[alsa-devel] [PATCH v2] ALSA: hda - restore BCLK M/N values when resuming HSW/BDW display controller

[Takashi Iwai]

At Fri, 27 Jun 2014 06:38:47 +0000,
Lin, Mengdong wrote:
> 
> Hi Takashi,
> 
> Sorry. This patch cannot handle the following case:
> If only embedded DP is connected on boot, the gfx driver will turn off the display power well on boot case before the audio driver probes and request this power.
> So the M/V values programmed by BIOS in EM4/5 registers will be lost at the very beginning. And so the audio driver will save invalid M/N values on suspend and restore these invalid values on resume. 
> One phenomenon is: if HDMI or DP monitor is connected after boot, audio playback rate is ~20% faster than normal.
> And I guess it's the patch cannot fix the Haswell issue https://bugzilla.kernel.org/show_bug.cgi?id=74861
> 
> There are two possible solutions:
> (1) Follows Windows model: Windows, gfx driver notifies BIOS on LPSP (when only eDP is connected) exit and BIOS reprograms those EM4/5 registers.
>    There is no such implementation in Linux gfx driver now. We need time to check the effort.
> 
> (2) Gfx driver notify audio driver on exiting LPSP and CDCLK value (by reading GPU registers), so audio driver can reprograms EM4/5 registers.
>   The open source interface between audio/gfx driver is still not ready. Would an private API to get CDCLK from gfx driver will be a possible workaround atm?
> 
> Could you share your opinion on this issue?

To me, the former sounds like an easier solution.  In the latter case,
it can be still an issue of module loading order.

In anyway, I keep the former patch as is, since it fixes actually some
cases although it's not perfect.  Please work on top of it for further
fixes.


thanks,

Takashi