On Fri, Aug 14, 2020 at 03:00:57PM +0200, Marcus Glocker wrote:
> Hi,
>
> Recently I took over the old iMac11,2 of my son, and what else to do
> with it other than installing OpenBSD and see what happens. The first
> thing which happened after the installation was that the screen
> remained dark after the radeondrm(4) KMS initialization.
>
> After some painful debugging, and exchanging forth and back with jsg@,
> I've noticed that when enabling DRMDEBUG, radeondrm(4) just works fine!
> Smells like a timing issue.
>
> After doing some more investigations I've figured out that when
> removing a specific debug printf in drm_dp_helper.c, just before an
> usleep_range() statement, it fails again:
>
> case DP_AUX_NATIVE_REPLY_DEFER:
> -> DRM_DEBUG_KMS("native defer\n");
> /*
> * We could check for I2C bit rate capabilities and if
> * available adjust this interval. We could also be
> * more careful with DP-to-legacy adapters where a
> * long legacy cable may force very low I2C bit rates.
> *
> * For now just defer for long enough to hopefully be
> * safe for all use-cases.
> */
> usleep_range(AUX_RETRY_INTERVAL, AUX_RETRY_INTERVAL + 100);
> continue;
>
> This usleep_range() expands to usleep_range(500, 600), and we wrap it in
> this function on OpenBSD:
>
> static inline void
> usleep_range(unsigned long min, unsigned long max)
> {
> DELAY(min);
> }
>
> In my case the 500us was too short, while the 600us works fine.
>
> Based on that jsg@ told me that there is an discussion on-going to
> change usleep_range() to calculate (min + max) / 2, which would change
> this specific usleep_range() line to 550us, which is enough in my case
> to make radeondrm(4) work fine on my iMac.
>
> We would like to understand if this diff has any negative impact on
> other devices. Can you please give it a spin therefore?
While we could increase AUX_RETRY_INTERVAL I prefer this change as linux
in practice seems to always add some about of delay on the minimum.
ok jsg@
>
>
> Index: dev/pci/drm/include/linux/delay.h
> ===================================================================
> RCS file: /cvs/src/sys/dev/pci/drm/include/linux/delay.h,v
> retrieving revision 1.2
> diff -u -p -u -p -r1.2 delay.h
> --- dev/pci/drm/include/linux/delay.h 8 Jun 2020 04:48:14 -0000 1.2
> +++ dev/pci/drm/include/linux/delay.h 14 Aug 2020 11:44:43 -0000
> @@ -20,7 +20,7 @@ ndelay(unsigned long nsecs)
> static inline void
> usleep_range(unsigned long min, unsigned long max)
> {
> - DELAY(min);
> + DELAY((min + max) / 2);
> }
>
> static inline void
>
>
