Re: KMS atomic state sets, full vs. minimal (Re: [PATCH v3 0/6] Add support for atomic async page-flips)
On Mon, Oct 03, 2022 at 11:48:49AM +0300, Pekka Paalanen wrote: > On Fri, 30 Sep 2022 18:45:09 +0300 > Ville Syrjälä wrote: > > > On Fri, Sep 30, 2022 at 06:37:00PM +0300, Pekka Paalanen wrote: > > > On Fri, 30 Sep 2022 18:09:55 +0300 > > > Ville Syrjälä wrote: > > > > > > > That would actively discourage people from even attempting the > > > > "just dump all the state into the ioctl" approach with async flips > > > > since even the props whose value isn't even changing would be rejected. > > > > > > > > > > About that. > > > > > > To me it looks like just a classic case of broken communication. > > > > > > The kernel developers assume that of course userspace will minimize the > > > state set to only those properties that change, because...? > > > > > > Userspace developers think that of course the kernel will optimize the > > > state set into minimal changes, because the kernel actually has the > > > authoritative knowledge of what the current state is, and the driver > > > actually knows which properties are costly and need to be optimized and > > > which ones don't matter. It has never even occurred to me that the > > > kernel would not compare next state to current state. > > > > > > No-one ever documented any expectations, did they? > > > > Do you really want that for async flips? Async flips can't be > > done atomically with anything else, so why are you even asking > > the kernel to do that? > > I'm not talking about async flips only. > > I'm talking about atomic commits in general. I don't think it's a good > idea to make async atomic commits behave fundamentally different from > sync atomic commits wrt. what non-changing state you are allowed to > list in your state set or not. > > Isn't there common DRM code to convert an atomic commit state set into > state objects? It probably starts by copying the current state, and > then playing through the commit state set, setting all listed > properties to their new values? Why wouldn't that loop maintain the > knowledge of what actually changed? Any such book keeping is entirely ad-hoc atm. It's also not super obvious how much of it is actually useful. You have to do a real commit on the crtc anyway if the crtc (or on any of its associated objects) is listed in the commit, so there's not necessarily much to be gained by tracking chages in all properties. And that behaviour again enters very muddy waters when combined with the async flip flag for the entire commit. The current approach being proposed seems to suggest that we can instead short circuit async commits when nothing has changed. That is not at all how sync atomic commits work. > When you copy the current data, reset all changed-flags to false. When > you apply each property from the commit set, compare the value and set > the changed-flag only if the value changes. > > This manufacturing of the new tentative state set happens at atomic > commit ioctl time before the ioctl returns, right? So the current state > is well-defined: any previous atomic sync or async commit can be assumed to > have succeeded even if it hasn't applied in hardware yet if the commit > ioctl for it succeeded, right? Yes. We could certainly try to fully track all changes in properties, but no has measured if there's any real benefit of doing so. -- Ville Syrjälä Intel
Re: KMS atomic state sets, full vs. minimal (Re: [PATCH v3 0/6] Add support for atomic async page-flips)
On Fri, 30 Sep 2022 18:45:09 +0300 Ville Syrjälä wrote: > On Fri, Sep 30, 2022 at 06:37:00PM +0300, Pekka Paalanen wrote: > > On Fri, 30 Sep 2022 18:09:55 +0300 > > Ville Syrjälä wrote: > > > > > That would actively discourage people from even attempting the > > > "just dump all the state into the ioctl" approach with async flips > > > since even the props whose value isn't even changing would be rejected. > > > > About that. > > > > To me it looks like just a classic case of broken communication. > > > > The kernel developers assume that of course userspace will minimize the > > state set to only those properties that change, because...? > > > > Userspace developers think that of course the kernel will optimize the > > state set into minimal changes, because the kernel actually has the > > authoritative knowledge of what the current state is, and the driver > > actually knows which properties are costly and need to be optimized and > > which ones don't matter. It has never even occurred to me that the > > kernel would not compare next state to current state. > > > > No-one ever documented any expectations, did they? > > Do you really want that for async flips? Async flips can't be > done atomically with anything else, so why are you even asking > the kernel to do that? I'm not talking about async flips only. I'm talking about atomic commits in general. I don't think it's a good idea to make async atomic commits behave fundamentally different from sync atomic commits wrt. what non-changing state you are allowed to list in your state set or not. Isn't there common DRM code to convert an atomic commit state set into state objects? It