Re: [PATCH 06/10] drm/tegra: Handle 64-bit return from drm_crtc_vblank_count()

Pandiyan, Dhinakaran
2018-02-07T21:32:35Z

On Wed, 2018-02-07 at 10:41 +0100, Thierry Reding wrote:
> On Wed, Feb 07, 2018 at 01:41:18AM +0000, Pandiyan, Dhinakaran wrote:
> > On Fri, 2018-02-02 at 21:12 -0800, Dhinakaran Pandiyan wrote:
> > > 570e86963a51 ("drm: Widen vblank count to 64-bits [v3]") changed the
> > > return type for drm_crtc_vblank_count() to u64. This could cause
> > > potential problems if the return value is used in arithmetic operations
> > > with a 32-bit reference HW vblank count. Explicitly typecasting this
> > > down to u32 either fixes a potential problem or serves to add clarity in
> > > case the implicit typecasting was already correct.
> > >
> > > Cc: Keith Packard <keithp@keithp.com>
> > > Cc: Thierry Reding <treding@nvidia.com>
> >
> >
> > Thierry,
> >
> > Can I get an Ack on this please?
> >
> > > Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
> > > ---
> > > drivers/gpu/drm/tegra/dc.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/tegra/dc.c b/drivers/gpu/drm/tegra/dc.c
> > > index b8403ed48285..49df2db2ad46 100644
> > > --- a/drivers/gpu/drm/tegra/dc.c
> > > +++ b/drivers/gpu/drm/tegra/dc.c
> > > @@ -1359,7 +1359,7 @@ static u32 tegra_dc_get_vblank_counter(struct drm_crtc *crtc)
> > > return host1x_syncpt_read(dc->syncpt);
> > >
> > > /* fallback to software emulated VBLANK counter */
> > > - return drm_crtc_vblank_count(&dc->base);
> > > + return (u32)drm_crtc_vblank_count(&dc->base);
>
> Isn't this the wrong way around? Shouldn't we instead make the
> ->get_vblank_counter() callback return u64 like drm_crtc_vblank_count()?

Here's how I understand this -

To use the software emulated vblank counter, the driver should set
max_vblank_count = 0 and the core takes care of calculating elapsed
vblanks.

->get_vblank_counter() is meant to return the hardware counter if
available, which would be a 32-bit value. Hence the explicit typecast to
32-bit for the fallback case too.

Having said that, I believe drm_crtc_accurate_vblank_count() is the
appropriate function for fallback.

-DK

>
> Thierry
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