[PATCH 1/2] drm/kms: Make i2c buses faster

[Keith Packard]

On Sat, 28 Jan 2012 11:07:09 +0100, Jean Delvare <jdelvare at suse.de> wrote:
> A udelay value of 20 leads to an I2C bus running at only 25 kbps. I2C
> devices can typically operate faster than this, 50 kbps should be fine
> for all devices (and compliant devices can always stretch the clock if
> needed.)
> 
> FWIW, the vast majority of framebuffer drivers set udelay to 10
> already. So set it to 10 in DRM drivers too, this will make EDID block
> reads faster. We might even lower the udelay value later if no problem
> is reported.

That runs the DDC at a whopping 50kbps, which is half of the maximum
rate specified in the DDC/CI standard. I don't know if we can count on
clock stretching (http://www.i2c-bus.org/clock-stretching/), but if so,
I don't know why we wouldn't just go to the standard 100kbps data rate
and be done with it.

Might be nice to see what frequency Windows uses for i2c; anyone want to
pull a vga cable apart and hook up a logic analyser?

-- 
keith.packard at intel.com
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 827 bytes
Desc: not available
URL: <http://lists.freedesktop.org/archives/dri-devel/attachments/20120128/8562ac5c/attachment.pgp>