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

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Message-ID: <20200402071801.GA1522@kekkonen.localdomain>
Date:   Thu, 2 Apr 2020 10:18:01 +0300
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Petr Mladek <pmladek@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-media@...r.kernel.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        laurent.pinchart@...asonboard.com, mchehab@...nel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 1/1] lib/vsprintf: Add support for printing V4L2 and DRM
 fourccs

Hi Hans,

Thank you for the review.

On Wed, Apr 01, 2020 at 04:13:51PM +0200, Hans Verkuil wrote:
> On 4/1/20 4:05 PM, 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@...nel.org>
> > Signed-off-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > ---
> >  Documentation/core-api/printk-formats.rst | 11 +++++++++
> >  lib/vsprintf.c                            | 29 +++++++++++++++++++++++
> >  2 files changed, 40 insertions(+)
> > 
> > diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
> > index 8ebe46b1af39..b6249f513c09 100644
> > --- a/Documentation/core-api/printk-formats.rst
> > +++ b/Documentation/core-api/printk-formats.rst
> > @@ -545,6 +545,17 @@ For printing netdev_features_t.
> >  
> >  Passed by reference.
> >  
> > +V4L2 and DRM fourcc code (pixel format)
> > +---------------------------------------
> > +
> > +::
> > +
> > +	%ppf
> > +
> > +Print a 4cc code used by V4L2 or DRM.
> 
> FourCC appears to be the more-or-less official name (https://en.wikipedia.org/wiki/FourCC)
> 
> I would explain about the -BE suffix for bigendian fourcc variants.

Agreed, I'll address these in v2.

> 
> > +
> > +Passed by reference.
> > +
> >  Thanks
> >  ======
> >  
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index 7c488a1ce318..b39f0ac317c5 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -1721,6 +1721,32 @@ char *netdev_bits(char *buf, char *end, const void *addr,
> >  	return special_hex_number(buf, end, num, size);
> >  }
> >  
> > +static noinline_for_stack
> > +char *pixel_format_string(char *buf, char *end, const u32 *fourcc,
> > +			  struct printf_spec spec, const char *fmt)
> > +{
> > +	char ch[2] = { 0 };
> 
> This can just be '{ };'

As Andy, I also do prefer { 0 }.

> 
> > +	unsigned int i;
> > +
> > +	if (check_pointer(&buf, end, fourcc, spec))
> > +		return buf;
> > +
> > +	switch (fmt[1]) {
> > +	case 'f':
> > +		for (i = 0; i < sizeof(*fourcc); i++) {
> > +			ch[0] = *fourcc >> (i << 3);
> 
> You need to AND with 0x7f, otherwise a big endian fourcc (bit 31 is set)
> will look wrong. Also, each character is standard 7 bit ascii, bit 7 isn't
> used except to indicate a BE variant.

Good point, will fix for v2.

> 
> > +			buf = string(buf, end, ch, spec);
> > +		}
> > +
> > +		if (*fourcc & BIT(31))
> > +			buf = string(buf, end, "-BE", spec);
> > +
> > +		return buf;
> > +	default:
> > +		return error_string(buf, end, "(%pp?)", spec);
> > +	}
> > +}
> > +
> >  static noinline_for_stack
> >  char *address_val(char *buf, char *end, const void *addr,
> >  		  struct printf_spec spec, const char *fmt)
> > @@ -2131,6 +2157,7 @@ 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
> > + * - 'pf' V4L2 or DRM pixel format.
> 
> I'd say 'FourCC format' instead of 'pixel format'.

Will fix.

-- 
Regards,

Sakari Ailus