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charset=UTF-8 Date: Mon, 24 Aug 2026 21:50:57 +0900 Message-Id: Cc: , , , "dri-devel" Subject: Re: [PATCH v3 1/2] gpu: nova-core: fix barrier usage in CPU->GSP messaging path From: "Eliot Courtney" To: "Gary Guo" , "Danilo Krummrich" , "Alice Ryhl" , "Alexandre Courbot" , "David Airlie" , "Simona Vetter" X-Mailer: aerc 0.21.0-0-g5549850facc2 References: <20260819-rust-barrier-v3-0-d5b7bd7e6624@garyguo.net> <20260819-rust-barrier-v3-1-d5b7bd7e6624@garyguo.net> In-Reply-To: <20260819-rust-barrier-v3-1-d5b7bd7e6624@garyguo.net> X-ClientProxiedBy: TY4PR01CA0060.jpnprd01.prod.outlook.com (2603:1096:405:370::13) To DS0PR12MB6413.namprd12.prod.outlook.com (2603:10b6:8:ce::10) Precedence: bulk X-Mailing-List: nova-gpu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR12MB6413:EE_|DS0PR12MB6486:EE_ X-MS-Office365-Filtering-Correlation-Id: b9c5eaf6-94a0-4417-91f6-08df01de599e X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|376014|366016|1800799024|10070799003|10067099003|11063799006|4143699003|56012099006|6133799003|18002099003|22082099003; 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Control dependency exists here which provide the > needed ordering, but it's best to avoid depending on it. > > A STORE->STORE ordering is needed after the command write and before the > write pointer advance. This is currently incorrectly done after the write > pointer advance (and before GSP notification), but this can cause issue i= f > GSP is still processing ring buffer, as it may observe the write pointer > advance before command write. Thus move this barrier to be before the wri= te > pointer advance. Note that barriers are not needed between write pointer > advance and GSP notification, as MMIO accessors already carries the > required barrier. > > Signed-off-by: Gary Guo > --- > drivers/gpu/nova-core/gsp/cmdq.rs | 15 ++++++++++++--- > 1 file changed, 12 insertions(+), 3 deletions(-) > > diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gs= p/cmdq.rs > index 6da728201281..70674d2d0f77 100644 > --- a/drivers/gpu/nova-core/gsp/cmdq.rs > +++ b/drivers/gpu/nova-core/gsp/cmdq.rs > @@ -27,6 +27,11 @@ > ptr, > sync::{ > aref::ARef, > + barrier::{ > + dma_mb, > + Full, > + Write, // > + }, > Mutex, // > }, > time::Delta, > @@ -272,6 +277,10 @@ fn new(dev: &device::Device) -> Resul= t { > (rx - 1, 0) > }; > =20 > + // ORDERING: LOAD->STORE ordering needed to order `gsp_read_ptr`= read before data write. > + // Control dependency can serve the same purpose here, but we do= n't want to rely on it. > + dma_mb(Full); > + > // SAFETY: > // - `data` was created from a valid pointer, and `rx` and `tx` = are in the > // `0..MSGQ_NUM_PAGES` range per the invariants of `cpu_write_= ptr` and `gsp_read_ptr`, > @@ -450,9 +459,6 @@ fn advance_cpu_write_ptr(&mut self, elem_count: u32) = { > let tx =3D io_project!(self.0, .cpuq.tx); > let wptr =3D MsgqTxHeader::write_ptr(tx).wrapping_add(elem_count= ) % MSGQ_NUM_PAGES; > MsgqTxHeader::set_write_ptr(tx, wptr); > - > - // Ensure all command data is visible before triggering the GSP = read. > - fence(Ordering::SeqCst); > } > } > =20 > @@ -683,6 +689,9 @@ fn send_single_command(&mut self, bar: Bar0<'_>, c= ommand: M) -> Result > dst.header.length(), > ); > =20 > + // ORDERING: STORE->STORE ordering needed to order `cpu_write_pt= r` write after data write. > + dma_mb(Write); > + Is there a reason this can't go into `advance_cpu_write_ptr`? > // All set - update the write pointer and inform the GSP of the = new command. > let elem_count =3D dst.header.element_count(); > self.seq +=3D 1;