# Re: \[PATCH v12 3/6\] fbmon: add videomode helpers

- List: linux-fbdev
- 2 messages, 2 authors
- Page: https://ratatoskr.run/linux-fbdev/2012/11/8065950/t

## Re: \[PATCH v12 3/6\] fbmon: add videomode helpers

- From: Laurent Pinchart \<hidden\>
- Date: 2012-11-22 08:50:10 
- Message: https://ratatoskr.run/linux-fbdev/2012/11/8065950.md

```text
Hi Sascha,

On Thursday 22 November 2012 07:20:00 Sascha Hauer wrote:
> On Wed, Nov 21, 2012 at 11:02:44PM +0100, Laurent Pinchart wrote:
> > Hi Steffen,
> > 
> > > +
> > > +	htotal = vm->hactive + vm->hfront_porch + vm->hback_porch +
> > > +		 vm->hsync_len;
> > > +	vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
> > > +		 vm->vsync_len;
> > > +	fbmode->refresh = (vm->pixelclock * 1000) / (htotal * vtotal);
> > 
> > This will fail if vm->pixelclock >= ((1 << 32) / 1000). The easiest
> > solution is probably to use 64-bit computation.
> 
> You have displays with a pixelclock > 4GHz?

vm->pixelclock is expressed in Hz. vm->pixelclock * 1000 will thus overflow if 
the clock frequency is >= ~4.3 MHz. I have displays with a clock frequency 
higher than that :-)

-- 
Regards,

Laurent Pinchart

```

## Re: \[PATCH v12 3/6\] fbmon: add videomode helpers

- From: Sascha Hauer \<hidden\>
- Date: 2012-11-22 08:53:42 
- Message: https://ratatoskr.run/linux-fbdev/2012/11/8065951.md

```text
On Thu, Nov 22, 2012 at 09:50:10AM +0100, Laurent Pinchart wrote:
> Hi Sascha,
> 
> On Thursday 22 November 2012 07:20:00 Sascha Hauer wrote:
> > On Wed, Nov 21, 2012 at 11:02:44PM +0100, Laurent Pinchart wrote:
> > > Hi Steffen,
> > > 
> > > > +
> > > > +	htotal = vm->hactive + vm->hfront_porch + vm->hback_porch +
> > > > +		 vm->hsync_len;
> > > > +	vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
> > > > +		 vm->vsync_len;
> > > > +	fbmode->refresh = (vm->pixelclock * 1000) / (htotal * vtotal);
> > > 
> > > This will fail if vm->pixelclock >= ((1 << 32) / 1000). The easiest
> > > solution is probably to use 64-bit computation.
> > 
> > You have displays with a pixelclock > 4GHz?
> 
> vm->pixelclock is expressed in Hz. vm->pixelclock * 1000 will thus overflow if 
> the clock frequency is >= ~4.3 MHz. I have displays with a clock frequency 
> higher than that :-)

If vm->pixelclock is in Hz, then the * 1000 above is wrong.

Sascha

-- 
Pengutronix e.K.                           |                             |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |
```