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

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Message-Id: <DFY8BWTRMOZN.2EBDVAOIJE4P0@nvidia.com>
Date: Mon, 26 Jan 2026 13:17:47 +0900
From: "Eliot Courtney" <ecourtney@...dia.com>
To: "Gary Guo" <gary@...yguo.net>, "Eliot Courtney" <ecourtney@...dia.com>,
 "Danilo Krummrich" <dakr@...nel.org>, "Alexandre Courbot"
 <acourbot@...dia.com>, "Alice Ryhl" <aliceryhl@...gle.com>, "David Airlie"
 <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>, "Alistair Popple"
 <apopple@...dia.com>
Cc: <nouveau@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>,
 <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
 "dri-devel" <dri-devel-bounces@...ts.freedesktop.org>
Subject: Re: [PATCH 3/4] gpu: nova-core: gsp: fix improper handling of empty
 slot in cmdq

On Sat Jan 24, 2026 at 3:31 AM JST, Gary Guo wrote:
> On Thu Jan 22, 2026 at 2:59 AM 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.
>
> Doesn't this mean that you're effectively wasting a slot? You can easily
> implementing a ring buffer that allows you to disambiguate empty/full while
> still using all slots.
>
> A common approach is to only do modulo/masking operation before accessing the
> slot. Then `write_ptr.wrapping_sub(read_ptr)` will give you the accurate length of
> things inside the ring buffer.

Yes, we are wasting a slot (the current code also does this). But,
this is the protocol between the CPU and GSP, so the GSP actually
expects this and we can't change the semantics easily, IIUC.