On Thu, Oct 3, 2019 at 1:20 PM Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
>
> I'm embarassed to say that even though I've touched
> vop_crtc_mode_fixup() twice and I swear I tested it, there's still a
> stupid glaring bug in it. Specifically, on veyron_minnie (with all
> the latest display timings) we want to be setting our pixel clock to
> 66,666,666.67 Hz and we tell userspace that's what we set, but we're
> actually choosing 66,000,000 Hz. This is confirmed by looking at the
> clock tree.
>
> The problem is that in drm_display_mode_from_videomode() we convert
> from Hz to kHz with:
>
> dmode->clock = vm->pixelclock / 1000;
>
> ...so when the device tree specifies a clock of 66666667 for the panel
> then DRM translates that to 66666000. The clock framework will always
> pick a clock that is _lower_ than the one requested, so it will refuse
> to pick 66666667 and we'll end up at 66000000.
>
> While we could try to fix drm_display_mode_from_videomode() to round
> to the nearest kHz and it would fix our problem,
I got a bit confused reading this and Doug straightened me out in a
sideband conversation.
To summarize, the drm_display_mode_from_videomode() call referenced
above is from panel-simple, and this downslotting is specific to
rockchip's clock driver. So I've asked to clarify these 2 points so
it's clear from the commit message that this patch is the best
solution. With that addressed,
Reviewed-by: Sean Paul <seanpaul@xxxxxxxxxxxx>
> it wouldn't help if
> the clock we actually needed was 60,000,001 Hz. We could
> alternatively have DRM always round up, but maybe this would break
> someone else who already baked in the assumption that DRM rounds down.
>
> Let's solve this by just adding 999 Hz before calling
> clk_round_rate(). This should be safe and work everywhere.
>
> NOTE: if this is picked to stable, it's probably easiest to first pick
> commit 527e4ca3b6d1 ("drm/rockchip: Base adjustments of the mode based
> on prev adjustments") which shouldn't hurt in stable.
>
> Fixes: b59b8de31497 ("drm/rockchip: return a true clock rate to adjusted_mode")
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
>
> drivers/gpu/drm/rockchip/rockchip_drm_vop.c | 37 +++++++++++++++++++--
> 1 file changed, 34 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_vop.c b/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
> index 613404f86668..84e3decb17b1 100644
> --- a/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
> +++ b/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
> @@ -1040,10 +1040,41 @@ static bool vop_crtc_mode_fixup(struct drm_crtc *crtc,
> struct drm_display_mode *adjusted_mode)
> {
> struct vop *vop = to_vop(crtc);
> + unsigned long rate;
>
> - adjusted_mode->clock =
> - DIV_ROUND_UP(clk_round_rate(vop->dclk,
> - adjusted_mode->clock * 1000), 1000);
> + /*
> + * Clock craziness.
> + *
> + * Key points:
> + *
> + * - DRM works in in kHz.
> + * - Clock framework works in Hz.
> + * - Rockchip's clock driver picks the clock rate that is the
> + * same _OR LOWER_ than the one requested.
> + *
> + * 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.
> + *
> + * 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. 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
> + * this in the actual clk_set_rate().
> + */
> + rate = clk_round_rate(vop->dclk, adjusted_mode->clock * 1000 + 999);
> + adjusted_mode->clock = DIV_ROUND_UP(rate, 1000);
>
> return true;
> }
> --
> 2.23.0.444.g18eeb5a265-goog
>
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