Re: [PATCH v4 7/8] drm/i2c: tda998x: register as a drm bridge

[Jyri Sarha <jsarha@ti.com>]

On 24/04/18 20:06, Russell King - ARM Linux wrote:
> On Tue, Apr 24, 2018 at 07:04:16PM +0300, Jyri Sarha wrote:
>> On 24/04/18 13:14, 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...
>>>
>>> (I think drm_panel might suffer from this issue too?)
>>
>> Yes it does and I took a shot at trying to fix it at the end of the
>> previous merge window, but gave up as I run out of time. I re-spun the
>> work now after reading this thread. I add you and Russell to cc.
> 
> Right, and these exact problems are what the component helper is
> there to sort out, in a subsystem independent way.
> 
> What is the problem with the component helper that people seem to
> be soo loathed to use it, instead preferring to come up with sub-
> standard and broken alternatives?
> 

Nothing but time. Embedding component helpers seamlessly into drm
framework does not sound like a couple of days job. Right now I simply
do not have time to take on a challenge like that. If someone does it I
am all for it.

However, I would not call device links substandard. They are in the
device core after all.

Best regards,
Jyri

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