[PATCH] drm/dp: Correctly mask DP_TRAINING_AUX_RD_INTERVAL values for DP 1.4

[Benson Leung]

Hi Matt,

On Tue, Mar 06, 2018 at 10:37:48AM -0800, matthew.s.atwood at intel.com wrote:
> From: Matt Atwood <matthew.s.atwood at intel.com>
> 
> DP_TRAINING_AUX_RD_INTERVAL with DP 1.3 spec changed bit scheme from 8
> bits to 7 bits in DPCD 0x000e. The 8th bit describes a new feature, for
> panels that use this new feature, this would cause a wait interval for
> clock recovery of at least 512 ms, much higher then spec maximum of 16 ms.
> This behavior is described in table 2-158 of DP 1.4 spec address 0000Eh.
> To avoid breaking panels that are not spec compliant we now warn on
> invalid values.
> 
> V2: commit title/message, masking all 7 bits, warn on out of spec values.
> 
> Signed-off-by: Matt Atwood <matthew.s.atwood at intel.com>

Tested-by: Benson Leung <bleung at chromium.org>

Tested this patch on a DP 1.3 panel which sets the
EXTENDED_RECEIVER_CAPABILITY_FIELD_PRESENT bit in DPCD 0000Eh. It has
a value of 0x80 in that field, indicating the extended caps, and 400us for
the Main-Link Channel Equalization phase.

Confirmed that link training passes normally where prior to this it would fail
after the driver waits too long.

Thanks for the fix!

-- 
Benson Leung
Staff Software Engineer
Chrome OS Kernel
Google Inc.
bleung at google.com
Chromium OS Project
bleung at chromium.org
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