Re: [PATCH 1/2] ALSA: hda - Continue probing even if i915 binding fails

[David Henningsson <david.henningsson@canonical.com>]

On 2015-06-12 08:06, Takashi Iwai wrote:
> At Fri, 12 Jun 2015 05:50:43 +0000,
> Lin, Mengdong wrote:
>>
>>
>>> -----Original Message-----
>>> From: Takashi Iwai [mailto:tiwai@suse.de]
>>> Sent: Friday, June 12, 2015 1:09 PM
>>> To: Lin, Mengdong
>>> Cc: alsa-devel@alsa-project.org; David Henningsson; Yang, Libin
>>> Subject: Re: [PATCH 1/2] ALSA: hda - Continue probing even if i915 binding fails
>>>
>>> At Fri, 12 Jun 2015 02:08:50 +0000,
>>> Lin, Mengdong wrote:
>>>>> -----Original Message-----
>>>>> From: Takashi Iwai [mailto:tiwai@suse.de]
>>>>> Sent: Wednesday, June 10, 2015 6:26 PM
>>>>>
>>>>> Currently snd-hda-intel driver aborts the probing of Intel HD-audio
>>>>> controller with i915 power well management when binding with i915
>>>>> driver via
>>>>> hda_i915_init() fails.  This is no big problem for Haswell and
>>>>> Broadwell where the HD-audio controllers are dedicated to HDMI/DP,
>>>>> thus i915 link is mandatory.  However, Skylake, Baytrail and
>>>>> Braswell have only one controller and both HDMI/DP and analog codecs
>>>>> share the same bus.  Thus, even if HDMI/DP isn't usable, we should keep
>>> the controller working for other codecs.
>>>>> For fixing this, this patch simply allows continuing the probing
>>>>> even if
>>>>> hda_i915_init() call fails.  This may leave stale sound components
>>>>> for HDMI/DP devices that are unbound with graphics.  We could abort
>>>>> the probing selectively, but from the code simplicity POV, it's
>>>>> better to continue in all cases.
>>>>>
>>>>> Reported-by: Libin Yang <libin.yang@intel.com>
>>>>> Signed-off-by: Takashi Iwai <tiwai@suse.de>
>>>>> ---
>>>>> This is for 4.1.  Appying to 4.2 may result in trivial conflicts.
>>>>>
>>>>>   sound/pci/hda/hda_intel.c | 3 ++-
>>>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/sound/pci/hda/hda_intel.c b/sound/pci/hda/hda_intel.c
>>>>> index fea198c58196..8a0af6770e1d 100644
>>>>> --- a/sound/pci/hda/hda_intel.c
>>>>> +++ b/sound/pci/hda/hda_intel.c
>>>>> @@ -1855,7 +1855,7 @@ static int azx_probe_continue(struct azx
>>>>> *chip) #ifdef CONFIG_SND_HDA_I915
>>>>>   		err = hda_i915_init(hda);
>>>>>   		if (err < 0)
>>>>> -			goto out_free;
>>>>> +			goto skip_i915;
>>>>>   		err = hda_display_power(hda, true);
>>>>>   		if (err < 0) {
>>>>>   			dev_err(chip->card->dev,
>>>>> @@ -1865,6 +1865,7 @@ static int azx_probe_continue(struct azx
>>>>> *chip) #endif
>>>>>   	}
>>>>>
>>>>> + skip_i915:
>>>>>   	err = azx_first_init(chip);
>>>>>   	if (err < 0)
>>>>>   		goto out_free;
>>>>> --
>>>>> 2.4.3
>>>> Thanks for your patch. But maybe we should not skip i915 for the
>>>> dedicated display HD-A controller on Haswell/Broadwell.
>>>>
>>>> For HSW/BDW, their HD-A controller for display audio (PCI dev#3) is
>>>> *partly* in GPU power domain:
>>>> - Its PCI configuration space is out of i915 power well, but in an
>>>> always-on  power well. So even if the BIOS disables the Intel GPU (PCI
>>>> dev#2), the HD-A  controller is still present, and we can still access
>>>> all standard registers in PCI  config space.
>>> Right, and changing this doesn't break things.
>>>
>>>> - Its audio functions are in the i915 power well. If hda_i915_init()
>>>> fails and
>>>>   I915 power well is unavailable, read/write to the audio register of
>>>> the  Controller will fail and we'll get kernel error messages. So I
>>>> feel it's better  to abort if hda_i915_init() fails for this HD-A controller.
>>> By skipping, the access to i915 power well won't happen later, since the
>>> component ops isn't set.
>>>
>>> I tested Haswell machines with nomodeset to simulate the situation, and there
>>> was no any kernel errors except for the first one indicating the failure of i915
>>> binding.
>>>
>>> I haven't tested Broadwell, though.
>> Hi Takashi,
>>
>> I think there is no error because the i915 power well is ON by default on
>> Haswell and BIOS does not touch this. However, it's possible that the BIOS may
>> disable this power well and depends on i915 driver to turn it on when need, or
>> just disable the power well when disabling the Intel GPU.
>>
>> I remember previously we met audio register I/O error when HSW introduced
>> this power well and i915 disabled it without sync with audio driver.
>>
>> Since we cannot expect the exact BIOS behavior, I feel it's safer not to use this
>> HD-A controller without i915 on HSW and BDW.

