Re: Intel graphics CPU usage - SDVO detect bogosity?

Andy Lutomirski
2010-08-16T14:37:36Z

[cc: intel-gfx]

Linus Torvalds wrote:
> On Sun, Aug 15, 2010 at 9:06 PM, Andy Lutomirski <luto@myrealbox.com> wrote:
>> You might be hitting the infamous hotplug storm [1]. The symptoms vary by
>> kernel version.
>
> Hmm. I don't think it's a storm. The drm.debug=4 thing shows things
> just every 10 seconds. That seems pretty controlled.

OK, probably not the hotplug storm. That's always been at least once per
second and sometimes several for me.

>
> Of course, it seems to be several milliseconds worth of work then, so
> it's not _too_ controlled. I must be missing some detail.
>
>> If I'm right, the outputs of intel_bios_dumper and intel_bios_reader could
>> be instructive (both are in intel-gpu-tools).
>
> My version of intel-gpu-tools must be old (fedora 12). It has
> intel_gpu_dump and intel_reg_write and some other apps, but not the
> bios-dumper/reader.
>
> Not that I could read the output of them anyway, I bet ;)
>
>> You could also try intel_reg_write 0x61110 0x0 and see if the problem stops
>> (at least until a suspend/resume cycle). That command turns off output
>> hotplug on the card, which has the side effect that the kernel will stop
>> acting on bogus interrupts.
>
> # intel_reg_write 0x61110 0x0
> Value before: 0x4000220
> Value after: 0x0
>
> but it doesn't seem to change any behavior. Still that sdvo probe
> every 10 s, and still 1% of CPU for kworker in top. But that kworker
> thing definitely has to be separate from the 10-second sdvo probe,
> because it shows up all the time (ie top updates every second).

Almost certainly not the hotplug storm bug.

The 10 seconds is probably the timer in output_poll_execute (drm_crtc_helper.c):

#define DRM_OUTPUT_POLL_PERIOD (10*HZ)

The code in output_poll_execute (which is in your perf output) looks wrong:

list_for_each_entry(connector, &dev->mode_config.connector_list, head) {

/* if this is HPD or polled don't check it -
TV out for instance */
if (!connector->polled)
continue;

else if (connector->polled & (DRM_CONNECTOR_POLL_CONNECT | DRM_CONNECTOR_POLL_DISCONNECT))
repoll = true;

[ so if we have DRM_CONNECTOR_POLL_CONNECT set we requeue the *entire*
work item. or if we have DRM_CONNECTOR_POLL_DISCONNECT and it's not
even connected, we still poll. ]

old_status = connector->status;
/* if we are connected and don't want to poll for disconnect
skip it */
if (old_status == connector_status_connected &&
!(connector->polled & DRM_CONNECTOR_POLL_DISCONNECT) &&
!(connector->polled & DRM_CONNECTOR_POLL_HPD))
continue;

[ but if the status isn't connector_status_connected, we poll it even if we're
only in here due to a *different* connector ]

status = connector->funcs->detect(connector);

[ ->detect is the expensive part ]

if (old_status != status)
changed = true;
}

But that's really the problem, because intel_sdvo_dvi_init contains:

connector->polled = DRM_CONNECTOR_POLL_CONNECT | DRM_CONNECTOR_POLL_DISCONNECT;

I don't know if SDVO is supposed to send hotplug interrupts because that section of the docs is incomplete.

--Andy