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

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Date:   Mon, 6 Apr 2020 10:37:58 +0300
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Hans Verkuil <hverkuil@...all.nl>, Petr Mladek <pmladek@...e.com>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        laurent.pinchart@...asonboard.com,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        mchehab@...nel.org, linux-media@...r.kernel.org
Subject: Re: [PATCH 1/1] lib/vsprintf: Add support for printing V4L2 and DRM
 fourccs

On Wed, 1 Apr 2020 18:13:32 +0300
Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:

> 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.  
> 
> %p4cc ?
> 

> Another possibility
> 
> 	u8 ch[8];
> 
> 	if (*fourcc & BIT(31)) {
> 		put_unaligned_be32(tmp, &ch[0]);
> 		strcpy(&ch[4], "-BE");
> 	} else {
> 		put_unaligned_le32(tmp, &ch[0]);
> 		strcpy(&ch[4], "-LE");
> 	}
> 	return string(buf, end, &ch[0], spec);
> 

Hi,

mind, if I guess right what that code does, I think this would confuse
the fourcc code endianness and the pixel data endianness. I think fourcc
codes are always crafted machine-endian, regardless of how the pixel
data is. At least, that is what drm_fourcc.h seems to be doing with
fourcc_code(). That has nothing to do with DRM_FORMAT_BIG_ENDIAN which
refers to the pixel data.


Thanks,
pq

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