Re: [PATCH v12 4/5] rust: drm: gem: Introduce shmem::SGTable

["Gary Guo" <gary@garyguo.net>]

On Thu Apr 23, 2026 at 4:27 PM BST, Alexandre Courbot wrote:
> On Fri Apr 24, 2026 at 12:09 AM JST, Gary Guo wrote:
>> On Thu Apr 23, 2026 at 4:01 PM BST, Alexandre Courbot wrote:
>>>
>>> I didn't like this `UnsafeCell<Option>` since the last time, but only figured how to replace it now:
>>>
>>>     sgt_res: SetOnce<Devres<SGTableMap<T>>>,
>>>
>>> It's actually designed for that! And lets you remove at least one unsafe
>>> statement, while simplifying `get_sg_table` quite a bit. With the other
>>> suggestions I have below, here is my version of `get_sg_table` for
>>> reference:
>>>
>>>     fn get_sg_table<'a>(
>>>         &'a self,
>>>         dev: &'a device::Device<Bound>,
>>>     ) -> Result<&'a Devres<SGTableMap<T>>> {
>>>         let _dma_resv = DmaResvGuard::new(self);
>>>
>>>         if let Some(devres) = self.sgt_res.as_ref() {
>>>             Ok(devres)
>>>         } else {
>>>             // Only called for the side-effect of populating the GEM SG table.
>>>             // SAFETY: We grabbed the lock required for calling this function above.
>>>             from_err_ptr(unsafe {
>>>                 bindings::drm_gem_shmem_get_pages_sgt_locked(self.as_raw_shmem())
>>>             })?;
>>>
>>>             // INVARIANT:
>>>             // - We called drm_gem_shmem_get_pages_sgt_locked above and checked that it
>>>             //   succeeded, fulfilling the invariant of `SGTableMap` that the object's `sgt` field
>>>             //   is initialized.
>>>             // - We store this Devres in the object itself and don't move it, ensuring that the
>>>             //   object it points to remains valid for the lifetime of the `SGTableMap`.
>>>             let devres =
>>>                 Devres::new(dev, init!(SGTableMap { obj: self.into() })).inspect_err(|_| {
>>>                     // We can't make sure that the pages for this object are unmapped on
>>>                     // driver-unbind, so we need to release the sgt
>>>                     // SAFETY:
>>>                     // - We grabbed the lock required for calling this function above
>>>                     // - We checked above that get_pages_sgt_locked() was successful
>>>                     unsafe { bindings::__drm_gem_shmem_free_sgt_locked(self.as_raw_shmem()) }
>>>                 })?;
>>>
>>>             self.sgt_res.populate(devres);
>>>
>>>             // PANIC: `populate` has just succeeded, guaranteeing that `sgt_res` is populated.
>>>             Ok(self.sgt_res.as_ref().unwrap())
>>>         }
>>>     }
>>>
>>> And if only we could populate the `SetOnce` with a `impl Init<T, E>`,
>>> then we could even remove the DMA reservation acquisition on the fast
>>> path, because `SetOnce` comes with its own locking and the DMA lock here
>>> is used outside of its intended scope. I'll try to push the necessary
>>> work for `SetOnce` and maybe we can do that as a follow-up patch.
>>
>> I have this sitting in my once_wip branch for while
>> https://github.com/nbdd0121/linux/commits/once_wip/
>> (the specific commit that adds init support is
>> https://github.com/nbdd0121/linux/commit/4aabdbcf20b11626c253f203745b1d55c37ab2ee).
>>
>> This was implemented for lazy revocable support which Alvin has picked up, see
>> https://lore.kernel.org/rust-for-linux/20260326-b4-tyr-debugfs-v1-1-074badd18716@linux.dev/
>
> Haha that's pretty close to what I wrote to test the code. Do you have
> plans to send it soon?

I mean.. Alvin has already sent it?

If you (or someone else) want to carry the patch in another series, by all means.

Best,
Gary