Re: [PATCH v4 2/9] gpu: nova-core: gsp: add mechanism to wait for space on command queue

[Alexandre Courbot]

On Tue Mar 3, 2026 at 12:07 PM JST, Eliot Courtney wrote:
> On Mon Mar 2, 2026 at 11:18 PM JST, Gary Guo wrote:
>> On Mon Mar 2, 2026 at 11:42 AM GMT, Eliot Courtney wrote:
>>> Add a timeout to `allocate_command` which waits for space on the GSP
>>> command queue. It uses a similar timeout to nouveau.
>>>
>>> This lets `send_command` wait for space to free up in the command queue.
>>> This is required to support continuation records which can fill up the
>>> queue.
>>
>> Any reason that this isn't implemented as a `poll_allocate_command` which just
>> returns a `EAGAIN`, and then a wrapper function that just waits for space to be
>> ready when it got one? This way the logic is cleaner.
>
> This kind of structure is what I originally did (see 
> https://lore.kernel.org/all/DGHRDFE9M6P7.L7JEOCLL3VS9@xxxxxxxxxx/)
> although it wasn't quite as nice since `allocate_command` takes
> &mut self and isn't callable from `read_poll_timeout`. Alex suggested
> merging these into one as it simplifies a few things. But maybe I am
> missing some better way to do it.

The rationale for this is that we don't have a case for polling for
command queue availability - the GSP is expected to consume all the data
sent to it rapidly, so we have a soft guarantee that the queue will be
ready soon. We also don't have a case for doing something else until the
queue is ready, AFAICT.

The current form is simpler code-wise, and if we ever need to break it
into two poll and wait methods, we can do it trivially when the need
arises.