drm/rockchip: Update crtc fixup to account for fractional clk change

After commit 928f9e2686 ("clk: fractional-divider: Hide
clk_fractional_divider_ops from wide audience") was merged it appears
that the DSI panel on my Odroid Go Advance stopped working. Upon closer
examination of the problem, it looks like it was the fixup in the
rockchip_drm_vop.c file was causing the issue. The changes made to the
clk driver appear to change some assumptions made in the fixup.

After debugging the working 5.14 kernel and the no-longer working
5.15 kernel, it looks like this was broken all along but still
worked, whereas after the fractional clock change it stopped
working despite the issue (it went from sort-of broken to very broken).

In the 5.14 kernel the dclk_vopb_frac was being requested to be set to
17000999 on my board. The clock driver was taking the value of the
parent clock and attempting to divide the requested value from it
(17000000/17000999 = 0), then subtracting 1 from it (making it -1),
and running it through fls_long to get 64. It would then subtract
the value of fd->mwidth from it to get 48, and then bit shift
17000999 to the left by 48, coming up with a very large number of
7649082492112076800. This resulted in a numerator of 65535 and a
denominator of 1 from the clk driver. The driver seemingly would
try again and get a correct 1:1 value later, and then move on.

Output from my 5.14 kernel (with some printfs for good measure):
[    2.830066] rockchip-drm display-subsystem: bound ff460000.vop (ops vop_component_ops)
[    2.839431] rockchip-drm display-subsystem: bound ff450000.dsi (ops dw_mipi_dsi_rockchip_ops)
[    2.855980] Clock is dclk_vopb_frac
[    2.856004] Scale 64, Rate 7649082492112076800, Oldrate 17000999, Parent Rate 17000000, Best Numerator 65535, Best Denominator 1, fd->mwidth 16
[    2.903529] Clock is dclk_vopb_frac
[    2.903556] Scale 0, Rate 17000000, Oldrate 17000000, Parent Rate 17000000, Best Numerator 1, Best Denominator 1, fd->mwidth 16
[    2.903579] Clock is dclk_vopb_frac
[    2.903583] Scale 0, Rate 17000000, Oldrate 17000000, Parent Rate 17000000, Best Numerator 1, Best Denominator 1, fd->mwidth 16

Contrast this with 5.15 after the clk change where the rate of 17000999
was getting passed and resulted in numerators/denomiators of 17001/
17000.

Output from my 5.15 kernel (with some printfs added for good measure):
[    2.817571] rockchip-drm display-subsystem: bound ff460000.vop (ops vop_component_ops)
[    2.826975] rockchip-drm display-subsystem: bound ff450000.dsi (ops dw_mipi_dsi_rockchip_ops)
[    2.843430] Rate 17000999, Parent Rate 17000000, Best Numerator 17018, Best Denominator 17017
[    2.891073] Rate 17001000, Parent Rate 17000000, Best Numerator 17001, Best Denominator 17000
[    2.891269] Rate 17001000, Parent Rate 17000000, Best Numerator 17001, Best Denominator 17000
[    2.891281] Rate 17001000, Parent Rate 17000000, Best Numerator 17001, Best Denominator 17000

I have tested the change extensively on my Odroid Go Advance (Rockchip
RK3326) and it appears to work well. However, this change will affect
all Rockchip SoCs that use this driver so I believe further testing
is warranted. Please note that without this change I can confirm
at least all PX30s with DSI panels will stop working with the 5.15
kernel.

Upon advice from Doug Anderson <dianders@chromium.org> it was decided
that we would first check if the clock rate can be set exactly as
requested, and only if it could not would we then add 999 to it and
attempt the process again. This way we can preserve the behavior for
clocks that still need it while resolving the specific issue for the
PX30 and DSI panels (since it is using a fractional clock).

Changes since v2:
 - Moved fixes to correct location.

Changes since v1:
 - Made the addition of 999 conditional based on whether the clock
   subsystem can set the actual clock rate as requested.
 - Updated the notes in the fixup routine to reflect this new behavior.
 - Added reference to original commit, as this has technically been
   broken since then however only now is it an issue due to the clock
   changes.

Fixes: 4e7cf74fa3 ("clk: fractional-divider: Export approximation algorithm to the CCF users")
Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210916202907.18394-1-macroalpha82@gmail.com
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

@@ -1174,26 +1174,24 @@ static bool vop_crtc_mode_fixup(struct drm_crtc *crtc,
 	 *
 	 * Action plan:
 	 *
-	 * 1. When DRM gives us a mode, we should add 999 Hz to it.  That way
-	 *    if the clock we need is 60000001 Hz (~60 MHz) and DRM tells us to
-	 *    make 60000 kHz then the clock framework will actually give us
-	 *    the right clock.
+	 * 1. Try to set the exact rate first, and confirm the clock framework
+	 *    can provide it.
 	 *
-	 *    NOTE: if the PLL (maybe through a divider) could actually make
-	 *    a clock rate 999 Hz higher instead of the one we want then this
-	 *    could be a problem.  Unfortunately there's not much we can do
-	 *    since it's baked into DRM to use kHz.  It shouldn't matter in
-	 *    practice since Rockchip PLLs are controlled by tables and
-	 *    even if there is a divider in the middle I wouldn't expect PLL
-	 *    rates in the table that are just a few kHz different.
+	 * 2. If the clock framework cannot provide the exact rate, we should
+	 *    add 999 Hz to the requested rate.  That way if the clock we need
+	 *    is 60000001 Hz (~60 MHz) and DRM tells us to make 60000 kHz then
+	 *    the clock framework will actually give us the right clock.
 	 *
-	 * 2. Get the clock framework to round the rate for us to tell us
+	 * 3. Get the clock framework to round the rate for us to tell us
 	 *    what it will actually make.
 	 *
-	 * 3. Store the rounded up rate so that we don't need to worry about
+	 * 4. Store the rounded up rate so that we don't need to worry about
 	 *    this in the actual clk_set_rate().
 	 */
-	rate = clk_round_rate(vop->dclk, adjusted_mode->clock * 1000 + 999);
+	rate = clk_round_rate(vop->dclk, adjusted_mode->clock * 1000);
+	if (rate / 1000 != adjusted_mode->clock)
+		rate = clk_round_rate(vop->dclk,
+				      adjusted_mode->clock * 1000 + 999);
 	adjusted_mode->clock = DIV_ROUND_UP(rate, 1000);
 
 	return true;