On Tue, Nov 22, 2016 at 08:23:38PM +0200, Laurent Pinchart wrote:
> Hi John,
>
> (CC'ing Daniel)
>
> On Tuesday 22 Nov 2016 10:07:53 John Stultz wrote:
> > On Tue, Nov 22, 2016 at 9:38 AM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> > > Interestingly, without the msleep added in this patch, removing the
> > > wait_event_interruptible_timeout() method in adv7511_wait_for_edid()
> > > and using the polling loop seems to make things just as reliable. So
> > > maybe something is off with the irq handling here instead?
> >
> > Ahhhh.. So I think the trouble here is the that when we fail waiting
> > for the irq, the backtrace is as follows:
> >
> > [ 8.318654] [<ffffff8008087c28>] dump_backtrace+0x0/0x1a0
> > [ 8.318661] [<ffffff8008087ddc>] show_stack+0x14/0x20
> > [ 8.318671] [<ffffff80084344f0>] dump_stack+0x90/0xb0
> > [ 8.318680] [<ffffff8008534650>] adv7511_get_edid_block+0x2c8/0x320
> > [ 8.318687] [<ffffff80085214a8>] drm_do_get_edid+0x78/0x280
> > [ 8.318693] [<ffffff8008534728>] adv7511_get_modes+0x80/0xd8
> > [ 8.318700] [<ffffff8008534794>] adv7511_connector_get_modes+0x14/0x20
> > [ 8.318710] [<ffffff8008500a54>]
> > drm_helper_probe_single_connector_modes+0x2bc/0x500
> > [ 8.318718] [<ffffff800850e400>] drm_fb_helper_hotplug_event+0x130/0x188
> > [ 8.318726] [<ffffff800850ee68>] drm_fbdev_cma_hotplug_event+0x10/0x20
> > [ 8.318733] [<ffffff8008535718>]
> > kirin_fbdev_output_poll_changed+0x20/0x58
> > [ 8.318740] [<ffffff8008500cc0>] drm_kms_helper_hotplug_event+0x28/0x38
> > [ 8.318748] [<ffffff80085010d8>] drm_helper_hpd_irq_event+0x138/0x180
> > [ 8.318754] [<ffffff8008533850>] adv7511_irq_process+0x78/0xd8
> > [ 8.318761] [<ffffff80085338c4>] adv7511_irq_handler+0x14/0x28
> > [ 8.318769] [<ffffff8008100060>] irq_thread_fn+0x28/0x68
> > [ 8.318775] [<ffffff8008100350>] irq_thread+0x128/0x1e8
> > [ 8.318782] [<ffffff80080d2e68>] kthread+0xd0/0xe8
> > [ 8.318788] [<ffffff8008082e80>] ret_from_fork+0x10/0x50
> >
> > So we're actually in irq handling the hotplug interrupt, which is why
> > we never get the irq notification when the edid is read.
> >
> > I suspect we need to use a workqueue to do the hotplug handling out of irq.
>
> Lovely :-)
>
> Quoting the DRM documentation:
>
> /**
> * drm_helper_hpd_irq_event - hotplug processing
> * @dev: drm_device
> *
> * Drivers can use this helper function to run a detect cycle on all
> connectors
> * which have the DRM_CONNECTOR_POLL_HPD flag set in their &polled member. All
> * other connectors are ignored, which is useful to avoid reprobing fixed
> * panels.
> *
> * This helper function is useful for drivers which can't or don't track
> hotplug
> * interrupts for each connector.
> *
> * Drivers which support hotplug interrupts for each connector individually
> and
> * which have a more fine-grained detect logic should bypass this code and
> * directly call drm_kms_helper_hotplug_event() in case the connector state
> * changed.
> *
> * This function must be called from process context with no mode
> * setting locks held.
> *
> * Note that a connector can be both polled and probed from the hotplug
> handler,
> * in case the hotplug interrupt is known to be unreliable.
> */
>
> So it looks like we should use drm_kms_helper_hotplug_event() instead.
>
> /**
> * drm_kms_helper_hotplug_event - fire off KMS hotplug events
> * @dev: drm_device whose connector state changed
> *
> * This function fires off the uevent for userspace and also calls the
> * output_poll_changed function, which is most commonly used to inform the
> fbdev
> * emulation code and allow it to update the fbcon output configuration.
> *
> * Drivers should call this from their hotplug handling code when a change is
> * detected. Note that this function does not do any output detection of its
> * own, like drm_helper_hpd_irq_event() does - this is assumed to be done by
> the
> * driver already.
> *
> * This function must be called from process context with no mode
> * setting locks held.
> */
>
> The function suffers from the same problem though, that it must be called from
> process context.
>
> Daniel, why do we have an API the is clearly related to interrupt handling but
> requires the caller to implement a workqueue ?
Because in general you need that workqueue anyway, and up to now there was
no driver ever who didn't have a work-queue already. Nesting workqueues
within workqueues seemed beyond silly, hence why I removed them in:
commit 69787f7da6b2adc4054357a661aaa1701a9ca76f
Author: Daniel Vetter <daniel.vetter@xxxxxxxx>
Date: Tue Oct 23 18:23:34 2012 +0000
drm: run the hpd irq event code directly
I guess we could talk about re-introducing a work-item based version of
drm_helper_hpd_irq_event. But for drm_kms_helper_hotplug_event I think it
doesn't make sense - if you call that you've probably just done a pile of
i2c transactions, and those can sleep. If you haven't done i2c
transactions, then it's not an external panel, and why exactly are you
handling hpd for them?
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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