devm actions and hw clenaup (was Re: [PATCH 01/11] drm: Add devm_drm_dev_init/register)
Noralf Trønnes
noralf at tronnes.org
Tue Jan 29 14:34:46 UTC 2019
Den 24.01.2019 18.57, skrev Daniel Vetter:
> On Thu, Jan 24, 2019 at 6:46 PM Greg KH <gregkh at linuxfoundation.org> wrote:
>>
>> On Thu, Jan 24, 2019 at 11:43:12AM +0100, Daniel Vetter wrote:
>>> On Wed, Jan 23, 2019 at 11:54:07AM +0100, Noralf Trønnes wrote:
>>>>
>>>>
>>>> Den 22.01.2019 20.30, skrev Daniel Vetter:
>>>>> On Tue, Jan 22, 2019 at 8:07 PM Noralf Trønnes <noralf at tronnes.org> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> Den 22.01.2019 10.32, skrev Daniel Vetter:
>>>>>>> On Mon, Jan 21, 2019 at 01:21:46PM +0100, Noralf Trønnes wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> Den 21.01.2019 10.55, skrev Daniel Vetter:
>>>>>>>>> On Mon, Jan 21, 2019 at 10:10:14AM +0100, Daniel Vetter wrote:
>>>>>>>>>> On Sun, Jan 20, 2019 at 12:43:08PM +0100, Noralf Trønnes wrote:
>>>>>>>>>>> This adds resource managed (devres) versions of drm_dev_init() and
>>>>>>>>>>> drm_dev_register().
>>>>>>>>>>>
>>>>>>>>>>> Also added is devm_drm_dev_register_with_fbdev() which sets up generic
>>>>>>>>>>> fbdev emulation as well.
>>>>>>>>>>>
>>>>>>>>>>> devm_drm_dev_register() isn't exported since there are no users.
>>>>>>>>>>>
>>>>>>>>>>> Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
>>>>>>>>>>
>>>>>>>>
>>>>>>>> <snip>
>>>>>>>>
>>>>>>>>>>> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
>>>>>>>>>>> index 381581b01d48..12129772be45 100644
>>>>>>>>>>> --- a/drivers/gpu/drm/drm_drv.c
>>>>>>>>>>> +++ b/drivers/gpu/drm/drm_drv.c
>>>>>>>>>>> @@ -36,6 +36,7 @@
>>>>>>>>>>>
>>>>>>>>>>> #include <drm/drm_client.h>
>>>>>>>>>>> #include <drm/drm_drv.h>
>>>>>>>>>>> +#include <drm/drm_fb_helper.h>
>>>>>>>>>>> #include <drm/drmP.h>
>>>>>>>>>>>
>>>>>>>>>>> #include "drm_crtc_internal.h"
>>>>>>>>>>> @@ -871,6 +872,111 @@ void drm_dev_unregister(struct drm_device *dev)
>>>>>>>>>>> }
>>>>>>>>>>> EXPORT_SYMBOL(drm_dev_unregister);
>>>>>>>>>>>
>>>>>>>>>>> +static void devm_drm_dev_init_release(void *data)
>>>>>>>>>>> +{
>>>>>>>>>>> + drm_dev_put(data);
>>>>>>>>>
>>>>>>>>> We need drm_dev_unplug() here, or this isn't safe.
>>>>>>>>
>>>>>>>> This function is only used to cover the error path if probe fails before
>>>>>>>> devm_drm_dev_register() is called. devm_drm_dev_register_release() is
>>>>>>>> the one that calls unplug. There are comments about this in the functions.
>>>>>>>
>>>>>>> I think I get a prize for being ignorant and blind :-/
>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>> +}
>>>>>>>>>>> +
>>>>>>>>>>> +/**
>>>>>>>>>>> + * devm_drm_dev_init - Resource managed drm_dev_init()
>>>>>>>>>>> + * @parent: Parent device object
>>>>>>>>>>> + * @dev: DRM device
>>>>>>>>>>> + * @driver: DRM driver
>>>>>>>>>>> + *
>>>>>>>>>>> + * Managed drm_dev_init(). The DRM device initialized with this function is
>>>>>>>>>>> + * automatically released on driver detach. You must supply a
>>>>>>>>>
>>>>>>>>> I think a bit more clarity here would be good:
>>>>>>>>>
>>>>>>>>> "... automatically released on driver unbind by callind drm_dev_unplug()."
