linux-kernel - Re: [PATCH v9 2/3] rust: add dma coherent allocator abstraction.

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Message-ID: <ddc185e6-3fee-40ea-ab55-b0f414e0f015@gmail.com>
Date: Thu, 23 Jan 2025 15:37:50 +0200
From: Abdiel Janulgue <abdiel.janulgue@...il.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: rust-for-linux@...r.kernel.org, daniel.almeida@...labora.com,
 dakr@...nel.org, robin.murphy@....com, daniel@...lak.dev,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Gary Guo <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Valentin Obst <kernel@...entinobst.de>,
 open list <linux-kernel@...r.kernel.org>, Christoph Hellwig <hch@....de>,
 Marek Szyprowski <m.szyprowski@...sung.com>, airlied@...hat.com,
 "open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v9 2/3] rust: add dma coherent allocator abstraction.

On 21/01/2025 21:56, Boqun Feng wrote:
> On Tue, Jan 21, 2025 at 10:47:46AM +0200, Abdiel Janulgue wrote:
> [...]
>> +
>> +    /// 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]> {
> 
> I don't think `read()` is a proper name here since this function only
> provides a slice for the caller to read. How about `as_slice()`? Or you
> can change the function signature to:
> 
> 	read(&self, offset, usize, count: usize, dst: &mut [T]) -> Result
> 

Thanks for the feedback! I've now changed this in v10 onwards.

/Abdiel

> Regards,
> Boqun
> 
>> +        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) })
>> +    }
>> +
> [...]