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Date:   Tue, 3 Nov 2020 11:55:08 +0100
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Maxime Ripard <maxime@cerno.tech>
Cc:     Thomas Zimmermann <tzimmermann@suse.de>,
        Tian Tao <tiantao6@hisilicon.com>,
        maarten.lankhorst@linux.intel.com, airlied@linux.ie,
        daniel@ffwll.ch, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] drm: Add the new api to install irq
Message-ID: <20201103105508.GD401619@phenom.ffwll.local>
Mail-Followup-To: Maxime Ripard <maxime@cerno.tech>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Tian Tao <tiantao6@hisilicon.com>,
        maarten.lankhorst@linux.intel.com, airlied@linux.ie,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
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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@hisilicon.com>
> > >> ---
> > >>  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 :-)

> 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
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch