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A LOAD->STORE ordering is needed after reading the read pointer from GSP and writing the command. 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 if 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 write 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 Reviewed-by: Eliot Courtney --- drivers/gpu/nova-core/gsp/cmdq.rs | 18 ++++++++++++++---- 1 file changed, 14 insertions(+), 4 deletions(-) diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/= cmdq.rs index 6da728201281..bfd61e678802 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, @@ -413,7 +418,12 @@ fn gsp_write_ptr(&self) -> u32 { // // - The returned value is within `0..MSGQ_NUM_PAGES`. fn gsp_read_ptr(&self) -> u32 { - MsgqRxHeader::read_ptr(io_project!(self.0, .gspq.rx)) % MSGQ_NUM_P= AGES + let ptr =3D MsgqRxHeader::read_ptr(io_project!(self.0, .gspq.rx)) = % MSGQ_NUM_PAGES; + + // ORDERING: LOAD->STORE ordering needed to order `gsp_read_ptr` r= ead before data write. + dma_mb(Full); + + ptr } =20 // Returns the index of the memory page the CPU can read the next mess= age from. @@ -447,12 +457,12 @@ fn cpu_write_ptr(&self) -> u32 { =20 // Informs the GSP that it can process `elem_count` new pages from the= command queue. fn advance_cpu_write_ptr(&mut self, elem_count: u32) { + // ORDERING: STORE->STORE ordering needed to order `cpu_write_ptr`= write after data write. + dma_mb(Write); 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Tue, 1 Sep 2026 20:59:43 +0000 From: Gary Guo Date: Tue, 01 Sep 2026 21:59:33 +0100 Subject: [PATCH v4 2/2] gpu: nova-core: fix barrier usage in GSP->CPU messaging path Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20260901-rust-barrier-v4-2-94427f445310@garyguo.net> References: <20260901-rust-barrier-v4-0-94427f445310@garyguo.net> In-Reply-To: <20260901-rust-barrier-v4-0-94427f445310@garyguo.net> To: Danilo Krummrich , Alice Ryhl , Alexandre Courbot , David Airlie , Simona Vetter Cc: nova-gpu@lists.linux.dev, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, Gary Guo X-Mailer: b4 0.15.2 X-Developer-Signature: v=1; a=ed25519-sha256; t=1788296382; l=2907; i=gary@garyguo.net; s=20221204; h=from:subject:message-id; bh=ZMyXNHTAh6vKIRfPgjk9v7wcxMKcn6bAbxX3mtXG9sw=; b=Eftfe7lKKOwB26dLdPpfDlXLYd/lTK11AjqJaIxj3iTTrMRc3jrIqDp2E92+MPYkZFnwG6rIx hHojum+v0S9D1v79t0OZ1eAy99rVPXLcVFqQ9qZhiHy1kywErN8ek78 X-Developer-Key: i=gary@garyguo.net; 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A LOAD->LOAD ordering is required after the write pointer read and the data read. Add it as this is currently missing. A LOAD->STORE ordering is required after the data read and the advance of read pointer. Currently a Rust `SeqCst` barrier is used, which roughly maps to `smp_mb(Full)`; this however does not order DMA operations (notably on ARM, the generate barrier orders inner shareable and not outer shareable, which is ordered by `dma_mb`). This ordering does not need to be in between read pointer read and write, because it's for ordering between the ring buffer data and the pointer; the RMW operation does not internally need a barrier (nor it has to be atomic, as CPU pointers are updated by CPU only), so move it to before the RMW sequence for clarity. Signed-off-by: Gary Guo Reviewed-by: Eliot Courtney --- drivers/gpu/nova-core/gsp/cmdq.rs | 23 +++++++++++------------ 1 file changed, 11 insertions(+), 12 deletions(-) diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/= cmdq.rs index bfd61e678802..14a711307654 100644 --- a/drivers/gpu/nova-core/gsp/cmdq.rs +++ b/drivers/gpu/nova-core/gsp/cmdq.rs @@ -2,13 +2,7 @@ =20 mod continuation; =20 -use core::{ - mem, - sync::atomic::{ - fence, - Ordering, // - }, -}; +use core::mem; =20 use kernel::{ device, @@ -30,6 +24,7 @@ barrier::{ dma_mb, Full, + Read, Write, // }, Mutex, // @@ -409,7 +404,12 @@ fn allocate_command(&mut self, size: usize, timeout: D= elta) -> Result u32 { - MsgqTxHeader::write_ptr(io_project!(self.0, .gspq.tx)) % MSGQ_NUM_= PAGES + let ptr =3D MsgqTxHeader::write_ptr(io_project!(self.0, .gspq.tx))= % MSGQ_NUM_PAGES; + + // ORDERING: LOAD->LOAD ordering needed to order `gsp_write_ptr` r= ead before data read. + dma_mb(Read); + + ptr } =20 // Returns the index of the memory page the GSP will read the next com= mand from. @@ -437,12 +437,11 @@ fn cpu_read_ptr(&self) -> u32 { =20 // Informs the GSP that it can send `elem_count` new pages into the me= ssage queue. fn advance_cpu_read_ptr(&mut self, elem_count: u32) { + // ORDERING: LOAD->STORE ordering needed to order `cpu_read_ptr` w= rite after data read. + dma_mb(Full); + let rx =3D io_project!(self.0, .cpuq.rx); let rptr =3D MsgqRxHeader::read_ptr(rx).wrapping_add(elem_count) %= MSGQ_NUM_PAGES; - - // Ensure read pointer is properly ordered. - fence(Ordering::SeqCst); - MsgqRxHeader::set_read_ptr(rx, rptr) } =20 --=20 2.54.0