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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 --- 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/gsp/= 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) -> Result<= Self> { (rx - 1, 0) }; =20 + // ORDERING: LOAD->STORE ordering needed to order `gsp_read_ptr` r= ead before data write. + // Control dependency can serve the same purpose here, but we don'= t want to rely on it. + dma_mb(Full); + // SAFETY: // - `data` was created from a valid pointer, and `rx` and `tx` ar= e in the // `0..MSGQ_NUM_PAGES` range per the invariants of `cpu_write_pt= r` 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 re= ad. - fence(Ordering::SeqCst); } } =20 @@ -683,6 +689,9 @@ fn send_single_command(&mut self, bar: Bar0<'_>, com= mand: M) -> Result dst.header.length(), ); =20 + // ORDERING: STORE->STORE ordering needed to order `cpu_write_ptr`= write after data write. + dma_mb(Write); + // All set - update the write pointer and inform the GSP of the ne= w command. let elem_count =3D dst.header.element_count(); self.seq +=3D 1; --=20 2.54.0 From nobody Mon Sep 28 17:49:01 2026 Received: from CWXP265CU008.outbound.protection.outlook.com (mail-ukwestazon11020118.outbound.protection.outlook.com [52.101.195.118]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6CFAC385D7C for ; Wed, 19 Aug 2026 17:28:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; 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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 | 19 ++++++++----------- 1 file changed, 8 insertions(+), 11 deletions(-) diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/= cmdq.rs index 70674d2d0f77..9fe393da6b10 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, // @@ -339,6 +334,9 @@ fn driver_write_area_size(&self) -> usize { (MSGQ_NUM_PAGES, tx) }; =20 + // ORDERING: LOAD->LOAD ordering needed to order `gsp_write_ptr` r= ead before data read. + dma_mb(Read); + // SAFETY: // - `data` was created from a valid pointer, and `rx` and `tx` ar= e in the // `0..MSGQ_NUM_PAGES` range per the invariants of `gsp_write_pt= r` and `cpu_read_ptr`, @@ -436,12 +434,11 @@ 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