>>>>>>>>>
>>>>>>>>>>> + * &drm_driver.release callback to control the finalization explicitly.
>>>>>>>>>
>>>>>>>>> I think a loud warning for these is in order:
>>>>>>>>>
>>>>>>>>> "WARNING:
>>>>>>>>>
>>>>>>>>> "In generally it is unsafe to use devm functions for drm structures
>>>>>>>>> because the lifetimes of &drm_device and the underlying &device do not
>>>>>>>>> match. This here works because it doesn't immediately free anything, but
>>>>>>>>> only calls drm_dev_unplug(), which internally decrements the &drm_device
>>>>>>>>> refcount through drm_dev_put().
>>>>>>>>>
>>>>>>>>> "All other drm structures must still be explicitly released in the
>>>>>>>>> &drm_driver.release callback."
>>>>>>>>>
>>>>>>>>> While thinking about this I just realized that with this design we have no
>>>>>>>>> good place to call drm_atomic_helper_shutdown(). Which we need to, or all
>>>>>>>>> kinds of things will leak badly (connectors, fb, ...), but there's no
>>>>>>>>> place to call it:
>>>>>>>>> - unbind is too early, since we haven't yet called drm_dev_unplug, and the
>>>>>>>>> drm_dev_unregister in there must be called _before_ we start to shut
>>>>>>>>> down anything.
>>>>>>>>> - drm_driver.release is way too late.
>>>>>>>>>
>>>>>>>>> Ofc for a real hotunplug there's no point in shutting down the hw (it's
>>>>>>>>> already gone), but for a driver unload/unbind it would be nice if this
>>>>>>>>> happens automatically and in the right order.
>>>>>>>>>
>>>>>>>>> So not sure what to do here really.
>>>>>>>>
>>>>>>>> How about this change: (it breaks the rule of pulling helpers into the
>>>>>>>> core, so maybe we should put the devm_ functions into the simple KMS
>>>>>>>> helper instead?)
>>>>>>>
>>>>>>> Yeah smells a bit much like midlayer ... What would work is having a pile
>>>>>>> more devm_ helper functions, so that we onion-unwrap everything correctly,
>>>>>>> and in the right order. So:
>>>>>>>
>>>>>>> - devm_drm_dev_init (always does a drm_dev_put())
>>>>>>>
>>>>>>> - devm_drm_poll_enable (shuts down the poll helper with a devm action)
>>>>>>>
>>>>>>> - devm_drm_mode_config_reset (does an atomic_helper_shutdown() as it's cleanup action)
>>>>>>>
>>>>>>> - devm_drm_dev_register (grabs an additional drm_dev_get() reference so it
>>>>>>> can call drm_dev_unplug() unconditionally).
>>>>>>>
>>>>>>
>>>>>> Beautiful! I really like this, it's very flexible.
>>>>>>
>>>>>> Where should devm_drm_mode_config_reset() live? It will pull in the
>>>>>> atomic helper...
>>>>>
>>>>> I think a new drm_devm.c helper would be nice for all this stuff.
>>>>> Especially since you can't freely mix devm-based setup/cleanup with
>>>>> normal cleanup I think it'd be good to have it all together in one
>>>>> place. And perhaps even a code example in the DOC: overview.
>>>>>
>>>>>>> We'd need to make sure some of the cleanup actions dtrt when the device is
>>>>>>> gone, but I think we can achieve that by liberally sprinkling
>>>>>>> drm_dev_enter/exit over them, e.g. the the cleanup action for
>>>>>>> drm_mode_config_reset would be:
>>>>>>>
>>>>>>> {
>>>>>>> if (drm_dev_enter())
>>>>>>> return;
>>>>>>>
>>>>>>> drm_atomic_helper_shutdown();
>>>>>>>
>>>>>>> drm_dev_exit();
>>>>>>> }
>>>>>>>
>>>>>>
>>>>>> drm_dev_enter() can only be used to check whether the drm_device is
>>>>>> registered or not, it doesn't say anything about the state of the parent
>>>>>> device.
>>>>>>
>>>>>> All we know is that the device is being unbound from the driver, we
>>>>>> don't know if it's the device that's being removed or if it's the driver
>>>>>> that's unregistered.
