Re: [PATCH] drm/rockchip: Round up _before_ giving to the clock framework

[Sean Paul <sean@poorly.run>]

On Thu, Oct 3, 2019 at 1:20 PM Douglas Anderson <dianders@chromium.org> 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@chromium.org>


> 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@chromium.org>
> ---
>
>  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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