Re: [PATCH v2 26/26] drm/bridge: establish a link between the bridge supplier and consumer

[Andrzej Hajda]

On 07.05.2018 15:43, Peter Rosin wrote:
> On 2018-05-07 14:59, Andrzej Hajda wrote:
>> On 04.05.2018 15:52, Peter Rosin wrote:
>>> If the bridge supplier is unbound, this will bring the bridge consumer
>>> down along with the bridge. Thus, there will no longer linger any
>>> dangling pointers from the bridge consumer (the drm_device) to some
>>> non-existent bridge supplier.
>> I understand rationales behind this patch, but it is another step into
>> making drm_dev one big driver with subcomponents, where drm will work
>> only if every subcomponent is working/loaded.
> The step is very small IMHO. Just a device-link, which is very easy to
> remove once whatever other solution is ready.
>
>>                                               Do we need to go this way?
> If the drivers expect the parts to be there, and there is no other safety
> net in place if they are not, what is the (short-term) alternative?
>
>> In case of many platforms such approach results in display turned on
>> very late on boot for example due to late initialization of some
>> regulator exposed by some i2c device, which is used by hdmi bridge. And
>> this hdmi bridge is just to provide alternative(rarely used) display
>> path, the main display path would work anyway.
> This patch does not contribute to any late init and any such delay is not
> affected by this. At all.
>
>> So the main question to drm maintainers is about evolution of bridges,
>> if drm_bridges should become mandatory components of drm device or they
>> could be added/removed dynamically?
> That is a much bigger question than this patch/series. Conflating the
> two is not fair IMHO. You could run this very same argument for every
> driver that gets added, since any additional driver will just make it
> harder to make everything dynamic. Should we stop development right
> away?
>
> Besides, as long as the drm devices are in fact acting as big static
> drivers (built from smaller parts), 

not true

> this should be considered a bug-fix
> that will prevent dereference of stale pointers.
>
> Or will some other solution appear and magically make all bridges and
> drm drivers capable of dynamic reconfiguration in the next few weeks?
> Yeah, right...

You are not changing single driver, you are changing framework and it
affects all the drivers using it, being more cautious about such patches
seems quite natural.

Anyway, I have realized that since drm_bridge_detach will remove the
link, so with properly written dynamic bridge removal, your patch should
not be a blocker.

Regards
Andrzej

>
> Cheers,
> Peter
>
>> Regards
>> Andrzej
>>
>>
>>> Signed-off-by: Peter Rosin <peda@xxxxxxxxxx>
>>> ---
>>>  drivers/gpu/drm/drm_bridge.c | 18 ++++++++++++++++++
>>>  include/drm/drm_bridge.h     |  2 ++
>>>  2 files changed, 20 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
>>> index 78d186b6831b..0259f0a3ff27 100644
>>> --- a/drivers/gpu/drm/drm_bridge.c
>>> +++ b/drivers/gpu/drm/drm_bridge.c
>>> @@ -26,6 +26,7 @@
>>>  #include <linux/mutex.h>
>>>  
>>>  #include <drm/drm_bridge.h>
>>> +#include <drm/drm_device.h>
>>>  #include <drm/drm_encoder.h>
>>>  
>>>  #include "drm_crtc_internal.h"
>>> @@ -127,12 +128,25 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
>>>  	if (bridge->dev)
>>>  		return -EBUSY;
>>>  
>>> +	if (encoder->dev->dev != bridge->odev) {
>>> +		bridge->link = device_link_add(encoder->dev->dev,
>>> +					       bridge->odev, 0);
>>> +		if (!bridge->link) {
>>> +			dev_err(bridge->odev, "failed to link bridge to %s\n",
>>> +				dev_name(encoder->dev->dev));
>>> +			return -EINVAL;
>>> +		}
>>> +	}
>>> +
>>>  	bridge->dev = encoder->dev;
>>>  	bridge->encoder = encoder;
>>>  
>>>  	if (bridge->funcs->attach) {
>>>  		ret = bridge->funcs->attach(bridge);
>>>  		if (ret < 0) {
>>> +			if (bridge->link)
>>> +				device_link_del(bridge->link);
>>> +			bridge->link = NULL;
>>>  			bridge->dev = NULL;
>>>  			bridge->encoder = NULL;
>>>  			return ret;
>>> @@ -159,6 +173,10 @@ void drm_bridge_detach(struct drm_bridge *bridge)
>>>  	if (bridge->funcs->detach)
>>>  		bridge->funcs->detach(bridge);
>>>  
>>> +	if (bridge->link)
>>> +		device_link_del(bridge->link);
>>> +	bridge->link = NULL;
>>> +
>>>  	bridge->dev = NULL;
>>>  }
>>>  
>>> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
>>> index b656e505d11e..804189c63a4c 100644
>>> --- a/include/drm/drm_bridge.h
>>> +++ b/include/drm/drm_bridge.h
>>> @@ -261,6 +261,7 @@ struct drm_bridge_timings {
>>>   * @list: to keep track of all added bridges
>>>   * @timings: the timing specification for the bridge, if any (may
>>>   * be NULL)
>>> + * @link: drm consumer <-> bridge supplier
>>>   * @funcs: control functions
>>>   * @driver_private: pointer to the bridge driver's internal context
>>>   */
>>> @@ -271,6 +272,7 @@ struct drm_bridge {
>>>  	struct drm_bridge *next;
>>>  	struct list_head list;
>>>  	const struct drm_bridge_timings *timings;
>>> +	struct device_link *link;
>>>  
>>>  	const struct drm_bridge_funcs *funcs;
>>>  	void *driver_private;
>>
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