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

[Russell King - ARM Linux]

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...)

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.

-- 
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