>>>>>
>>>>> You're right, both paths will have called drm_dev_unplug by then.
>>>>> Silly me. I really liked my idea :-)
>>>>>
>>>>>> I have looked at the various call chains:
>>>>>>
>>>>>> driver_unregister ->
>>>>>> bus_remove_driver ->
>>>>>> driver_detach ->
>>>>>> device_release_driver_internal
>>>>>>
>>>>>> device_unregister ->
>>>>>> device_del ->
>>>>>> bus_remove_device ->
>>>>>> device_release_driver ->
>>>>>> device_release_driver_internal
>>>>>>
>>>>>> sysfs: unbind_store ->
>>>>>> device_release_driver ->
>>>>>> device_release_driver_internal
>>>>>>
>>>>>> The only way I've found to differentiate between these in a cleanup
>>>>>> action is that device_del() uses the bus notifier to signal
>>>>>> BUS_NOTIFY_DEL_DEVICE before calling bus_remove_device(). Such a
>>>>>> notifier could be used to set a drm_device->parent_removed flag.
>>>>>
>>>>> Hm, this might upset Greg KH's code taste ... maybe there's a better
>>>>> way to do this, but best to prototype a patch with this, send it to
>>>>> him and ask how to :-)
>>>>>
>>>>
>>>> I'll leave this to the one that needs it. The tinydrm drivers doesn't
>>>> need to touch hw after DRM unregister.
>>>>
>>>>>> Why is it necessary to call drm_atomic_helper_shutdown() here? Doesn't
>>>>>> everything get disabled when userspace closes? It does in my tinydrm
>>>>>> world :-)
>>>>>
>>>>> Iirc fbdev/fbcon can result in leaks ... at least we've had patches
>>>>> where drivers leaked drm_connector and drm_framebuffer objects, and
>>>>> they've been fixed by calling drm_atomic_helper_shutdown() in the
>>>>> unload path. Maybe this is cargo-culting, but it goes way back to
>>>>> pre-atomic, where drivers called drm_helper_force_disable_all().
>>>>>
>>>>> If you try to move the fbcon to your tinydrm drivers (con2fb is
>>>>> apparently the cmdline tool you need, never tried it, I only switch
>>>>> the kernel's console between fbcon and dummycon and back, not what
>>>>> fbcon drivers itself), then I think you should be able to reproduce.
>>>>> And maybe you have a better idea how to deal with this all.
>>>>>
>>>>> Note also that there's been proposals floating around to only close an
>>>>> drm_framebuffer, not also remove it (like the current RMFB ioctl
>>>>> does), with that closing userspace would not necessarily lead to a
>>>>> full cleanup.
>>>>>
>>>>> Another thing (which doesn't apply to drm_simple_display_pipe drivers)
>>>>> is if you have the display on, but no planes showing (i.e. all black).
>>>>> Then all the fbs will be cleaned up, but drm_connector will be
>>>>> leaking. That's a case where you need drm_atomic_helper_shutdown()
>>>>> even if fbcon/fbdev isn't even enabled.
>>>>>
>>>>
>>>> Ok, this means that I don't need to call drm_atomic_helper_shutdown() in
>>>> tinydrm. DRM userspace disables the pipe on close and the generic fbdev
>>>> emulation also releases everything.
>>>> Even so, maybe I should use devm_drm_mode_config_reset() after all to
>>>> keep drivers uniform, to avoid confusion: why doesn't he use it?
>>>
>>> Hm maybe there is an official way to solve this, pulling in Greg+Rafael.
>>>
>>> Super short summary: We want to start using devm actions to clean up drm
>>> drivers. Here's the problem:
>>> - For a driver unload/unbind without hotunplug, we want to properly clean
>>> up the hardware and shut it all down.
>>
>> Then do it on probe/disconnect.
>>
>>> - But if the device is unplugged already, that's probably not the best
>>> idea, and we only want to clean up the kernel's resources/allocations.
>>
>> Again, probe/disconnect will be called either way.
>>
>> But as you note, memory will NOT be freed by the devm stuff if you
>> manually unbind a driver from a device.
>>
>> So don't touch hardware there, it's not going to work :)
>>
>>> What's the recommendation here? I see a few options:
>>>
>>> - Make sure everything can deal with this properly. Hotunplug can happen
>>> anytime, so there's a race no matter what.
