Re: [PATCH v3 06/14] Documentation: drm/bridge: add document for analogix_dp

From: Yakir Yang
Date: Tue Aug 25 2015 - 10:23:58 EST

Hi Thierry,

å 2015/8/25 22:16, Thierry Reding åé:
On Tue, Aug 25, 2015 at 09:48:01PM +0800, Yakir Yang wrote:
Hi Thierry & Rob,

å 2015/8/25 21:27, Rob Herring åé:
On Tue, Aug 25, 2015 at 4:15 AM, Thierry Reding <treding@xxxxxxxxxx> wrote:
On Sun, Aug 23, 2015 at 06:23:14PM -0500, Rob Herring wrote:
On Wed, Aug 19, 2015 at 9:50 AM, Yakir Yang <ykk@xxxxxxxxxxxxxx> wrote:
+ -analogix,link-rate:
+ max link rate supported by the eDP controller.
+ LINK_RATE_1_62GBPS = 0x6, LINK_RATE_2_70GBPS = 0x0A,
+ LINK_RATE_5_40GBPS = 0x14
Same here. I'd rather see something like "link-rate-mbps" and use the
actual rate.
There is no need whatsoever to hard-code this in DT. (e)DP provides the
means to detect what rate the link supports and the specification
provides guidance on how to select an appropriate one.
Good, even better.
I do think we still need keep this DT prop yet.

I think drm_dp_help.c could get the "panel" max link-rate and lane-count,
but it's not enough, we still need knew the "eDP controller" max link-rate
and lane-count.

Let me show the exact example that happened in my side. When I connect
my board to my 2K DP-1.2 TV. Analogix dp driver would get the max link-rate
from dpcd, and the max link-rate is 5.4Gbps. So if I just set eDP controller
to 5.4Gbps, the DP TV just broken, do not light up normally.

This reason why TV broken is the max link-rate which support by RK3288 eDP
controller is 2.7Gbps. Here are the exact words that RK3288 eDP TRM said:

*ïïCompliant with DisplayPortTM Specification, Version 1.2.
ïïCompliant with eDPTM Specification, Version 1.3.
ïïHDCP v1.3 amendment for DisplayPortTM Revision 1.0.
ïïMain link containing 4 physical lanes of 2.7/1.62 Gbps/lane

Beside I haven't found there are some registers would indicate the eDP
max link-rate and lane-count, so this is why I still instance that we need
this DT
prop to indicata "Max rate controller support".

So, I wish you could agree with me on this point.
Your driver should know what link rates it supports and restrict itself
to use those. This is implied by the compatible string and doesn't need
to be duplicated into device tree.

Oh, yeah, good idea :-D
Thanks for your point out.

- Yakir


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