Re: [PATCH v11 2/3] rust: add dma coherent allocator abstraction.

[Daniel Almeida <daniel.almeida@collabora.com>]

> 
> I mean, I imagine that you could make the syntax
> `dma_read!(my_alloc[7])` read the entire struct. I'm not sure which
> safe methods you are referring to, because right now there is only the
> unsafe as_slice().
> 
> Alice

I mean this:

+    /// Writes data to the region starting from `offset`. `offset` is in units of `T`, not the
+    /// number of bytes.
+    pub fn write(&self, src: &[T], offset: usize) -> Result {
+        if offset + src.len() >= self.count {
+            return Err(EINVAL);
+        }
+        // SAFETY:
+        // - The pointer is valid due to type invariant on `CoherentAllocation`
+        // and we've just checked that the range and index is within bounds.
+        // - `offset` can't overflow since it is smaller than `self.count` and we've checked
+        // that `self.count` won't overflow early in the constructor.
+        unsafe {
+            core::ptr::copy_nonoverlapping(src.as_ptr(), self.cpu_addr.add(offset), src.len())
+        };
+        Ok(())
+    }
+}

…and the similar read() method that was apparently removed, i.e.:

+    /// Reads data from the region starting from `offset` as a slice.
+    /// `offset` and `count` are in units of `T`, not the number of bytes.
+    ///
+    /// Due to the safety requirements of slice, the data returned should be regarded by the
+    /// caller as a snapshot of the region when this function is called, as the region could
+    /// be modified by the device at anytime. For ringbuffer type of r/w access or use-cases
+    /// where the pointer to the live data is needed, `start_ptr()` or `start_ptr_mut()`
+    /// could be used instead.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that no hardware operations that involve the buffer are currently
+    /// taking place while the returned slice is live.
+    pub unsafe fn read(&self, offset: usize, count: usize) -> Result<&[T]> {
+        if offset + count >= self.count {
+            return Err(EINVAL);
+        }
+        // SAFETY: The pointer is valid due to type invariant on `CoherentAllocation`,
+        // we've just checked that the range and index is within bounds. The immutability of the
+        // of data is also guaranteed by the safety requirements of the function.
+        Ok(unsafe { core::slice::from_raw_parts(self.cpu_addr.wrapping_add(offset), count) })
+    }

I don’t think there’s anything wrong with these, so maybe your solution can come in as a convenience
when one wants to save on the amount of copying? IMHO the two methods above co-exist with what
you propose.

The unsafe `as_slice()` and `as_slice_mut()` should also stay, in my opinion. They can be used, for example,
in codec drivers.

— Daniel