Re: [PATCH v3 1/2] rust: sizes: add DeviceSize trait for device address space constants

[John Hubbard]

On 4/1/26 2:46 AM, Alice Ryhl wrote:
> On Tue, Mar 31, 2026 at 03:43:18PM -0700, John Hubbard wrote:
>> The SZ_* constants are usize, matching the CPU pointer width. But
>> device address spaces have their own widths (32-bit MMIO windows,
>> 64-bit GPU framebuffers, etc.), so drivers end up casting these
>> constants with SZ_1M as u64 or helper functions. This adds
>> boilerplate with no safety benefit.
>>
>> Add a DeviceSize trait with associated SZ_* constants, implemented
>> for u32, u64, and usize. With the trait in scope, callers write
>> u64::SZ_1M or u32::SZ_4K to get the constant in their device's
>> native width. All SZ_* values fit in a u32, so every implementation
>> is lossless. Each impl has a const assert to catch any future
>> constant that would overflow.
>>
>> A define_sizes! macro generates everything from a single internal
>> list of names. The macro takes the target types as arguments, so
>> adding a new target type requires changing only the call site.
>>
>> Suggested-by: Danilo Krummrich <dakr@xxxxxxxxxx>
>> Link: https://lore.kernel.org/all/DGB9G697GSWO.3VBFGU5MKFPMR@xxxxxxxxxx/
>> Link: https://lore.kernel.org/all/DGHI8WRKBQS9.38910L6FIIZTE@xxxxxxxxxx/
>> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> 
> The name `DeviceSize` seems overly specific to the use-case you had. It

Yes, actually this name has been worrying me from the start. Because
it is not necessary to tie it, conceptually, to devices at all.

> also makes it sound like something you would implement *for* the device
> type rather than for the integer type. Why not name it something more
> generic such as SizeConstants?

Yes, thanks, I do think SizeConstants is more accurate.

I'm inclined to make that change, unless someone else tells me not
to...let's see.


thanks,
-- 
John Hubbard