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sw: Fix snrt_dma_wait implementation (pulp-platform#153)
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The previous implementation of `snrt_dma_wait` was incorrect as it had an infinite loop in the case that `tid` was the last completed transaction. Looking at the documentation, the snippet in custom_instructions.md shows the correct way of implementing the transaction check (get last completed tid and loop if it's less than the one we are waiting for) except it had its register operands swapped.
This PR uses that implementation instead, fixes the small bug in the documentation and changes one of the waits in `dma_simple.c` to make sure we don't regress either.

---------

Co-authored-by: Luca Colagrande <[email protected]>
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zero9178 and colluca authored Jul 1, 2024
1 parent 44ca8c4 commit 17aa42a
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Showing 5 changed files with 32 additions and 7 deletions.
2 changes: 1 addition & 1 deletion docs/rm/custom_instructions.md
Original file line number Diff line number Diff line change
Expand Up @@ -104,7 +104,7 @@ The DMSTATI instruction can be used to implement a blocking wait for the complet

dmcpyi a0, ...
1: dmstati t0, 0
bltu a0, t0, 1b
bltu t0, a0, 1b

Similarly, waiting for the completion of *all* DMA transfers:

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7 changes: 3 additions & 4 deletions sw/snRuntime/src/dma.h
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Expand Up @@ -51,7 +51,7 @@ inline snrt_dma_txid_t snrt_dma_start_1d_wideptr(uint64_t dst, uint64_t src,

return reg_txid;
} else {
return -1;
return 0;
}
}

Expand Down Expand Up @@ -129,7 +129,7 @@ inline snrt_dma_txid_t snrt_dma_start_2d_wideptr(uint64_t dst, uint64_t src,

return reg_txid;
} else {
return -1;
return 0;
}
}

Expand All @@ -151,8 +151,7 @@ inline void snrt_dma_wait(snrt_dma_txid_t tid) {
( 0b000 << 12) | \
( (5) << 7) | \
(0b0101011 << 0) \n"
"sub t0, t0, %0 \n"
"blez t0, 1b \n" ::"r"(tid)
"bltu t0, %0, 1b \n" ::"r"(tid)
: "t0");
}

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25 changes: 25 additions & 0 deletions sw/tests/dma_empty_transfer.c
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@@ -0,0 +1,25 @@
// Copyright 2020 ETH Zurich and University of Bologna.
// Licensed under the Apache License, Version 2.0, see LICENSE for details.
// SPDX-License-Identifier: Apache-2.0

#include <snrt.h>

// Allocate a buffer in the main memory which we will use to copy data around
// with the DMA.
uint32_t buffer[32];

int main() {
if (snrt_global_core_idx() != 8) return 0; // only DMA core

// Populate source buffer.
uint32_t buffer_src[32];
for (uint32_t i = 0; i < 32; i++) {
buffer_src[i] = i + 1;
}

// Start a DMA transfer of zero size.
snrt_dma_txid_t id = snrt_dma_start_1d(buffer, buffer_src, 0);
snrt_dma_wait(id);

// Test is successful if the previous transfer doesn't block.
}
4 changes: 2 additions & 2 deletions sw/tests/dma_simple.c
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Expand Up @@ -21,8 +21,8 @@ int main() {
}

// Copy data to main memory.
snrt_dma_start_1d(buffer, buffer_src, sizeof(buffer));
snrt_dma_wait_all();
snrt_dma_txid_t id = snrt_dma_start_1d(buffer, buffer_src, sizeof(buffer));
snrt_dma_wait(id);

// Check that the main memory buffer contains the correct data.
for (uint32_t i = 0; i < 32; i++) {
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1 change: 1 addition & 0 deletions target/snitch_cluster/sw/run.yaml
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Expand Up @@ -9,6 +9,7 @@ runs:
simulators: [vsim, vcs, verilator] # banshee fails with exit code 0x4
- elf: tests/build/barrier.elf
- elf: tests/build/data_mover.elf
- elf: tests/build/dma_empty_transfer.elf
- elf: tests/build/dma_simple.elf
- elf: tests/build/event_unit.elf
- elf: tests/build/fence_i.elf
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