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

[Raag Jadav]

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()?

Raag