[Linaro-mm-sig] Re: [PATCH v12 4/5] rust: drm: gem: Introduce shmem::SGTable

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/4aabdbcf20b11626c253f203745b1d55c37...).

This was implemented for lazy revocable support which Alvin has picked up, see https://lore.kernel.org/rust-for-linux/20260326-b4-tyr-debugfs-v1-1-074badd1...

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