On 2018-04-24 12:14, Peter Rosin wrote:
> On 2018-04-24 10:08, Russell King - ARM Linux wrote:
>> On Tue, Apr 24, 2018 at 08:58:42AM +0200, Peter Rosin wrote:
>>> On 2018-04-23 18:08, Russell King - ARM Linux wrote:
>>>> On Mon, Apr 23, 2018 at 09:23:00AM +0200, Peter Rosin wrote:
>>>>> static int tda998x_remove(struct i2c_client *client)
>>>>> {
>>>>> - component_del(&client->dev, &tda998x_ops);
>>>>> + struct device *dev = &client->dev;
>>>>> + struct tda998x_bridge *bridge = dev_get_drvdata(dev);
>>>>> +
>>>>> + drm_bridge_remove(&bridge->bridge);
>>>>> + component_del(dev, &tda998x_ops);
>>>>> +
>>>>
>>>> I'd like to ask a rather fundamental question about DRM bridge support,
>>>> because I suspect that there's a major fsckup here.
>>>>
>>>> The above is the function that deals with the TDA998x device being
>>>> unbound from the driver. With the component API, this results in the
>>>> DRM device correctly being torn down, because one of the hardware
>>>> devices has gone.
>>>>
>>>> With DRM bridge, the bridge is merely removed from the list of
>>>> bridges:
>>>>
>>>> void drm_bridge_remove(struct drm_bridge *bridge)
>>>> {
>>>> mutex_lock(&bridge_lock);
>>>> list_del_init(&bridge->list);
>>>> mutex_unlock(&bridge_lock);
>>>> }
>>>> EXPORT_SYMBOL(drm_bridge_remove);
>>>>
>>>> and the memory backing the "struct tda998x_bridge" (which contains
>>>> the struct drm_bridge) will be freed by the devm subsystem.
>>>>
>>>> However, there is no notification into the rest of the DRM subsystem
>>>> that the device has gone away. Worse, the memory that is still in
>>>> use by DRM has now been freed, so further use of the DRM device
>>>> results in a use-after-free bug.
>>>>
>>>> This is really not good, and to me looks like a fundamental problem
>>>> with the DRM bridge code. I see nothing in the DRM bridge code that
>>>> deals with the lifetime of a "DRM bridge" or indeed the lifetime of
>>>> the actual device itself.
>>>>
>>>> So, from what I can see, there seems to be a fundamental lifetime
>>>> issue with the design of the DRM bridge code. This needs to be
>>>> fixed.
>>>
>>> Oh crap. A gigantic can of worms...
>>
>> Yes, it's especially annoying for me, having put the effort in to
>> the component helper to cover all these cases.
>>
>>> Would a patch (completely untested btw) along this line of thinking make
>>> any difference whatsoever?
>>
>> It looks interesting - from what I can see of the device links code,
>> it would have the effect of unbinding the DRM device just before
>> TDA998x is unbound, so that's an improvement.
>>
>> However, from what I can see, the link vanishes at that point (as
>> DL_FLAG_AUTOREMOVE is set), and re-binding the TDA998x device results
>> in nothing further happening - the link will be recreated, but there
>> appears to be nothing that triggers the "consumer" to rebind at that
>> point. Maybe I've missed something?
>
> Right, auto-remove is a no-go. So, improving on the previous...
Heh, I didn't address the rebind triggering part at all, and while I'm by
no means responsible or have any deep knowledge, I thought this was true:
- driver .remove for the device owning the drm_bridge is what typically
calls drm_bridge_remove()
- driver .remove is called as part of the device being unbound from the
driver by the bus (i2c in this case)
- by registering a link to the consumer, this unbinding will trigger the
removal of this main drm consumer device as part of the unbinding
- so everything aboput the drm device will be torn down, and everything
will thus have to be reprobed to get things back
But I could easily have misunderstood just about everything in the above...
And maybe it's really inconvenient to have to trigger a reprobe of the
whole drm cluster? Maybe all drm driver parts should be components?
I have no idea.
Cheers,
Peter
PS. compile-tested the below and drm_bridge.c needs to
#include <drm/drm_device.h>
> (I think drm_panel might suffer from this issue too?)
>
> Cheers,
> Peter
>
> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> index 1638bfe9627c..b1365cfee445 100644
> --- a/drivers/gpu/drm/drm_bridge.c
> +++ b/drivers/gpu/drm/drm_bridge.c
> @@ -121,12 +121,17 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
> if (bridge->dev)
> return -EBUSY;
>
> + bridge->link = device_link_add(encoder->dev->dev, bridge->owner, 0);
> + if (!bridge->link)
> + return -EINVAL;
> +
> bridge->dev = encoder->dev;
> bridge->encoder = encoder;
>
> if (bridge->funcs->attach) {
> ret = bridge->funcs->attach(bridge);
> if (ret < 0) {
> + device_link_del(bridge->link);
> bridge->dev = NULL;
> bridge->encoder = NULL;
> return ret;
> @@ -153,6 +158,8 @@ void drm_bridge_detach(struct drm_bridge *bridge)
> if (bridge->funcs->detach)
> bridge->funcs->detach(bridge);
>
> + device_link_del(bridge->link);
> +
> bridge->dev = NULL;
> }
>
> diff --git a/drivers/gpu/drm/i2c/tda998x_drv.c b/drivers/gpu/drm/i2c/tda998x_drv.c
> index b8cb6237a38b..29eba4e9a39d 100644
> --- a/drivers/gpu/drm/i2c/tda998x_drv.c
> +++ b/drivers/gpu/drm/i2c/tda998x_drv.c
> @@ -1857,6 +1857,7 @@ tda998x_probe(struct i2c_client *client, const struct i2c_device_id *id)
> bridge->dev = dev;
> dev_set_drvdata(dev, bridge);
>
> + bridge->bridge.owner = dev;
> bridge->bridge.funcs = &tda998x_bridge_funcs;
> #ifdef CONFIG_OF
> bridge->bridge.of_node = dev->of_node;
> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> index 682d01ba920c..b8f33aba3216 100644
> --- a/include/drm/drm_bridge.h
> +++ b/include/drm/drm_bridge.h
> @@ -224,6 +224,8 @@ struct drm_bridge_funcs {
>
> /**
> * struct drm_bridge - central DRM bridge control structure
> + * @owner: device that owns the bridge
> + * @link: drm consumer <-> bridge supplier
> * @dev: DRM device this bridge belongs to
> * @encoder: encoder to which this bridge is connected
> * @next: the next bridge in the encoder chain
> @@ -233,6 +235,8 @@ struct drm_bridge_funcs {
> * @driver_private: pointer to the bridge driver's internal context
> */
> struct drm_bridge {
> + struct device *owner;
> + struct device_link *link;
> struct drm_device *dev;
> struct drm_encoder *encoder;
> struct drm_bridge *next;
>
>
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