On Fri, Jul 30, 2021 at 6:43 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote: > > Hi, > > The background is that I'm reviewing Wedson's PR on IoMem for > Rust-for-Linux project: > > https://github.com/Rust-for-Linux/linux/pull/462 > > readX() and writeX() are used to provide Rust code to read/write IO > memory. And I want to find whether we need to check the alignment of the > pointer. I wonder whether the addresses passed to readX() and writeX() > need to be aligned to the size of the accesses (e.g. the parameter of > readl() has to be a 4-byte aligned pointer). > > The only related information I get so far is the following quote in > Documentation/driver-io/device-io.rst: > > On many platforms, I/O accesses must be aligned with respect to > the access size; failure to do so will result in an exception or > unpredictable results. > > Does it mean all readX() and writeX() need to use aligned addresses? > Or the alignment requirement is arch-dependent, i.e. if the architecture > supports and has enabled misalignment load and store, no alignment > requirement on readX() and writeX(), otherwise still need to use aligned > addresses. > > I know different archs have their own alignment requirement on memory > accesses, just want to make sure the requirement of the readX() and > writeX() APIs. I am not aware of any driver that requires unaligned access on __iomem pointers, and since it definitely doesn't work on most architectures, I think having an unconditional alignment check makes sense. What would the alignment check look like? Is there a way to annotate a pointer that is 'void __iomem *' in C as having a minimum alignment when it gets passed into a function that uses readl()/writel() on it? Arnd
