Re: [PATCH v2] drm: Add the new api to install irq

[Daniel Vetter]

On Tue, Nov 03, 2020 at 12:25:06PM +0100, Thomas Zimmermann wrote:
> Hi
> 
> Am 03.11.20 um 11:55 schrieb Daniel Vetter:
> > On Tue, Nov 03, 2020 at 11:38:32AM +0100, Maxime Ripard wrote:
> >> On Tue, Nov 03, 2020 at 11:10:27AM +0100, Thomas Zimmermann wrote:
> >>> Hi
> >>>
> >>> Am 03.11.20 um 10:52 schrieb Maxime Ripard:
> >>>> On Tue, Nov 03, 2020 at 10:10:41AM +0800, Tian Tao wrote:
> >>>>> Add new api devm_drm_irq_install() to register interrupts,
> >>>>> no need to call drm_irq_uninstall() when the drm module is removed.
> >>>>>
> >>>>> v2:
> >>>>> fixed the wrong parameter.
> >>>>>
> >>>>> Signed-off-by: Tian Tao <tiantao6@xxxxxxxxxxxxx>
> >>>>> ---
> >>>>>  drivers/gpu/drm/drm_drv.c | 23 +++++++++++++++++++++++
> >>>>>  include/drm/drm_drv.h     |  3 ++-
> >>>>>  2 files changed, 25 insertions(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> >>>>> index cd162d4..0fe5243 100644
> >>>>> --- a/drivers/gpu/drm/drm_drv.c
> >>>>> +++ b/drivers/gpu/drm/drm_drv.c
> >>>>> @@ -39,6 +39,7 @@
> >>>>>  #include <drm/drm_color_mgmt.h>
> >>>>>  #include <drm/drm_drv.h>
> >>>>>  #include <drm/drm_file.h>
> >>>>> +#include <drm/drm_irq.h>
> >>>>>  #include <drm/drm_managed.h>
> >>>>>  #include <drm/drm_mode_object.h>
> >>>>>  #include <drm/drm_print.h>
> >>>>> @@ -678,6 +679,28 @@ static int devm_drm_dev_init(struct device *parent,
> >>>>>  	return ret;
> >>>>>  }
> >>>>>  
> >>>>> +static void devm_drm_dev_irq_uninstall(void *data)
> >>>>> +{
> >>>>> +	drm_irq_uninstall(data);
> >>>>> +}
> >>>>> +
> >>>>> +int devm_drm_irq_install(struct device *parent,
> >>>>> +			 struct drm_device *dev, int irq)
> >>>>> +{
> >>>>> +	int ret;
> >>>>> +
> >>>>> +	ret = drm_irq_install(dev, irq);
> >>>>> +	if (ret)
> >>>>> +		return ret;
> >>>>> +
> >>>>> +	ret = devm_add_action(parent, devm_drm_dev_irq_uninstall, dev);
> >>>>> +	if (ret)
> >>>>> +		devm_drm_dev_irq_uninstall(dev);
> >>>>> +
> >>>>> +	return ret;
> >>>>> +}
> >>>>> +EXPORT_SYMBOL(devm_drm_irq_install);
> >>>>> +
> >>>>
> >>>> Shouldn't we tie the IRQ to the drm device (so with drmm_add_action)
> >>>> instead of tying it to the underlying device?
> >>>
> >>> If the HW device goes away, there won't be any more interrupts. So it's
> >>> similar to devm_ functions for I/O memory. Why would you use the drmm_
> >>> interface?
> >>
> >> drm_irq_install/uninstall do more that just calling request_irq and
> >> free_irq though, they will also run (among other things) the irq-related
> >> hooks in the drm driver (irq_preinstall, irq_postinstall irq_uninstall)
> >> and wake anything waiting for a vblank to occur, so we need the DRM
> >> device and driver to still be around when we run drm_irq_uninstall.
> >> That's why (I assume) you have to pass the drm_device as an argument and
> >> not simply the device.
> > 
> > drm_device is guaranteed to outlive devm_, plus the hooks are meant to
> > shut down hw. hw isn't around anymore when we do drmm_ cleanup, at least
> > not in full generality.
> > 
> >> This probably works in most case since you would allocate the drm_device
> >> with devm_drm_dev_alloc, and then run drm_irq_install, so in the undoing
> >> phase you would have first drm_irq_uninstall to run, and everything is
> >> fine.
> >>
> >> However, if one doesn't use devm_drm_dev_alloc but would use
> >> devm_drm_irq_install, you would have first remove being called that
> >> would free the drm device, and then drm_irq_uninstall which will use a
> >> free'd pointer.
> > 
> > Don't do that, it's broken :-)
> 
> Umm, I just saw that hibmc doesn't use devm_drm_dev_alloc. So maybe we
> have to convert it first before using the managed irq function. OTOH
> converting it is a good idea in any case, so why not. :)

Yeah that doesn't work as-is.

I guess the second option would be if devm_drm_dev_irqinstall would take a
drm_dev_get() reference of its own. Not sure that's a good idea, but it
would make the thing a bit more flexible.
-Daniel

> 
> Best regards
> Thomas
> 
> > 
> >> So yeah, even though the interrupt line itself is tied to the device,
> >> all the logic we have around the interrupt that is dealt with in
> >> drm_irq_install is really tied to the drm_device. And since we tie the
> >> life of drm_device to its underlying device already (either implicitly
> >> through devm_drm_dev_alloc, or explictly through manual allocation in
> >> probe and free in remove) we can't end up in a situation where the
> >> drm_device outlives its device.
> > 
> > Most drivers get their drm_device lifetime completely wrong. That's why I
> > typed drmm_ stuff. So this isn't a surprise at all, but it might motiveate
> > a bunch more people to fix this up correctly.
> > 
> > Also, these drm_irq functions are 100% optional helpers (should maybe
> > rename them to make this clearer) for modern drivers. They're only built
> > in for DRIVER_LEGACY, because hysterical raisins.
> > -Daniel
> > 
> 
> -- 
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch