Re: [PATCH v2 3/4] gpu: nova-core: gsp: fix improper handling of empty slot in cmdq

["Alexandre Courbot" <acourbot@nvidia.com>]

On Tue Jan 27, 2026 at 3:26 AM JST, Gary Guo wrote:
> On Fri Jan 23, 2026 at 12:12 PM GMT, Eliot Courtney wrote:
>> The current code hands out buffers that go all the way up to and
>> including `rx - 1`, but we need to maintain an empty slot to prevent the
>> ring buffer from wrapping around into having 'tx == rx', which means
>> empty.
>>
>> Also add more rigorous no-panic proofs.
>>
>> Fixes: 75f6b1de8133 ("gpu: nova-core: gsp: Add GSP command queue bindings and handling")
>> Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
>> ---
>>  drivers/gpu/nova-core/gsp/cmdq.rs | 33 +++++++++++++++++++--------------
>>  1 file changed, 19 insertions(+), 14 deletions(-)
>>
>> diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/cmdq.rs
>> index 09c28eeb6f12..aa8758fc7723 100644
>> --- a/drivers/gpu/nova-core/gsp/cmdq.rs
>> +++ b/drivers/gpu/nova-core/gsp/cmdq.rs
>> @@ -227,21 +227,26 @@ fn new(dev: &device::Device<device::Bound>) -> Result<Self> {
>>          // PANIC: per the invariant of `cpu_write_ptr`, `tx` is `< MSGQ_NUM_PAGES`.
>>          let (before_tx, after_tx) = gsp_mem.cpuq.msgq.data.split_at_mut(tx);
>>  
>> -        if rx <= tx {
>> -            // The area from `tx` up to the end of the ring, and from the beginning of the ring up
>> -            // to `rx`, minus one unit, belongs to the driver.
>> -            if rx == 0 {
>> -                let last = after_tx.len() - 1;
>> -                (&mut after_tx[..last], &mut before_tx[0..0])
>> -            } else {
>> -                (after_tx, &mut before_tx[..rx])
>> -            }
>> +        // The area starting at `tx` and ending at `rx - 2` modulo MSGQ_NUM_PAGES, inclusive,
>> +        // belongs to the driver for writing.
>> +        if rx == 0 {
>> +            // Since `rx` is zero, leave an empty slot at end of the buffer.
>> +            let last = after_tx.len() - 1;
>> +            (&mut after_tx[..last], &mut before_tx[0..0])
>
> Does the address actually matter? Otherwise I would find `&mut []` easier to
> understand than an empty indexing.

The address doesn't matter, and indeed I am not sure why we did that
(possibly a lifetime issue with some previous version of the code?).

Your suggestion seems to work fine, so Eliot feel free to include it in
your series (as a separate patch please to make sure we can isolate the
effects of both changes).

>
>> +        } else if rx > tx {
>> +            // The area is contiguous and we leave an empty slot before `rx`.
>> +            // PANIC:
>> +            // - The index `rx - tx - 1` is non-negative because `rx > tx` in this branch.
>> +            // - The index does not exceed `after_tx.len()` (which is `MSGQ_NUM_PAGES - tx`)
>> +            //   because `rx < MSGQ_NUM_PAGES` by the `gsp_read_ptr` invariant.
>> +            (&mut after_tx[..(rx - tx - 1)], &mut before_tx[0..0])
>>          } else {
>> -            // The area from `tx` to `rx`, minus one unit, belongs to the driver.
>> -            //
>> -            // PANIC: per the invariants of `cpu_write_ptr` and `gsp_read_ptr`, `rx` and `tx` are
>> -            // `<= MSGQ_NUM_PAGES`, and the test above ensured that `rx > tx`.
>> -            (after_tx.split_at_mut(rx - tx).0, &mut before_tx[0..0])
>> +            // The area is discontiguous and we leave an empty slot before `rx`.
>> +            // PANIC:
>> +            // - The index `rx - 1` is non-negative because `rx != 0` in this branch.
>> +            // - The index does not exceed `before_tx.len()` (which equals `tx`) because
>> +            //   `rx <= tx` in this branch.
>> +            (after_tx, &mut before_tx[..(rx - 1)])
>
> If this is written with get_disjoint_mut, the indices would be so much easier to
> understand... To bad that that API is only available from 1.86 onwards.

I for one am longing for `split_at_checked`... :)