drm/atmel-hlcdc: allow selecting a higher pixel-clock than requested

But only if the highest pixel-clock frequency lower than requested
is significantly less accurate than the lowest frequency higher than
requested.

I pulled "10 times" as the discriminator out of the hat, and went with
that.

This is useful, if e.g. the target pixel-clock is 65MHz and the sys_clk
is 132MHz. In this case the highest possible pixel-clock lower than the
requested 65MHz is 52.8MHz, which is almost 20% off (and outside the
spec for the panel). The lowest possible pixel-clock higher than 65MHz
is 66MHz, which is a *much* better match, and only 1.5% off.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180824092458.13165-3-peda@axentia.se

drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c

@@ -116,6 +116,18 @@ static void atmel_hlcdc_crtc_mode_set_nofb(struct drm_crtc *c)
 		div = DIV_ROUND_UP(prate, mode_rate);
 		if (ATMEL_HLCDC_CLKDIV(div) & ~ATMEL_HLCDC_CLKDIV_MASK)
 			div = ATMEL_HLCDC_CLKDIV_MASK;
+	} else {
+		int div_low = prate / mode_rate;
+
+		if (div_low >= 2 &&
+		    ((prate / div_low - mode_rate) <
+		     10 * (mode_rate - prate / div)))
+			/*
+			 * At least 10 times better when using a higher
+			 * frequency than requested, instead of a lower.
+			 * So, go with that.
+			 */
+			div = div_low;
 	}
 
 	cfg |= ATMEL_HLCDC_CLKDIV(div);