Re: [PATCH] drm/i915/hotplug: Fixing storm handling for digital ports

[Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>]

On 7/1/2015 6:08 PM, Daniel Vetter wrote:
> On Tue, Jun 30, 2015 at 03:47:41PM +0300, Ville Syrjälä wrote:
>> On Tue, Jun 30, 2015 at 03:30:16PM +0300, Jani Nikula wrote:
>>> On Tue, 30 Jun 2015, Daniel Vetter <daniel@ffwll.ch> wrote:
>>>> On Tue, Jun 30, 2015 at 01:19:57PM +0300, Jani Nikula wrote:
>>>>> On Tue, 30 Jun 2015, Daniel Vetter <daniel@ffwll.ch> wrote:
>>>>>> On Tue, Jun 30, 2015 at 08:45:48AM +0530, Sivakumar Thulasimani wrote:
>>>>>>>
>>>>>>> On 6/29/2015 10:07 PM, Daniel Vetter wrote:
>>>>>>>> On Mon, Jun 29, 2015 at 04:30:40PM +0530, Sivakumar Thulasimani wrote:
>>>>>>>>> From: "Thulasimani, Sivakumar" <sivakumar.thulasimani@intel.com>
>>>>>>>>>
>>>>>>>>> HPD storm is detected in intel_hpd_irq_handler and disabled for respective
>>>>>>>>> port immediately but polling is enabled only in i915_hotplug_work_func and
>>>>>>>>> not in i915_digport_work_func. This will result in disabled hpd never enabled
>>>>>>>>> back again. This is fixed by calling the appropriate storm disable function
>>>>>>>>> that will handle the rest of the sequence (both polling enable and reenabling
>>>>>>>>> of HPD later).
>>>>>>>>> ---
>>>>>>>>>   drivers/gpu/drm/i915/intel_hotplug.c |    4 ++++
>>>>>>>>>   1 file changed, 4 insertions(+)
>>>>>>>>>
>>>>>>>>> diff --git a/drivers/gpu/drm/i915/intel_hotplug.c b/drivers/gpu/drm/i915/intel_hotplug.c
>>>>>>>>> index 3c53aac..8e18587 100644
>>>>>>>>> --- a/drivers/gpu/drm/i915/intel_hotplug.c
>>>>>>>>> +++ b/drivers/gpu/drm/i915/intel_hotplug.c
>>>>>>>>> @@ -205,6 +205,10 @@ static void i915_digport_work_func(struct work_struct *work)
>>>>>>>>>   	dev_priv->hotplug.long_port_mask = 0;
>>>>>>>>>   	short_port_mask = dev_priv->hotplug.short_port_mask;
>>>>>>>>>   	dev_priv->hotplug.short_port_mask = 0;
>>>>>>>>> +
>>>>>>>>> +	/* Disable hotplug on connectors that hit an irq storm. */
>>>>>>>>> +	intel_hpd_irq_storm_disable(dev_priv);
>>>>>>>> digport_work_func schedules the hotplug handler for everything not
>>>>>>>> handled, which should result in this getting called. It really shouldn't
>>>>>>>> matter when exactly it gets called.
>>>>>>>>
>>>>>>>> Can you please provide more data and details for your analysis? Like bug
>>>>>>>> reports, backtraces and dmesg traces showing that the handler is stuck and
>>>>>>>> similar things.
>>>>>>>>
>>>>>>>> Also your patch is missing the s-o-b line.
>>>>>>>> -Daniel
>>>>>>>>
>>>>>>> there is no bug filed for this, it was observed as part of code analysis
>>>>>>> (that is provided below)
>>>>>>> I'll try to get more info as soon as i get access to a system.
>>>>>>>
>>>>>>> short answer:
>>>>>>> the issue will be seen during hpd storm, where the last HPD is handled
>>>>>>> inside intel_dp_hpd_pulse.
>>>>>>> so i915_hotplug_work_func will not be queued thus missing the storm_disable
>>>>>>> call.
>>>>>>>
>>>>>>> long answer :
>>>>>>> To give a bit more background, lets assume that we get a call to
>>>>>>> intel_hpd_irq_handler, with params long pulse for DP panel in PORT_B
>>>>>>> on a HSW/BDW system during HPD storm scenario.
>>>>>>> The following sequence will take place
>>>>>>> *) is_dig_port will be set and will result in queue_dig being set as well
>>>>>>> *) intel_hpd_irq_storm_detect will detect that this is 6th hpd call within
>>>>>>> the
>>>>>>> HPD_STORM_DETECT_PERIOD and so will mark the HPD status of PORT_B to
>>>>>>> HPD_MARK_DISABLED
>>>>>>> *) This will result in HPD for PORT_B being disabled immediately(masked in
>>>>>>> case of LPT)
>>>>>>> *) i915_digport_work_func will be queued at the end of this function, since
>>>>>>> queue_dig is set
>>>>>>> *) once in the i915_digport_work_func, hpd_pulse func pointer will be
>>>>>>> executed since it is defined for DP
>>>>>>> *) intel_dp_hpd_pulse, will have long_hpd set and since the panel is plugged
>>>>>>> in still,
>>>>>>> ISR will be high and so will return true.
>>>>>>> *) intel_dp_get_dpcd, will succeed since DP is connected
>>>>>>> *) finally IRQ_HANDLED will be returned
>>>>>>> *) once call exits intel_hpd_irq_handler, HPD on port B will never be
>>>>>>> enabled again
>>>>>>> (unmasked in case of LPT) and no more hot plug notifications.
>>>>>> The assumption of the storm code is that when there is a DP sink, a storm
>>>>>> will never happen. We need that since otherwise the mst code (which
>>>>>> creates ridiculous amounts of hpds on the DP port) will run into the storm
>>>>>> detection code all the time.
>>>>>>
>>>>>> Might be better to document this design assumption somewhere, but it is
>>>>>> baked in. Hence my question whether you've seen this happen in the real
>>>>>> world - DP storms haven't been observed yet afaik, and it would be a much
>>>>>> more serious problem.
>>>>> The dp short hotplug irqs (used by mst) are not caught by the irq storm
>>>>> code, but the long hotplug irqs are.
>>>> We assume there's no DP hotplug storms ever, whether long or short pulses.
>>>> Trying to fix that will require serious rework since we need to wait until
>>>> dig_port_work has run to know whether the hpd was a real one or just
>>>> fluctuation, and only update storm statistic then. And once we do DP is
>>>> essentially broken, which means we also need to enable polling for dp aux
>>>> short pulses (which will probably piss off some sink device).
>>>>
>>>> In short: If you have a hpd storm, and there's something DP-like
>>>> connected, you're screwed. Until we have real-world evidence of this
>>>> happening updating comments is really the only thing we need.
>>> In that case we should update the code to never do hotplug irq storm
>>> detection on dp long hpd, which we currently do.
>> The HPD pin is shared for DP and HDMI so we can't disable HPD just for
>> HDMI when a storm is detected.
> Yup this is the crux. The real fix really is wiring up IRQ_NONE handling
> all the way to be able to differentiate storms from normal/expected irq
> load. But that really has to wait until this happens in reality somewhere
> with a DP sink.
> -Daniel
>
when checking internally i was informed that DP hotplug storm is also 
normal like HDMI
if not more frequent, but for now i'll wait till this comes as a real 
world issue and drop this
patch.

-- regards, Sivakumar
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