linux-kernel - Re: [PATCH 4.4 030/103] drm/amdgpu: Make display watermark calculations more accurate

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Message-ID: <1496918999.1989.13.camel@codethink.co.uk>
Date:   Thu, 08 Jun 2017 11:49:59 +0100
From:   Ben Hutchings <ben.hutchings@...ethink.co.uk>
To:     Mario Kleiner <mario.kleiner.de@...il.com>
Cc:     Alex Deucher <alexander.deucher@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 4.4 030/103] drm/amdgpu: Make display watermark
 calculations more accurate

On Tue, 2017-06-06 at 19:46 +0200, Mario Kleiner wrote:
> On Thu, Jun 1, 2017 at 1:13 PM, Ben Hutchings
> <ben.hutchings@...ethink.co.uk> wrote:
> > On Tue, 2017-05-23 at 22:08 +0200, Greg Kroah-Hartman wrote:
> >> 4.4-stable review patch.  If anyone has any objections, please let me know.
> >>
> >> ------------------
> >>
> >> From: Mario Kleiner <mario.kleiner.de@...il.com>
> >>
> >> commit d63c277dc672e0c568481af043359420fa9d4736 upstream.
> >>
> >> Avoid big roundoff errors in scanline/hactive durations for
> >> high pixel clocks, especially for >= 500 Mhz, and thereby
> >> program more accurate display fifo watermarks.
> > [...]
> >> --- a/drivers/gpu/drm/amd/amdgpu/dce_v10_0.c
> >> +++ b/drivers/gpu/drm/amd/amdgpu/dce_v10_0.c
> >> @@ -1237,14 +1237,14 @@ static void dce_v10_0_program_watermarks
> >>  {
> >>       struct drm_display_mode *mode = &amdgpu_crtc->base.mode;
> >>       struct dce10_wm_params wm_low, wm_high;
> >> -     u32 pixel_period;
> >> +     u32 active_time;
> >>       u32 line_time = 0;
> >>       u32 latency_watermark_a = 0, latency_watermark_b = 0;
> >>       u32 tmp, wm_mask, lb_vblank_lead_lines = 0;
> >>
> >>       if (amdgpu_crtc->base.enabled && num_heads && mode) {
> >> -             pixel_period = 1000000 / (u32)mode->clock;
> >> -             line_time = min((u32)mode->crtc_htotal * pixel_period, (u32)65535);
> >> +             active_time = 1000000UL * (u32)mode->crtc_hdisplay / (u32)mode->clock;
> >> +             line_time = min((u32) (1000000UL * (u32)mode->crtc_htotal / (u32)mode->clock), (u32)65535);
> > [...]
> >
> > Won't these multiplications overflow if a >4K display is attached to a
> > 32-bit machine?
> >
> > Ben.
> >
> 
> Yes, indeed > 4k causes a new overflow problem! Thanks for catching this.
> I will prepare a follow-up patch on top of this one, to use ...
> 
> active_time = (u32) div_u64((u64) mode->crtc_hdisplay * 1000000,
> (u32)mode->clock);
> line_time = (u32) div_u64((u64) mode->crtc_htotal * 1000000, (u32)mode->clock);
> line_time = min(line_time, (u32) 65535);
> 
> ...instead.
> 
> Ok?

I think that would work.

Ben.

-- 
Ben Hutchings
Software Developer, Codethink Ltd.