Re: [PATCH] drm/i915: Enable scanline read for gen9 dsi

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: "Shankar, Uma" <uma.shankar@intel.com>
Cc: "intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>,
	"Srinivas, Vidya" <vidya.srinivas@intel.com>
Subject: Re: [PATCH] drm/i915: Enable scanline read for gen9 dsi
Date: Wed, 13 Sep 2017 20:36:57 +0300	[thread overview]
Message-ID: <20170913173657.GU4914@intel.com> (raw)
In-Reply-To: <E7C9878FBA1C6D42A1CA3F62AEB6945F7EFFB7A1@BGSMSX104.gar.corp.intel.com>

On Wed, Sep 13, 2017 at 08:24:38AM +0000, Shankar, Uma wrote:
> 
> 
> >-----Original Message-----
> >From: Ville Syrjälä [mailto:ville.syrjala@linux.intel.com]
> >Sent: Tuesday, September 12, 2017 8:36 PM
> >To: Shankar, Uma <uma.shankar@intel.com>
> >Cc: intel-gfx@lists.freedesktop.org; Srinivas, Vidya <vidya.srinivas@intel.com>
> >Subject: Re: [PATCH] drm/i915: Enable scanline read for gen9 dsi
> >
> >On Tue, Sep 12, 2017 at 02:21:42PM +0000, Shankar, Uma wrote:
> >>
> >>
> >> >-----Original Message-----
> >> >From: Ville Syrjälä [mailto:ville.syrjala@linux.intel.com]
> >> >Sent: Tuesday, September 12, 2017 7:43 PM
> >> >To: Shankar, Uma <uma.shankar@intel.com>
> >> >Cc: intel-gfx@lists.freedesktop.org; Srinivas, Vidya
> >> ><vidya.srinivas@intel.com>
> >> >Subject: Re: [PATCH] drm/i915: Enable scanline read for gen9 dsi
> >> >
> >> >On Tue, Sep 12, 2017 at 01:40:58PM +0000, Shankar, Uma wrote:
> >> >> If we have multiple scans on the same frame (no new flip being
> >> >> issued). Prev timestamp value which is read from Frametime Stamp
> >> >> will remain same, but current time stamp will keep on incrementing.
> >> >
> >> >The frame timestamp should get sampled on every vblank, whereas the
> >> >flip timestamp only when a flip occurs. Are you using the correct timestamp
> >register?
> >> >
> >>
> >> Yes, we are using what is there in the patch.
> >> Name Pipe A Frame Time Stamp
> >> Symbol PIPE_FRMTMSTMP_A
> >> Start 0x70048
> >> End 0x7004B
> >>
> >> Its behaving as FLIP Timestamp though (not being updated on every
> >vblank_start).
> >> Atleast with the readback what we get on APL.
> >
> >Then it's broken and probably can't be used without having a decent idea of how
> >long the frame actually is. Which probably means we'd need something like what
> >Chris suggested.
> >
> 
> Hi Ville,
> On further experiments we figured out that, frame time stamp  is not updated if the
> vblank interrupt gets disabled (which is currently controlled through vblank get and put).
> We tried to forcefully enable vblank interrupt by doing an extra  vblank get during crtc_enable.
> By doing this,  we see that frame timestamp is updating at every vblank.

Well, that's rather unfortuante. I guess we'd have to keep the vblank
interrupt unmasked all the time. Hopefully we could still disable it in
IER.

So we'd unmask the interrupt permanently, and just toggle the IER bit as
needed. That should be doable but somewhat annoying because it's exactly
the opposite of what we do normally.

-- 
Ville Syrjälä
Intel OTC
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