On Sun, Feb 22, 2026 at 03:46:48PM -0300, Daniel Almeida wrote: > > > > On 22 Feb 2026, at 14:57, Boris Brezillon <[email protected]> > > wrote: > > > > On Fri, 20 Feb 2026 13:55:31 -0300 > > Daniel Almeida <[email protected]> wrote: > > > >>> On 20 Feb 2026, at 13:21, Boris Brezillon <[email protected]> > >>> wrote: > >>> > >>> On Fri, 20 Feb 2026 12:25:43 -0300 > >>> Daniel Almeida <[email protected]> wrote: > >>> > >>>>>> + > >>>>>> + // Checks and updates the seat state based on the slot it points > >>>>>> to > >>>>>> + // (if any). Returns a Seat with a value matching the slot state. > >>>>>> + fn check_seat(&mut self, locked_seat: &LockedSeat<T, MAX_SLOTS>) > >>>>>> -> Seat { > >>>>>> + let new_seat = match locked_seat.access(self) { > >>>>>> + Seat::Active(seat_info) => { > >>>>>> + let old_slot_idx = seat_info.slot as usize; > >>>>>> + let slot = &self.slots[old_slot_idx]; > >>>>>> + > >>>>>> + if kernel::warn_on!( > >>>>>> + !matches!(slot, Slot::Active(slot_info) if > >>>>>> slot_info.seqno == seat_info.seqno) > >>>>>> + ) { > >>>>>> + Seat::NoSeat > >>>>>> + } else { > >>>>>> + Seat::Active(SeatInfo { > >>>>>> + slot: seat_info.slot, > >>>>>> + seqno: seat_info.seqno, > >>>>>> + }) > >>>>>> + } > >>>>>> + } > >>>>>> + > >>>>>> + Seat::Idle(seat_info) => { > >>>>>> + let old_slot_idx = seat_info.slot as usize; > >>>>>> + let slot = &self.slots[old_slot_idx]; > >>>>>> + > >>>>>> + if !matches!(slot, Slot::Idle(slot_info) if > >>>>>> slot_info.seqno == seat_info.seqno) { > >>>>>> + Seat::NoSeat > >>>>>> + } else { > >>>>>> + Seat::Idle(SeatInfo { > >>>>>> + slot: seat_info.slot, > >>>>>> + seqno: seat_info.seqno, > >>>>>> + }) > >>>>>> + } > >>>>>> + } > >>>>>> + > >>>>>> + _ => Seat::NoSeat, > >>>>>> + }; > >>>>>> + > >>>>>> + // FIXME: Annoying manual copy. The original idea was to not > >>>>>> add Copy+Clone to SeatInfo, > >>>>>> + // so that only slot.rs can change the seat state, but there > >>>>>> might be better solutions > >>>>>> + // to prevent that. > >>>>> > >>>>> Okay, I guess we want some inputs from Daniel and/or Alice on that one. > >>>>> > >>>> > >>>> Hm, I'd say we shouldn't implement Clone to avoid any possibility of > >>>> holding on > >>>> to stale state by duplicating it. > >>> > >>> Okay, so basically what we have now. > >>> > >>>> > >>>> Why do we need to return Seat from this function? Can't we simply write > >>>> locked_seat in place and not return anything? > >>> > >>> We do both actually. IIRC, the reason is that LockedBy borrows &self if > >>> we want to read the locked_seat, which prevents us from calling methods > >>> taking a &mut ref from a `match(locked_seat.access())`. > >> > >> > >> I am referring to this change: > >> > >> --- a/drivers/gpu/drm/tyr/slot.rs > >> +++ b/drivers/gpu/drm/tyr/slot.rs > >> @@ -242,8 +242,8 @@ fn evict_slot(&mut self, slot_idx: usize, locked_seat: > >> &LockedSeat<T, MAX_SLOTS> > >> } > >> > >> // Checks and updates the seat state based on the slot it points to > >> - fn check_seat(&mut self, locked_seat: &LockedSeat<T, MAX_SLOTS>) -> > >> Seat { > >> + fn check_seat(&mut self, locked_seat: &LockedSeat<T, MAX_SLOTS>) { > >> let new_seat = match locked_seat.access(self) { > >> Seat::Active(seat_info) => { > >> let old_slot_idx = seat_info.slot as usize; > >> @@ -278,26 +278,7 @@ fn check_seat(&mut self, locked_seat: &LockedSeat<T, > >> MAX_SLOTS>) -> Seat { > >> _ => Seat::NoSeat, > >> }; > >> > >> - // FIXME: Annoying manual copy. The original idea was to not add > >> Copy+Clone to SeatInfo, > >> - // so that only slot.rs can change the seat state, but there > >> might be better solutions > >> - // to prevent that. > >> - match &new_seat { > >> - Seat::Active(seat_info) => { > >> - *locked_seat.access_mut(self) = Seat::Active(SeatInfo { > >> - slot: seat_info.slot, > >> - seqno: seat_info.seqno, > >> - }) > >> - } > >> - Seat::Idle(seat_info) => { > >> - *locked_seat.access_mut(self) = Seat::Idle(SeatInfo { > >> - slot: seat_info.slot, > >> - seqno: seat_info.seqno, > >> - }) > >> - } > >> - _ => *locked_seat.access_mut(self) = Seat::NoSeat, > >> - } > >> - > >> - new_seat > >> + *locked_seat.access_mut(self) = new_seat; > > > > That one requires Copy support, or am I missing something? > > No. Copy and Clone produce a new value and both the old and new values remain > valid. The line above is moving a value into a new location, invalidating the > previous one. This means that Copy is not required. > > > > >> } > >> > >> Or even shorter: > >> > >> fn check_seat(&mut self, locked_seat: &LockedSeat<T, MAX_SLOTS>) { > >> let (slot_idx, seqno, is_active) = match locked_seat.access(self) { > >> Seat::Active(info) => (info.slot as usize, info.seqno, true), > >> Seat::Idle(info) => (info.slot as usize, info.seqno, false), > >> _ => return, > >> }; > >> > >> let valid = if is_active { > >> !kernel::warn_on!(!matches!(&self.slots[slot_idx], > >> Slot::Active(s) if s.seqno == seqno)) > >> } else { > >> matches!(&self.slots[slot_idx], Slot::Idle(s) if s.seqno == > >> seqno) > >> }; > >> > >> if !valid { > >> *locked_seat.access_mut(self) = Seat::NoSeat; > >> } > >> } > > > > Did you try that? Last I tried, I was hitting a wall because the caller > > of check_seat() does a match on the updated seat, and inside this > > match, it calls functions that need a &mut self, and the borrow checker > > rightfully points the invalid &self then &mut self borrow pattern. > > Yes, I compiled-tested all changes I suggested. My changes compile because > they > intentionally avoid doing what you said above. > > The key here is that your borrows do not overlap anymore. The code I showed > first borrows immutably, and then returns this tuple: (slot_idx, seqno, > is_active). > The immutable borrow then ends, since this tuple is copied (not borrowed) from > its source. This benefits from the fact that primitive types are Copy. > > Note that you can borrow both mutably and immutably in the same scope just > fine. The > borrows just can’t be alive at the same time. When the borrow checker rejects > your code, > it shows you why incompatible borrows overlap (i.e.: check the “first used > here…later > used here” part of the error). > > > > >> > >> access vs access_mut() does not matter here: since the owner is &mut self > >> anyways we know we have exclusive access to the LockedSeat throughout the > >> whole > >> function. > > > > I agree, but LockedBy is picky, and last I tried I couldn't make it > > work without the annoying update+return-copy-of-seat dance you see > > here. Maybe I missed something obvious and it does indeed work with > > your suggested changes, dunno. > > Rewriting things so they pass the borrow checker is common in Rust. Sometimes > it can be done rather easily; other times the design is just broken and needs > to be reworked. Luckily this one fell in the first category. > > This benefits from the fact that no one can race us between reading this tuple > > (slot_idx, seqno, is_active) > > ..and using it. That’s because we’re taking &mut self as a proxy in LockedBy, > so > we’re sure we have exclusive access in this scope. > > If you don’t have any complaints about the code I sent (i.e.: convoluted, > wrong > logic, etc), I suggest switching to it.
I’ve tested the short check_seat() function and can confirm that it correctly compares Seat and Slot seqno as well as emits the kernel warning when we have a mismatch on an active Seat. So I’ll the simplified check_seat() function in v2 unless there are any more issues to address. > > > — Daniel >
