Re: [PATCH v9 1/4] drm: Introduce device wedged event

["Christian König" <christian.koenig@amd.com>]

Am 25.11.24 um 06:56 schrieb Aravind Iddamsetty:
> On 22/11/24 21:32, Raag Jadav wrote:
>> On Fri, Nov 22, 2024 at 11:09:32AM +0100, Christian König wrote:
>>> Am 22.11.24 um 08:07 schrieb Raag Jadav:
>>>> On Mon, Nov 18, 2024 at 08:26:37PM +0530, Aravind Iddamsetty wrote:
>>>>> On 15/11/24 10:37, Raag Jadav wrote:
>>>>>> Introduce device wedged event, which notifies userspace of 'wedged'
>>>>>> (hanged/unusable) state of the DRM device through a uevent. This is
>>>>>> useful especially in cases where the device is no longer operating as
>>>>>> expected and has become unrecoverable from driver context. Purpose of
>>>>>> this implementation is to provide drivers a generic way to recover with
>>>>>> the help of userspace intervention without taking any drastic measures
>>>>>> in the driver.
>>>>>>
>>>>>> A 'wedged' device is basically a dead device that needs attention. The
>>>>>> uevent is the notification that is sent to userspace along with a hint
>>>>>> about what could possibly be attempted to recover the device and bring
>>>>>> it back to usable state. Different drivers may have different ideas of
>>>>>> a 'wedged' device depending on their hardware implementation, and hence
>>>>>> the vendor agnostic nature of the event. It is up to the drivers to
>>>>>> decide when they see the need for recovery and how they want to recover
>>>>>> from the available methods.
>>>>>>
>>>>>> Prerequisites
>>>>>> -------------
>>>>>>
>>>>>> The driver, before opting for recovery, needs to make sure that the
>>>>>> 'wedged' device doesn't harm the system as a whole by taking care of the
>>>>>> prerequisites. Necessary actions must include disabling DMA to system
>>>>>> memory as well as any communication channels with other devices. Further,
>>>>>> the driver must ensure that all dma_fences are signalled and any device
>>>>>> state that the core kernel might depend on are cleaned up. Once the event
>>>>>> is sent, the device must be kept in 'wedged' state until the recovery is
>>>>>> performed. New accesses to the device (IOCTLs) should be blocked,
>>>>>> preferably with an error code that resembles the type of failure the
>>>>>> device has encountered. This will signify the reason for wegeding which
>>>>>> can be reported to the application if needed.
>>>>> should we even drop the mmaps we created?
>>>> Whatever is required for a clean recovery, yes.
>>>>
>>>> Although how would this play out? Do we risk loosing display?
>>>> Or any other possible side-effects?
>>> Before sending a wedge event all DMA transfers of the device have to be
>>> blocked.
>>>
>>> So yes, all display, mmap() and file descriptor connections you had with the
>>> device would need to be re-created.
>> Does it mean we'd have to rely on userspace to unmap()?
>
> I'm not sure of display, but at least all user mappings can be destroyed
> using drm_vma_node_unmap.

That is not really correct. The mappings are not destroy, they are 
invalidated.

On access a page fault is generated and TTM should redirect the access 
to the dummy page.

The userspace application still has to unmap the VMA to destroy it.

Regards,
Christian.

>
> Thanks,
> Aravind.
>> Raag