Re: [PATCH 3/3] drm/i915: Read HDMI EDID only when required

["Sharma, Shashank" <shashank.sharma@intel.com>]

Regards
Shashank

On 7/1/2015 6:26 PM, Daniel Vetter wrote:
> On Tue, Jun 30, 2015 at 09:49:57PM +0530, Shashank Sharma wrote:
>>> Userspace always sets force. Are you sure this actually improves anything?
>> Yes we do. We have had this code for commercial projects, and that's really
>>   important to have proper interrupt handling as well as to avoid race
>> condition between multiple HDMI detects from interrupt handler and
>> userspace detect calls. This is a must for HDMI compliance also.
>
> There's no race, we have locks for this. And we already have a little bit
> of edid caching, and you're code won't add more caching for the force =
> true case. Which is why I wondered whether you've really seen improvements
> with this on latest upstream, and not just an older android tree.
This caching is not useful, as in every detect call, we are doing a 
unset EDID, and doing a set EDID again. The actual reason behind 
multiple HDMI EDID reads is detect() getting called from user space 
only.So every time there is a detect call, there is HDMI EDID read.

We should read EDID only on hotplug, cache it, and reuse it until hot 
unplug.
>
>> Actually the plan is to use this force for GEN < 6 HW only, where the
>> hotplug doesn't work reliably (I remember our last conversation on some old
>> HW which doesn't support HPD properly). For vlv+, we can (will) use only
>> the cached EDID.
>
> Ok, once more: HDMI hpd is unreliable everywhere. I have a gen7 here which
> is half-busted it seems, and we've found examples for every single
> platform that supports hdmi out there. The problem isn't necessarily spec
> compliant HDMI sinks, but all the other crap ppl like to plug in.
>
> Yes this means we'll not be spec compliant, but if we have reality vs.
> spec, reality wins. At least in upstream.

We have tested HPD with several HDMI monitors for VLV, CHV SKL and now 
for BXT also. We are getting reliable hotplugs and unplugs across these 
platforms, with accurate information on long/short pulses. Can you 
please give some details about what is your observation ?

Its very important for the Android projects to comply with the spec due 
to certification pressure from customers. And we can get a common path 
for us, if we know what exactly is the problem. But for sure we cant 
ignore this factor that compliance is essential.
>
>>> Also the goal should be to keep things cache for a few calls from
>>> userspace (since often it pokes a few times in a row unfortuantely), for
>>> which we need a proper timeout to clear the edid again.
>>
>> Can you please let us know why ? Why do we need to clear this EDID caching
>> ? We should clear it only in the next hot-unplug, and maintain this cached
>> EDID for all userspace detect operations. I believe as long as we have the
>> state machine maintained, we need not to clear it.
>
> hotplug is not reliable, at least not outside of labs and validation
> testers. And your code here throws the cached edid away every time force =
> true is set, which is pretty much always. At least on upstream.
>
> The only place where we don't set force is in the poll worker, and that's
> only run when we have a hpd storm.
> -Daniel
The new code doesn't throw away cached EDID for platform's > GEN6
but the current code does that, in every detect call.

The current state machine is:
=============================
1. Hotplug -> Unset EDID, Read EDID, Set edid
2. all detect calls -> Unset EDID, read EDID, Set EDID
3. Hotunplug -> Unset EDID

The state machine I am suggesting is:
=====================================
1. Hotplug -> throw away cached EDID, cache new one probing DDC
2. all detect() calls ->
	use cached EDID only
3. hot unplug -> throw away cached EDID,

