On Fri, May 05, 2017 at 08:39:31PM +0800, Icenowy Zheng wrote:

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+	/* Set base coordinates */
+	DRM_DEBUG_DRIVER("Layer coordinates X: %d Y: %d\n",
+			 state->crtc_x, state->crtc_y);
+	regmap_write(mixer->engine.regs,
+		     SUN8I_MIXER_CHAN_UI_LAYER_COORD(chan, layer),
+		     SUN8I_MIXER_COORD(state->crtc_x, state->crtc_y));

X and Y are fixed point numbers. You want to keep only the higher

16

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bits there.

Do you mean "lower 16 bits"? Thus should I (x & 0xffff) or ((u16)x) ?

Nevermind, I got confused with src_x and src_y.

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P.S. The negative coordinates are broken, how should I deal with it?

or

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is the coordinates promised to be not negative?

Adjust the buffer base address, and use a shorter line. You have such
an example in the sun4i code.

Are they these two lines:

paddr += (state->src_x >> 16) * bpp;
paddr += (state->src_y >> 16) * fb->pitches[0];

I think I copied them here, so I don't need to mind this problem any
more, right?

Hmmm, yes, probably. That's pretty easy to test anyway, you just need
to set up a plane with a negative base coordinate.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com