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A location > has no say in which accesses are allowed, so a register cannot be > declared read-only or write-only. > > Add the `io::access` module with the `Mode` trait and the `ReadWrite`, > `ReadOnly` and `WriteOnly` modes, and add an `Access` associated type to > `IoLoc`. Projecting through a location applies its mode: `ReadWrite` > leaves the view as it is, while `ReadOnly` and `WriteOnly` wrap it in the > `ReadOnly` and `WriteOnly` views. The bounds of the generic accessors now > apply to the backend of the restricted view, so a write to a read-only > location, or a read from a write-only one, does not compile. > > Raw offsets are `ReadWrite`. `OffsetLoc` takes the mode as a type > parameter that defaults to `ReadWrite`, and `RegisterArray` gets an > `Access` type, which `register!` sets to `ReadWrite` for now. > > No existing location changes its mode, so there is no functional change. > A following patch lets `register!` declare read-only and write-only > registers. So far, without considering access modes, we have a clean mapping after the register projection series that a `register!()` macro just generates a cust= om projection. However, this commit makes it special again and this isn't poss= ible with struct definitions, so it breaks feature composibility. It is also som= ewhat surprising in the sense that it changes the base view type. What about making RO/WO wrapper types on actual data? Something like Mmio<'_, ReadOnly> and Mmio<'_, WriteOnly> Then we can implement impl IoRead> for B where B: IoRead {} and impl IoWrite> for B where B: IoWrite {} ReadOnly will not implement FromBytes, and WriteOnly will not implement IntoBytes, to avoid people using them with `copy_read`/`copy_write` API. This way, the design works for struct too. One issue of this, though, is th= at you cannot project from `Mmio<'_, ReadOnly>` to `Mmio<'_, ReadOnly>` currently. However, I believe this is solved with the "pl= ace wrapper" concept in field projection. Regardless, before we proceed with any design, I'd like to hear opinions fr= om Benno on how this interacts with the field projection design as language feature. Best, Gary > > Signed-off-by: Daniel Almeida > --- > rust/kernel/io.rs | 42 +++++++++++++++++-------- > rust/kernel/io/access.rs | 76 ++++++++++++++++++++++++++++++++++++++++= ++++++ > rust/kernel/io/register.rs | 18 +++++++++-- > rust/macros/io/register.rs | 1 + > 4 files changed, 121 insertions(+), 16 deletions(-)