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From: "Li, Sun peng (Leo)" <Sunpeng.Li@amd.com>
To: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: "Zuo, Jerry" <Jerry.Zuo@amd.com>,
	"amd-gfx@lists.freedesktop.org" <amd-gfx@lists.freedesktop.org>,
	dri-devel <dri-devel@lists.freedesktop.org>
Subject: Re: [RFC 0/2] Add AUX device entries for DP MST devices
Date: Tue, 16 Apr 2019 15:28:24 +0000	[thread overview]
Message-ID: <042b83bf-e256-2ff2-c7a6-b3128699b829@amd.com> (raw)
In-Reply-To: <312744c9-53b8-76e4-1fc0-c3bba556e0d0@amd.com>

>> Hmm. My MST-foo is admittedly weak so I'm not sure. A quick trawl through
>> the spec didn't provide any solid explanations either :( However eg.
>> "Figure 2-83: Example Multi-function MST Branch-Sink Device Enumeration"
>> in the DP 1.4 spec does appear to show kind of virtual DPCD thing behind
>> a logical port. But I'm not really sure what than means.
> 
> Good point, I'm gonna dig more into that. It sounds like we should be
> able to access that with the relative addressing as defined by the spec
> (2.11.5). I'll have to see why that's currently getting nacked though.
> 

It looks like DPCD reads on logical ports work on some devices, and not
others... I swapped my MST display out with a different one, and it read
just fine.

More specifically - in a daisy chain setup with 2 MST displays - with
the one that works at the end:

# auxrw.py read /dev/drm_dp_aux4_mst\:0-1-8 0x30-0x3f -> ACK
# auxrw.py read /dev/drm_dp_aux6_mst\:0-1 0x30-0x3f -> ACK
(The GUIDs returned are identical)

With the one that doesn't at the end:

# auxrw.py read /dev/drm_dp_aux4_mst\:0-1-8 0x30-0x3f -> *NAK*
# auxrw.py read /dev/drm_dp_aux6_mst\:0-1 0x30-0x3f -> ACK

So it seems there's some device dependent behavior here. I'm not sure if
there's a better way of handling this besides exposing all the
downstream ports: If it works, great. If not, just don't use it?

Leo
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  reply	other threads:[~2019-04-16 15:28 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-12 16:05 [RFC 0/2] Add AUX device entries for DP MST devices sunpeng.li
2019-04-12 16:05 ` [PATCH 1/2] drm/dp_aux: Use non-cyclic idr, add suffix option for aux device sunpeng.li
     [not found]   ` <1555085131-8716-2-git-send-email-sunpeng.li-5C7GfCeVMHo@public.gmane.org>
2019-04-16 22:16     ` Lyude Paul
     [not found]       ` <d616d0485a41b93e3da660b936af64968b88ec5a.camel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-04-17 23:10         ` Li, Sun peng (Leo)
     [not found]           ` <dcbb8aaa-0855-b2a6-9aa1-d42ce441740a-5C7GfCeVMHo@public.gmane.org>
2019-04-22 17:39             ` Lyude Paul
     [not found] ` <1555085131-8716-1-git-send-email-sunpeng.li-5C7GfCeVMHo@public.gmane.org>
2019-04-12 16:05   ` [PATCH 2/2] drm/dp_mst: Register aux-dev nodes for MST ports sunpeng.li-5C7GfCeVMHo
2019-04-16 22:22     ` Lyude Paul
     [not found]       ` <a67eaacd5601de0cab21b04d7432454892e47304.camel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-04-17 13:09         ` Ville Syrjälä
2019-04-12 17:30   ` [RFC 0/2] Add AUX device entries for DP MST devices Ville Syrjälä
     [not found]     ` <20190412173011.GK3888-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
2019-04-15 18:50       ` Li, Sun peng (Leo)
2019-04-16 15:28         ` Li, Sun peng (Leo) [this message]
     [not found]           ` <042b83bf-e256-2ff2-c7a6-b3128699b829-5C7GfCeVMHo@public.gmane.org>
2019-04-16 16:55             ` Ville Syrjälä

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