[PATCH v4 14/16] drm: bridge: analogix/dp: try force hpd after plug in lookup failed

[Yakir Yang]

Hi Thierry & Rob,

Sorry, apologize for the delay in replying  :-)


On 09/07/2015 04:20 PM, Thierry Reding wrote:
> On Sun, Sep 06, 2015 at 11:59:08AM +0800, Yakir Yang wrote:
>> Hi Thierry,
>>
>> 在 09/03/2015 05:04 PM, Thierry Reding 写道:
>>> On Thu, Sep 03, 2015 at 12:27:47PM +0800, Yakir Yang wrote:
>>>> Hi Rob,
>>>>
>>>> 在 09/03/2015 04:17 AM, Rob Herring 写道:
>>>>> On Tue, Sep 1, 2015 at 1:14 AM, Yakir Yang <ykk at rock-chips.com> wrote:
>>>>>> Some edp screen do not have hpd signal, so we can't just return
>>>>>> failed when hpd plug in detect failed.
>>>>> This is a property of the panel (or connector perhaps), so this
>>>>> property should be located there. At least, it is a common issue and
>>>>> not specific to this chip. We could have an HDMI connector and failed
>>>>> to hook up HPD for example. A connector node is also where hpd-gpios
>>>>> should be located instead (and are already defined by
>>>>> ../bindings/video/hdmi-connector.txt). Perhaps we need a eDP connector
>>>>> binding, too.
>>>> Yep, I agree with your front point, it is a property of panel, not specific
>>>> to eDP controller, so this code should handle in connector logic.
>>>  From your description it sounds more like this is in fact a property of
>>> the panel. Or maybe I should say "quirk". If the panel doesn't generate
>>> the HPD signal, then that should be a property of the panel, not the
>>> connector. The eDP specification mandates that connectors have a HPD
>>> signal, though it allows the "HPD conductor in the connector cable" to
>>> be omitted if not used by the source. I'd consider the cable to belong
>>> to the panel rather than the connector, so absence of HPD, either
>>> because the cable doesn't have the conductor or because the panel does
>>> not generate the signal, should be a quirk of the panel.
>>>
>>> That said you could have a panel that supports HPD connected via a cable
>>> that doesn't transmit it, so this would be a per-board variant and hence
>>> should be a device tree property rather than hard-coded in some panel
>>> driver.
>>>
>>> Conversely, if the panel isn't capable of generating an HPD signal, then
>>> I don't think it would be appropriate to make it a DT property. It would
>>> be better to hard-code it in the driver, lest someone forget to set the
>>> property in DT and get stuck with a device that isn't operational.
>> Oh, you're right, if it's a cable quirk, then DT property would be okay, if
>> it
>> is a problem of panel, then maybe hard-code in driver would be better.
>>
>> After look up for the document of panel "innolux,n116bge", I haven't see
>> any description of hot plug signal, and even not found in PIN ASSIGNMENT.
>> So I believe it's a panel problem, that's to say it should handle in panel
>> driver.
> The datasheet that I have for that panel lists HPD as pin 17. Also I
> used to have a setup with that panel and I distinctly remember hotplug
> working just fine. Perhaps this is an issue with a specific variant of
> the panel? Or perhaps this is indeed a problem with the cable that's
> connecting the panel to the board. It could be one of those cases where
> they left out the HPD conductor to save money.

You're right, I guess I just download the wrong datasheet "N116BGE-L41.pdf"
which the video interfaces is "LVDS", thanks for you point out.

And I double checked with the guys who work with this screen vendor, he said
that it's the fault that vendor missed HPD pin on the screen board, and 
vendor
have fixed this problem later.

But there are still some machine didn't contain the HPD signal, and you also
mention that in some cases where vendor would left out the HPD conductor,
so I still wish to support those "quirk" screen in the later version. 
But I wish
you could share your opinion whether this could exist in the mainline
kernel.

If the answer is no, okay, I would remove this from the next versions. 
but If
the answer is yes, wow, I may still can use the DT property to satisfied 
this
demand (I guess it's okay to keep the DT property way from previous 
discussion).

Thanks,
- Yakir

>
> Thierry
>
>
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