linux-kernel - Re: [PATCH v12 2/3] rust: io: mem: add a generic iomem abstraction

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Message-Id: <72809F34-033A-4A72-B56D-519D1D089299@collabora.com>
Date: Mon, 14 Jul 2025 20:39:15 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 Trevor Gross <tmgross@...ch.edu>,
 Danilo Krummrich <dakr@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
 Bjorn Helgaas <bhelgaas@...gle.com>,
 Mika Westerberg <mika.westerberg@...ux.intel.com>,
 Ying Huang <huang.ying.caritas@...il.com>,
 Benno Lossin <lossin@...nel.org>,
 linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v12 2/3] rust: io: mem: add a generic iomem abstraction

Alice,

> On 14 Jul 2025, at 08:51, Alice Ryhl <aliceryhl@...gle.com> wrote:
> 
> On Thu, Jul 10, 2025 at 10:58:13AM -0300, Daniel Almeida wrote:
>> Hi Alice,
>> 
>>>> +impl<const SIZE: usize> IoMem<SIZE> {
>>>> +    fn ioremap(resource: &Resource) -> Result<Self> {
>>>> +        let size = resource.size();
>>>> +        if size == 0 {
>>>> +            return Err(EINVAL);
>>>> +        }
>>>> +
>>>> +        let res_start = resource.start();
>>>> +
>>>> +        let addr = if resource
>>>> +            .flags()
>>>> +            .contains(io::resource::flags::IORESOURCE_MEM_NONPOSTED)
>>>> +        {
>>>> +            // SAFETY:
>>>> +            // - `res_start` and `size` are read from a presumably valid `struct resource`.
>>>> +            // - `size` is known not to be zero at this point.
>>>> +            unsafe { bindings::ioremap_np(res_start, size as usize) }
>>> 
>>> Here you cast from ResourceSize to usize. Are you sure that is correct?
>>> I thought those types could be different.
>> 
>> This seems to what C is doing as well, i.e.:
>> 
>> static void __iomem *__devm_ioremap(struct device *dev, resource_size_t offset,
>>     resource_size_t size, <---------
>>     enum devm_ioremap_type type)
>> {
>> 
>>        […]
>> 
>> case DEVM_IOREMAP_NP:
>> addr = ioremap_np(offset, size);
>> break;
>> }
>> 
>> 
>> Where:
>> 
>> `static inline void __iomem *ioremap_np(phys_addr_t offset, size_t size)`
>> 
>> IOW: this stems from the mix and match of types used the C API itself.
>> 
>> What do you suggest here? Maybe a try_into() then?
> 
> What a mess. It looks like there aren't any 32-bit architectures that
> define ioremap_np. This means that sometimes this cast will be lossy,
> but in those cases the function body just returns NULL and doesn't read
> the size.
> 
> I would probably cast to an underscore instead of explicitly mentioning
> the target type and make a comment about it.
> 
> Alice

You replied here but v13 was already out [0]. Can we shift the dicussion
there? I ended up using try_into(), but feel free to suggest the cast to _
+ the TODO if you still feel like this is the right approach.

-- Daniel

[0] https://lore.kernel.org/rust-for-linux/20250711-topics-tyr-platform_iomem-v13-0-06328b514db3@collabora.com/