78e8fc6b2e
Seems I've been a bit dense with regards to the start of vblank vs. the scanline counter / pixel counter. After staring at the pixel counter on gen4 I came to the conclusion that the start of vblank interrupt and scanline counter increment happen at the same time. The scanline counter increment is documented to occur at start of hsync, which means that the start of vblank interrupt must also trigger there. Looking at the pixel counter value when the scanline wraps from vtotal-1 to 0 confirms that, as the pixel counter at that point reads hsync_start. This also clarifies why we see need the +1 adjustment to the scaline counter. The counter actually starts counting from vtotal-1 on the first active line. I also confirmed that the frame start interrupt happens ~1 line after the start of vblank, but the frame start occurs at hblank_start instead. We only use the frame start interrupt on gen2 where the start of vblank interrupt isn't available. The only important thing to note here is that frame start occurs after vblank start, so we don't have to play any additional tricks to fix up the scanline counter. The other thing to note is the fact that the pixel counter on gen3-4 starts counting from the start of horizontal active on the first active line. That means that when we get the start of vblank interrupt, the pixel counter reads (htotal*(vblank_start-1)+hsync_start). Since we consider vblank to start at (htotal*vblank_start) we need to add a constant (htotal-hsync_start) offset to the pixel counter, or else we risk misdetecting whether we're in vblank or not. I talked a bit with Art Runyan about these topics, and he confirmed my findings. And that the same rules should hold for platforms which don't have the pixel counter. That's good since without the pixel counter it's rather difficult to verify the timings to this accuracy. So the conclusion is that we can throw away all the ISR tricks I added, and just increment the scanline counter by one always. Reviewed-by: Sourab Gupta <sourabgupta@gmail.com> Reviewed-by: Akash Goel <akash.goels@gmail.com> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> |
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