>>
>> Yes.
>>
>>> - Check with the device model whether the struct device is disappearing or
>>> whether we're just dealing with a driver unbind (no idea how to do
>>> that), and act accordingly.
>>
>> You don't know that, sorry. Just do any hardware stuff on disconnect.
>> Assuming your hardware is still present :)
>>
>>> - Fundamental question: Touching the hw from devm actions, is that ok? If
>>> not, then the pretty nifty plan laid out in this thread wont work.
>>
>> Nope, that's not going to work, the device could either be long gone, or
>> you will not be called due to unbind happening from userspace.
>>
>> But really, unbind from userspace is very very rare, it's a developer
>> thing mostly. Oh and a virtual driver thing, but those people are crazy
>> :)
>>
>>> - Something completely different?
>>
>> Do it in disconnect :)
>
> Ah, I forgot to mention the important constraint :-) disconnect/unbind
> should be the following sequence:
>
> 1. Unregister all the userspace interfaces (there's a lot of them) and
> make sure all the pending ioctls are done so that from now on
> userspace sees lots of -EIO (in case it still has fd open, which is
> going to be the normal for hotunplug.
>
> 2. Shut down hw and all ongoing operations (only relevant for unbind,
> but needs to be able to cope with sudden hotunplug on top anyway).
>
> 3. Clean up the kernel mess and release everything.
>
> Probe is exactly the other way round, so would perfectly fit into the
> devm onion cleanup. See in the commented earlier replies above how
> that would match in details, but tldr; if we have to do 2. in
> disconnect, then we also have to do 1. in disconnected, and only doing
> 3. through devm is almost not worth the bother. But if we could do all
> three through devm then simple drivers wouldn't even need any
> disconnect/unbind callback at all. That's our motivation for trying to
> come up with an answer that's not "do it in disconnect". "do it in
> disconnect" is how we do it all today already.
>
> Yes we're trying to make tiny drivers even smaller, we have enough
> nowadays that this stuff would be worth it :-)
>
I think a solution is to say that drivers that want to touch hw on
disconnect needs to use device_driver->remove to do that.
This is an example driver that doesn't need to touch hw because it's so
simple that userspace has disabled the pipeline:
static void drm_driver_release(struct drm_device *drm)
{
drm_mode_config_cleanup(drm);
drm_dev_fini(drm);
kfree(drm);
}
static struct drm_driver drm_driver = {
.release = drm_driver_release,
/* ... */
};
static int driver_probe(struct device *dev)
{
struct drm_device *drm;
int ret;
drm = kzalloc(sizeof(*drm), GFP_KERNEL);
if (!drm)
return -ENOMEM;
ret = devm_drm_dev_init(dev, drm, &drm_driver);
if (ret) {
kfree(drm);
return ret;
}
drm_mode_config_init(drm);
/* Aquire various resources, all managed by devres */
drm_mode_config_reset(drm);
return devm_drm_dev_register(drm);
}
struct device_driver driver = {
.probe = driver_probe,
};
A driver that wants to touch hardware on disconnect, can look like this:
static void drm_driver_release(struct drm_device *drm)
{
drm_mode_config_cleanup(drm);
drm_dev_fini(drm);
kfree(drm);
}
static struct drm_driver drm_driver = {
.release = drm_driver_release,
/* ... */
};
static int driver_probe(struct device *dev)
{
struct drm_device *drm;
int ret;
drm = kzalloc(sizeof(*drm), GFP_KERNEL);
if (!drm)
return -ENOMEM;
ret = devm_drm_dev_init(dev, drm, &drm_driver);
if (ret) {
kfree(drm);
return ret;
}
drm_mode_config_init(drm);
/* Aquire various resources, all managed by devres */
drm_mode_config_reset(drm);
ret = drm_dev_register(drm);
if (ret)
return ret;
drm_dev_get(dev); /* If using drm_dev_unplug() */
dev_set_drvdata(dev, drm);
return 0;
}
/* This function is called before devres_release_all() */
static int driver_remove(struct device *dev)
{
struct drm_device *drm = dev_get_drvdata(dev);
drm_dev_unplug(drm); OR drm_dev_unregister(drm);
drm_atomic_helper_shutdown(drm)
return 0;
}
struct device_driver driver = {
.probe = driver_probe,
.remove = driver_remove,
};
Noralf.
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