Re: [PATCH v2 1/2] drm/input_helper: Add new input-handling helper

[Daniel Vetter]

On Mon, Nov 22, 2021 at 11:43:42AM +0200, Pekka Paalanen wrote:
> On Fri, 19 Nov 2021 17:11:07 +0100
> Daniel Vetter <daniel@xxxxxxxx> wrote:
> 
> > On Fri, Nov 19, 2021 at 04:04:28PM +0000, Simon Ser wrote:
> > > On Friday, November 19th, 2021 at 16:53, Daniel Vetter <daniel@xxxxxxxx> wrote:
> > >   
> > > > Random idea ... should we perhaps let userspace connect the boosting? I.e.
> > > > we do a bunch of standardized boost targets (render clocks, display sr
> > > > exit), and userspace can then connect it to whichever input device it
> > > > wants to?  
> > > 
> > > On IRC we discussed having user-space hand over a FD to the kernel. When the FD
> > > becomes readable, the kernel triggers the boost.
> > > 
> > > This would let user-space use e.g. an input device, an eventfd, or an epoll FD
> > > with any combination of these as the boost signal.  
> > 
> > Can userspace filter eventfd appropriately like we do here? And can they
> > get at that maybe 2nd eventfd from logind or whatever there is on distros
> > where /dev access is locked down for compositors/users.
> 
> (Mind, eventfd is a specific thing, see 'man eventfd', and evdev/input
> device fd is different.)

Yeah I was a bit sloppy, but I knew.

> I don't think any of that is any problem when userspace prepares an
> epoll fd to be given to the boosting machinery. The boosting machinery
> could have several different targets as well, PSR vs. GPU clocks and
> whatnot.
> 
> I envision a compositor to maintain an epoll fd for boosting by
> adding/removing the same device fds to it that it already uses in its
> operations. I don't see any need to open new device fds just for
> boosting. It's only the epoll fd given to the kernel and after that the
> epoll set can still be changed, right?
> 
> The boosting machinery would never actually read or write the
> registered fd(s), so it would not interfere with the normal operations.
> But it also means the fd will remain readable until userspace services
> it. Userspace may need to set up that epoll set very carefully to have
> it work right (e.g. edge-triggered?).
> 
> If your input handling is in a different process than the DRM poking
> for some reason, the epoll fd should still work if:
> - it is possible to use SCM_RIGHTS to pass the epollfd from the
>   input process to the DRM process, and
> - you cannot extract the watched fds from an epoll fd.
> 
> Do we have those assumptions today?
> 
> Then the attack surface in the DRM process is limited to changing the
> epoll set of which fds can trigger boosting, but the DRM process can do
> that anyway. I also presume the input process can still add and remove
> fds from the epoll set even afterwards.
> 
> > I do agree that if we can do this generically maybe we should, but also
> > the use-case for input boosting is pretty well defined. I think it's just
> > about making sure that compositors is in control, and that we don't make
> > it worse (e.g. with the sr exit adding latency when the compositor can
> > redraw quickly enough).
> 
> The epollfd design sounds very good to me. One can register an
> arbitrary set of fds with it, and use even eventfds in the set to have
> purely software triggers.

Yeah I think just allowing to internall poll on any arbitrary fd sounds
like a neat interface. Userspace should then be able to do whatever it
wants to.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch