EDID modes unavailable when no connector/crtc available at boot

[Tony Prisk]

On 11/08/13 20:42, Dave Airlie wrote:
> On Sun, Aug 11, 2013 at 2:41 PM, Tony Prisk <linux at prisktech.co.nz> wrote:
>> I am working on the HDMI driver for the i.MX6 as part of the larger DRM
>> driver written by Sascha Hauer and need a little advice. I seem to be
>> missing one important part of the subsystem that I haven't been able to
>> resolve.
>>
> fbcon is limited by boot sizes as at least with dynamic memory
> management and how fbdev works resizing the allocation is nearly
> impossible to do race free, since fbdev will hand out mmaps to
> userspace and that stops you from ever moving anything once the device
> is open.
>
> But this is only for the fbdev emulation, a real kms application
> should be able to use a larger size no problems.
>
> Dave.
It seems to be worse than just a fbcon issue as far as I can tell.

I am making an assumption, but I believe 
'/sys/class/drm/card0-HDMI-A-1/modes' should list all the supported 
modes of the connector (regardless of fbcon).
Using 'cat /sys/class/drm/card0-HDMI-A-1/modes', it appears the 
supported modes are being limited by fbcon

1) HDMI Cable connected at bootup (fb @ 1920x1080)
cat /sys/class/drm/card0-HDMI-A-1/modes
1920x1080
1280x720
1280x720
720x576
720x480
640x480

2) HDMI Cable NOT connected at bootup (fb @ 1024x768), cable is then 
connected after userspace has started (still in console)
cat /sys/class/drm/card0-HDMI-A-1/modes
720x576
720x480
640x480


Following back through the source:

static struct drm_connector_funcs imx_hdmi_connector_funcs = {
     .fill_modes = drm_helper_probe_single_connector_modes,
     ...
};

static struct drm_connector_helper_funcs imx_hdmi_connector_helper_funcs = {
     .get_modes = imx_hdmi_connector_get_modes,
     .mode_valid = imx_hdmi_connector_mode_valid,
     ...
};


It appears that only drm_helper_probe_single_connector_modes() calls 
.get_modes() and .mode_valid()
.fill_modes() is called from drm_fb_helper_probe_connector_modes(), 
which is called from drm_fb_helper_hotplug_event()
drm_fb_helper_hotplug_event() sets max_width to fb_helper->fb->width, 
and max_height to fb_helper->fb->height.
fb->width is 1024 if booted without the cable connected, hence the 
clipping of the values.

Regards
Tony Prisk