probably starts by copying the current state, and then playing through the commit state set, setting all listed properties to their new values? Why wouldn't that loop maintain the knowledge of what actually changed? When you copy the current data, reset all changed-flags to false. When you apply each property from the commit set, compare the value and set the changed-flag only if the value changes. This manufacturing of the new tentative state set happens at atomic commit ioctl time before the ioctl returns, right? So the current state is well-defined: any previous atomic sync or async commit can be assumed to have succeeded even if it hasn't applied in hardware yet if the commit ioctl for it succeeded, right? Thanks, pq pgp0OLizrAqiL.pgp Description: OpenPGP digital signature
Re: KMS atomic state sets, full vs. minimal (Re: [PATCH v3 0/6] Add support for atomic async page-flips)
On Fri, Sep 30, 2022 at 06:45:09PM +0300, Ville Syrjälä wrote: > On Fri, Sep 30, 2022 at 06:37:00PM +0300, Pekka Paalanen wrote: > > On Fri, 30 Sep 2022 18:09:55 +0300 > > Ville Syrjälä wrote: > > > > > That would actively discourage people from even attempting the > > > "just dump all the state into the ioctl" approach with async flips > > > since even the props whose value isn't even changing would be rejected. > > > > About that. > > > > To me it looks like just a classic case of broken communication. > > > > The kernel developers assume that of course userspace will minimize the > > state set to only those properties that change, because...? > > > > Userspace developers think that of course the kernel will optimize the > > state set into minimal changes, because the kernel actually has the > > authoritative knowledge of what the current state is, and the driver > > actually knows which properties are costly and need to be optimized and > > which ones don't matter. It has never even occurred to me that the > > kernel would not compare next state to current state. > > > > No-one ever documented any expectations, did they? > > Do you really want that for async flips? Async flips can't be > done atomically with anything else, so why are you even asking > the kernel to do that? Also what if you want plane 1 to do an async flip, and plane 2 to do a sync flip? With the single async flag per commit you will end up with one of the following results: plane 1 plane 2 outcome can asynccan asyncasync flip on both planes (totally not what you wanted) can asynccan't async -EINVAL (what you actually wanted but got unfairly rejected) can't async can async-EINVAL can't async can't async -EINVAL Those last two are actually reasonable outcomes since the plane where you did want to async flip didn't support it. But the first two are just nonsense results. -- Ville Syrjälä Intel
Re: KMS atomic state sets, full vs. minimal (Re: [PATCH v3 0/6] Add support for atomic async page-flips)
On Fri, Sep 30, 2022 at 06:37:00PM +0300, Pekka Paalanen wrote: > On Fri, 30 Sep 2022 18:09:55 +0300 > Ville Syrjälä wrote: > > > That would actively discourage people from even attempting the > > "just dump all the state into the ioctl" approach with async flips > > since even the props whose value isn't even changing would be rejected. > > About that. > > To me it looks like just a classic case of broken communication. > > The kernel developers assume that of course userspace will minimize the > state set to only those properties that change, because...? > > Userspace developers think that of course the kernel will optimize the > state set into minimal changes, because the kernel actually has the > authoritative knowledge of what the current state is, and the driver > actually knows which properties are costly and need to be optimized and > which ones don't matter. It has never even occurred to me that the > kernel would not compare next state to current state. > > No-one ever documented any expectations, did they? Do you really want that for async flips? Async flips can't be done atomically with anything else, so why are you even asking the kernel to do that? -- Ville Syrjälä Intel
KMS atomic state sets, full vs. minimal (Re: [PATCH v3 0/6] Add support for atomic async page-flips)
On Fri, 30 Sep 2022 18:09:55 +0300 Ville Syrjälä wrote: > That would actively discourage people from even attempting the > "just dump all the state into the ioctl" approach with async flips > since even the props whose value isn't even changing would be rejected. About that. To me it looks like just a classic case of broken communication. The kernel developers assume that of course userspace will minimize the state set to only those properties that change, because...? Userspace developers think that of course the kernel will optimize the state set into minimal changes, because the kernel actually has the authoritative knowledge of what the current state is, and the driver actually knows which properties are costly and need to be optimized and which ones don't matter. It has never even occurred to me that the kernel would not compare next state to current state. No-one ever documented any expectations, did they? Thanks, pq pgpzHiBxOm78R.pgp Description: OpenPGP digital signature
