linux-kernel - Re: [PATCH] drm/rockchip: Round up _before

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Message-ID: <CAD=FV=UnC_b1DfN0Aq1bcw_1Oz95Kc-2hkkEULY84+bMfd4sNw@mail.gmail.com>
Date:   Thu, 3 Oct 2019 11:48:57 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Sean Paul <sean@...rly.run>
Cc:     Heiko Stuebner <heiko@...ech.de>, David Airlie <airlied@...ux.ie>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Sean Paul <seanpaul@...omium.org>,
        Ryan Case <ryandcase@...omium.org>,
        Tomasz Figa <tfiga@...omium.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] drm/rockchip: Round up _before_ giving to the clock framework

Hi,

On Thu, Oct 3, 2019 at 11:37 AM Sean Paul <sean@...rly.run> wrote:
>
> On Thu, Oct 3, 2019 at 1:20 PM Douglas Anderson <dianders@...omium.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@...omium.org>

Thanks for the review!  Hopefully people don't mind the quick spin...

https://lore.kernel.org/r/20191003114726.v2.1.Ib233b3e706cf6317858384264d5b0ed35657456e@changeid

-Doug