If BIOS disables the power well, then we would get the "audio register 
I/O error" in the boot phase, there will be no codecs detected, and thus 
no broken sound card showing up.

The only issue is if the i915 driver disables the power well, but the 
case that the i915 should be functioning enough to turn off the power 
well at some point, and at the same time so broken that we can't bind to 
it, seems extremely unlikely to me.

Either the i915 driver works, or it does not. In both cases, continuing 
probing is the more useful option.

> Fair enough.  Then I'm going to queue the patch below in addition to
> for-linus branch.

We'll have to agree to disagree on that, then.

>
>
> thanks,
>
> Takashi
>
> -- 8< --
> From: Takashi Iwai <tiwai@suse.de>
> Subject: [PATCH] ALSA: hda - Abort the probe without i915 binding for HSW/BDW
>
> The previous patch tried to continue the probe if i915 binding fails.
> For for simplicity reason, we haven't implemented abort even for
> controller chips that are dedicated for HDMI/DP on HSW and BDW.
> However, Mengdong suggested that this can be dangerous; BIOS may
> disable gfx power well although the PCI entry for HD-audio is left,
> and this may result in the unexpected behavior, kernel errors, etc.
>
> For avoiding this situation, abort the probe at i915 binding failure
> only for HSW/BDW chips selectively.  For other chips, it still
> continues.
>
> Fixes: bf06848bdbe5 ('ALSA: hda - Continue probing even if i915 binding fails')
> Reported-by: Mengdong Lin <mengdong.lin@intel.com>
> Signed-off-by: Takashi Iwai <tiwai@suse.de>
> ---
>   sound/pci/hda/hda_intel.c | 18 ++++++++++++++++--
>   1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/sound/pci/hda/hda_intel.c b/sound/pci/hda/hda_intel.c
> index 8a0af6770e1d..a244ba706317 100644
> --- a/sound/pci/hda/hda_intel.c
> +++ b/sound/pci/hda/hda_intel.c
> @@ -340,6 +340,11 @@ enum {
>   #define use_vga_switcheroo(chip)	0
>   #endif
>   
> +#define CONTROLLER_IN_GPU(pci) (((pci)->device == 0x0a0c) || \
> +					((pci)->device == 0x0c0c) || \
> +					((pci)->device == 0x0d0c) || \
> +					((pci)->device == 0x160c))
> +
>   static char *driver_short_names[] = {
>   	[AZX_DRIVER_ICH] = "HDA Intel",
>   	[AZX_DRIVER_PCH] = "HDA Intel PCH",
> @@ -1854,8 +1859,17 @@ static int azx_probe_continue(struct azx *chip)
>   	if (chip->driver_caps & AZX_DCAPS_I915_POWERWELL) {
>   #ifdef CONFIG_SND_HDA_I915
>   		err = hda_i915_init(hda);
> -		if (err < 0)
> -			goto skip_i915;
> +		if (err < 0) {
> +			/* if the controller is bound only with HDMI/DP
> +			 * (for HSW and BDW), we need to abort the probe;
> +			 * for other chips, still continue probing as other
> +			 * codecs can be on the same link.
> +			 */
> +			if (CONTROLLER_IN_GPU(pci))
> +				goto out_free;
> +			else
> +				goto skip_i915;
> +		}
>   		err = hda_display_power(hda, true);
>   		if (err < 0) {
>   			dev_err(chip->card->dev,