *            a certain separator (' ' by default):
>   *              C colon
> @@ -2223,6 +2246,8 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
>  		return restricted_pointer(buf, end, ptr, spec);
>  	case 'N':
>  		return netdev_bits(buf, end, ptr, spec, fmt);
> +	case '4':
> +		return fourcc_string(buf, end, ptr, spec, fmt);
>  	case 'a':
>  		return address_val(buf, end, ptr, spec, fmt);
>  	case 'd':

-- 
Regards,

Laurent Pinchart