i.MX6 HDMI aspect ratio issue within X11

[Marco Trillo <martri@arantia.com>]

Hi all,

I'm facing a curious HDMI aspect ratio issue within X11 on i.MX6.
My setup is Yocto 1.6.1 `Daisy' with xserver-xorg-1.14.4 and kernel 
3.10.17-1.0.0-wandboard.

If I boot with video=mxcfb0:dev=hdmi,1280x720M@50,bpp=32 in 
/proc/cmdline, I get a 720P50 mode with the HDMI aspect ratio flag set 
to 16/9. The mode is "S:1280x720p-50" as listed in 
/sys/class/graphics/fb0/mode.

If I then start the X server, the mode is changed to "U:1280x720p-50" 
and the HDMI aspect ratio flag is set to 4/3.

This "U:1280x720p-50" seems to be a new mode, as it was not previously 
listed in /sys/class/graphics/fb0/modes, but it is listed now in 
addition to the previous  "S:1280x720p-50". Being a new mode, it lacks 
the imx-specific FB_VMODE_ASPECT_16_9 flag and the mxc_hdmi driver 
defaults to 4/3 aspect ratio.

However, this "U:1280x720p-50" seems to be identical to the 
"S:1280x720p-50" mode (both modes when listed with "fbset -i" appear 
identical). In fact, the kernel seems to confuse the two modes, as if I 
change manually to "S:1280x720p-50" in /sys/class/graphics/fb0/mode I 
still get the 4/3 aspect ratio flag (with other S:___ modes I get the 
correct flag).

The strange part is that if I boot with ...@60... instead of ...@50... 
(video=mxcfb0:dev=hdmi,1280x720M@60,bpp=32), the mode is now 
"S:1280x720p-60", but it is now preserved when starting X, including the 
16/9 aspect ratio flag.

The Xorg.0.log does not contain anything strange:

[374954.464] (II) VIVANTE(0): Output DISP3 BG using initial mode 
S:1280x720p-50
[374954.594] (II) VIVANTE(0): imxDisplayPreInit: virtual set 1280 x 720, 
display width 0
[374954.594] (II) VIVANTE(0): FBDevPreInit: adjust display width 1280
[374954.594] (**) VIVANTE(0): PreInit done
[374954.594] (--) Depth 24 pixmap format is 32 bpp
[374954.594] (II) VIVANTE(0): Init mode for fb device
[374954.673] (II) VIVANTE(0): hardware: DISP3 BG (video memory: 16330kB)

(When booting with ...@60... it is the same except that the first line 
ends with "S:1280x720p-60").

What could be causing such a strange issue? Did anyone ran into 
something similar when running X11 or is it something specific in my setup?

Thanks in advance,
Marco.