Re: [PATCH] drm/dp: add module parameter for the dpcd access max retries

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Quoting Feng Tang (2018-05-07 22:26:34)
> Hi Chris,
> 
> Thanks for the prompt review!
> 
> On Mon, May 07, 2018 at 11:40:45AM +0100, Chris Wilson wrote:
> > Quoting Feng Tang (2018-05-07 11:36:09)
> > > To fulfil the Dell 4K monitor, the dpcd max retries has been bumped
> > > from 7 to 32, which may hurt the boot/init time for some platforms,
> > > as the 32 retries may take hundreds of ms.
> > 
> > If we need that many retries, so be it. No modparam, the driver just has
> > to work.
> 
> I understand your point. The retry numer was originally 7, and worked
> fine untill the Dell 4K monitor which changes to 32.  According to my test,
> each retry will take about 8ms on the A3960 based NUC.
> 
> One of our product need to boot up within a given time limit, this
> 32 retries will take about 1/3 of the budget (about 270ms), that's
> why I would try to make it a parameter.

The essence is that probing whether a monitor is connected should not be
blocking boot. If an async probe tries and fails to find a monitor,
fine - no one will notice. If it does take 270ms to find a monitor, it
turns on 200ms after userspace kicks in, just like any other hotplug.
-Chris
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