[alsa-devel] [Alsa-user] hdmi sound rerouted?

Clemens Ladisch clemens at ladisch.de
Mon Sep 30 09:05:25 CEST 2013

Deucher, Alexander wrote:
>> Even the latest driver lacks the PCI IDs of many HDMI HDA devices
>> (aa50/aa58/aa60/aa68/aa80/aa88/aa90/aa98: Cypress/Juniper/Redwood/Cedar/
>> Cayman+Antilles/Barts/Turks/Caicos).
>> Alex, are these devices that require disabling snooping?
> Sorry what does snooping mean in this context?

Coherent memory accesses, i.e., not setting bit 18 in the stream
descriptor control register; the HDA spec says:
| Traffic Priority (TP):  If set to a 1, the stream will be treated as
| preferred traffic if the underlying bus supports it.  If set to a 0,
| the traffic will be handled on a “best effort” basis.  [...]
| On PCI Express, for example, setting the TP bit to a 1 might cause
| the controller to generate non-snooped isochronous traffic, while
| a PCI implementation may ignore this bit.
| It is permitted to implement this bit as RO if there is no
| controllability available.

So "require" might be wrong, this is just an optimization.  Anyway,
devices up to aa48 use snooping, while from aaa0 (Tahiti XT) forward,
the driver sets TP (<http://git.kernel.org/linus/1815b34a6267>).
The question is, what was the first device to support this?


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