OR if you want to support some old platforms:
=============================================
1. Hotplug -> throw away cached EDID, cache new one probing DDC
2. all detect() calls -> (Support old HW with unstable HPD)
	if (gen > GEN6)
		use cached EDID only
	else
		probe DDC and read EDID, update cache
3. hot unplug -> throw away cached EDID,
>
>>
>>> -Daniel
>>
>> Regards
>> Shashank
>>
>> On Tue, Jun 30, 2015 at 4:36 PM, Daniel Vetter <daniel@ffwll.ch> wrote:
>>
>>> On Tue, Jun 30, 2015 at 11:13:58AM +0530, Sonika Jindal wrote:
>>>> From: Shashank Sharma <contactshashanksharma@gmail.com>
>>>>
>>>> This patch makes sure that the HDMI detect function
>>>> reads EDID only when its forced to do it. All the other
>>>> times, it uses the connector->detect_edid which was cached
>>>> during hotplug handling in the hdmi_probe() function. As the
>>>> probe function gets called before detect in the interrupt handler
>>>> and handles the EDID cacheing part, its absolutely safe to assume
>>>> that presence of EDID reflects monitor connected and viceversa.
>>>>
>>>> This will save us from many race conditions between hotplug/unplug
>>>> detect call handler threads and userspace calls for the same.
>>>> The previous patch in this patch series explains this in detail.
>>>>
>>>> Signed-off-by: Shashank Sharma <contactshashanksharma@gmail.com>
>>>> ---
>>>>   drivers/gpu/drm/i915/intel_hdmi.c |   26 ++++++++++++++++++++------
>>>>   1 file changed, 20 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/intel_hdmi.c
>>> b/drivers/gpu/drm/i915/intel_hdmi.c
>>>> index 064ddd8..1fb6919 100644
>>>> --- a/drivers/gpu/drm/i915/intel_hdmi.c
>>>> +++ b/drivers/gpu/drm/i915/intel_hdmi.c
>>>> @@ -1362,19 +1362,33 @@ static enum drm_connector_status
>>>>   intel_hdmi_detect(struct drm_connector *connector, bool force)
>>>>   {
>>>>        enum drm_connector_status status;
>>>> +     struct intel_connector *intel_connector =
>>>> +                             to_intel_connector(connector);
>>>>
>>>>        DRM_DEBUG_KMS("[CONNECTOR:%d:%s]\n",
>>>>                      connector->base.id, connector->name);
>>>> +     /*
>>>> +      * There are many userspace calls which probe EDID from
>>>> +      * detect path. In case on multiple hotplug/unplug, these
>>>> +      * can cause race conditions while probing EDID. Also its
>>>> +      * waste of CPU cycles to read the EDID again and again
>>>> +      * unless there is a real hotplug.
>>>> +      * So until we are forced, check connector status
>>>> +      * based on availability of cached EDID. This will avoid many of
>>>> +      * these race conditions and timing problems.
>>>> +      */
>>>> +     if (force)
>>>
>>> Userspace always sets force. Are you sure this actually improves anything?
>>> Also the goal should be to keep things cache for a few calls from
>>> userspace (since often it pokes a few times in a row unfortuantely), for
>>> which we need a proper timeout to clear the edid again.
>>> -Daniel
>>>
>>>> +             intel_hdmi_probe(intel_connector->encoder);
>>>>
>>>> -     intel_hdmi_unset_edid(connector);
>>>> -
>>>> -     if (intel_hdmi_set_edid(connector)) {
>>>> +     if (intel_connector->detect_edid) {
>>>>                struct intel_hdmi *intel_hdmi =
>>> intel_attached_hdmi(connector);
>>>> -
>>>> -             hdmi_to_dig_port(intel_hdmi)->base.type =
>>> INTEL_OUTPUT_HDMI;
>>>>                status = connector_status_connected;
>>>> -     } else
>>>> +             hdmi_to_dig_port(intel_hdmi)->base.type =
>>> INTEL_OUTPUT_HDMI;
>>>> +             DRM_DEBUG_DRIVER("hdmi status = connected\n");
>>>> +     } else {
>>>>                status = connector_status_disconnected;
>>>> +             DRM_DEBUG_DRIVER("hdmi status = disconnected\n");
>>>> +     }
>>>>
>>>>        return status;
>>>>   }
>>>> --
>>>> 1.7.10.4
>>>>
>>>> _______________________________________________
>>>> Intel-gfx mailing list
>>>> Intel-gfx@lists.freedesktop.org
>>>> http://lists.freedesktop.org/mailman/listinfo/intel-gfx
>>>
>>> --
>>> Daniel Vetter
>>> Software Engineer, Intel Corporation
>>> http://blog.ffwll.ch
>>>
>
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