[Intel-gfx] [PATCH] Defined NM doesn't work on KBL and uses automatic N/M.

[Wang, Quanxian]

Hi, Jani

Thanks for your comment. My comment below.

-----Original Message-----
From: Jani Nikula [mailto:jani.nikula at linux.intel.com] 
Sent: Friday, May 26, 2017 4:31 PM
To: Wang, Quanxian <quanxian.wang at intel.com>; intel-gfx at lists.freedesktop.org
Cc: Yang, Libin <libin.yang at intel.com>; Wang, Quanxian <quanxian.wang at intel.com>
Subject: Re: [PATCH] Defined NM doesn't work on KBL and uses automatic N/M.

On Fri, 26 May 2017, Quanxian Wang <quanxian.wang at intel.com> wrote:
> According to the bspec, when set N/M, should disable and enable 
> transcoder which attaching DP audio. However with that implementation 
> will affect performance more.
>
> But with current implementation, KBL could not work with defined N/M.
> By default, KBL will use automatic N/M.
>
> Signed-off-by: Quanxian Wang <quanxian.wang at intel.com>
> Reviewed-by: Libin Yang <libin.yang at intel.com>
> Reviewed-by: Jani Nikula <jani.nikula at intel.com>

I have *NOT* reviewed this patch. Even if it's based on my suggestion in [1], it does not automatically signify review.
[Wang, Quanxian] Sorry, I accept your suggestion. So in order to respect what you said, I input your name here.

The patch subject should start with "drm/i915/audio:".
[Wang, Quanxian] Ok.

The DP spec always expresses the values as M/N, more specifically Maud/Naud, so it's confusing to see them expressed as N/M.
[Wang, Quanxian] accept it.

I still have trouble understanding the commit message. I am wondering if something was fixed in Kabylake and beyond, since according to
[Wang, Quanxian] in Ubuntu 17.04, we do audio testing on KBL/SKL/APL and more, the audio doesn't work on KBL specially, and other platform works fine. Even if with latest upstream kernel, but it works on 4.8 kernel. Therefore It is a regression issue. After investigation, we found it is caused by  commit "6014ac122ed081feca99217bc57b2e15c7fc1a51] drm/i915/audio: set proper N/M in modeset". Therefore I provided this patch.
After getting suggestion from you and Libin, we do more testing on 4K monitor for audio delay issue. Also talked with Libin why not use transcoder disable/enable process described in bspec, it will affect performance.
This is the whole story. You can check LCK number 3791 in Intel JIRA about that.

6014ac122ed0 ("drm/i915/audio: set proper N/M in modeset") we need this for certain modes. In particular, should the condition check for KBL || gen >= 10?

[Wang, Quanxian] Basically if upstream could provide a reasonable patch, it will appreciate since you are expert in this area.
Thanks for your understanding.

BR,
Jani.


[1] http://mid.mail-archive.com/87pogs1lz7.fsf@intel.com



> ---
>  drivers/gpu/drm/i915/intel_audio.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/intel_audio.c 
> b/drivers/gpu/drm/i915/intel_audio.c
> index d805b6e..ba4994e 100644
> --- a/drivers/gpu/drm/i915/intel_audio.c
> +++ b/drivers/gpu/drm/i915/intel_audio.c
> @@ -289,6 +289,10 @@ hsw_dp_audio_config_update(struct intel_crtc *intel_crtc, enum port port,
>  	enum pipe pipe = intel_crtc->pipe;
>  	u32 tmp;
>  
> +	/* It doesn't work on KBL and uses automatic N/M. */
> +	if (IS_KABYLAKE(dev_priv))
> +		nm = NULL;
> +
>  	if (nm)
>  		DRM_DEBUG_KMS("using Maud %u, Naud %u\n", nm->m, nm->n);
>  	else

--
Jani Nikula, Intel Open Source Technology Center