[U-Boot] [PATCH 4/4] sunxi: video: Force h/vsync active high when using ext. vga dac

[Hans de Goede]

Hi,

On 28-01-15 10:13, Ian Campbell wrote:
> On Sun, 2015-01-25 at 15:35 +0100, Hans de Goede wrote:
>> On both my A13-OLinuxIno and my A13-OLinuxIno-Micro, the vga output gives an
>> unstable image when active low v or hsync is used, so when an external vga dac
>> is used force v and hsync active high independent of what the modeline says.
>
> Does this not just mean the modeline is wrong and should be fixed or an
> alternative provided which is correct for these systems?

I would rather not, the modeline uses is a vesa standard timing, so it
certainly is correct.

The problem seems to be specific with the OLinuxIno A13 (normal & micro)
boards. I've just looked up the schematics and they use an opendrain driver
for the vga sync lines, and with sync pulses it is the logical high->low
edge of the pulse which counts for the timing, which with an active low
sync is being driven by the pull-up, and that simply seems to not drive
it hard enough to get a stable image.

> In the code comment you say "some dacs", what about ones which don't
> have this issue or, worse, ones which have the inverse issue and don't
> work with active high?
>
> At the least I'd say this aught to be controllable via Kconfig.

I agree this should be a Kconfig and only turned on on the Olinuxino
13 boards. I'll do a v2.

Regards,

Hans