Re: OMAP4 Panda DVI problem

[Roger Quadros <rogerq@ti.com>]

Hi Tomi,

On 06/14/2013 04:52 PM, Tomi Valkeinen wrote:
> Hi Tony, Roger,
> 
> I recently raised the i2c issue with reading EDID from a DVI monitor on
> Panda, when booting with DT. I just couldn't get reading EDID work at
> all reliably.
> 
> I now see problems with non-DT also. Booting with 3.10-rc5, with DSS
> compiled into the kernel, reading EDID fails during the boot. I can
> later read the EDID fine from userspace.
> 
> With help from Roger, we found the issue.
> 
> DVI cable has a +5V line, that the source (i.e. Panda) has to provide.
> This +5V is required to read EDID, but according to the documentation I
> think it's actually required to show video also (although I guess this
> could depend on the monitor).
> 
> I think for OMAP3 boards the +5V is always-on (I haven't looked at OMAP3
> boards yet, though). For Panda, the +5V comes from DC_HST_5V, controlled
> with a GPIO, which is used for USB and currently managed by USB in
> arch/arm/mach-omap2/usb-host.c, which creates a fixed voltage hsusbX_vcc
> regulator.
> 
> So, basically, if USB is disabled, DVI doesn't work. This is why I see
> problems with DT (no USB), and non-DT (USB enabled after DVI is enabled).
> 
> To fix this properly I need to add regulator handling to the DVI driver,
> and we need to share the same supply with USB and DVI.
> 
> However, while the above shouldn't be particularly difficult, it's still
> not simple. And additional complications come with DT boot, as DSS
> doesn't support DT yet. We have the hack that enables DSS for Panda and
> SDP board when booting with DT, but I don't think it's possible to
> manage the shared regulator easily in that setup (ie. USB and the
> regulator being described in DT, DSS and DVI added the old way in
> dss-common.c).
> 
> So I think the simplest solution is to make DC_HST_5V always-on. This
> should be fixed for 3.10 also.

I am fine with this.

> 
> Any idea if DC_HST_5V is a big power sink if it's always enabled? If
> it's a big one, I could look at how to hack the DVI driver so that it
> would find the regulator. I'm quite sure I won't be neat, though.
> 

DC_HST_5V is going to be active if USB is active. So I wouldn't be much
worried if this supply is left always ON for people needing USB host &
ethernet. (NOTE, ethernet is part of the same USB hub chip)

cheers,
-roger