[PATCH 13/22] drm: bridge: dw-hdmi: Replace device type with platform quirks

[Laurent Pinchart]

Hi Russell,

On Friday 02 Dec 2016 16:08:17 Russell King - ARM Linux wrote:
> On Fri, Dec 02, 2016 at 05:43:43PM +0200, Laurent Pinchart wrote:
> > DW_HDMI_QUIRK_FC_INVIDCONF is indeed a bad name, I'll fix that.
> > 
> > Do you know why this function needs to write to the HDMI_FC_INVIDCONF
> > register  four times in the normal case, and once only for IMX6DL ?
> 
> (I don't have much time at present, I'm buried in ARM64 crud trying to
> get a platform to boot, and working out how to debug an ARM64 platform
> that even earlycon doesn't work on... no printascii() types of easy
> debug facilities on ARM64 make this job several orders of magnitude
> harder than it needs to be...)

Thanks all the more for taking time to reply.

> It prevents a magenta line down the left hand side of the screen, which
> is caused when the frame composer in the HDMI Tx gets confused -
> according to the errata, it does a load of maths when you write to the
> frame composer registers, and sometimes it doesn't update properly.
> 
> So, the four writes (and the number is critical) is there to persuade
> the IP to do the maths with the right values so the internal timings
> are correct.
> 
> The rather confusing thing is - it's actually IMX6Q which has the more
> severe errata, not IMX6DL - the workaround of four writes is applied
> in the 6Q case.
> 
> It's covered by ERR004308 in the IMX6Q Errata document (search for
> IMX6DQCE).  It isn't mentioned in the IMX6DL documentation, but it
> seems that similar workaround of one write is necessary there.
> 
> Some of this was determined by experimentation in conjunction with the
> errata documentation - I remember it took a while to get it right so
> that we didn't ever see the magenta line.

That's interesting. I'll test the different options on my Renesas platform (no 
write, one write, four writes) and see what is needed. Could anyone perform 
the same tests on a Rockchip RK3288 system ?

-- 
Regards,

Laurent Pinchart