On Mon, Mar 27, 2023 at 8:15 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
From: Arnd Bergmann <arnd@xxxxxxxx>
For a DMA_BIDIRECTIONAL transfer, the caches have to be cleaned
first to let the device see data written by the CPU, and invalidated
after the transfer to let the CPU see data written by the device.
riscv also invalidates the caches before the transfer, which does
not appear to serve any purpose.
Yes, we can't guarantee the CPU pre-load cache lines randomly during
dma working.
But I've two purposes to keep invalidates before dma transfer:
- We clearly tell the CPU these cache lines are invalid. The caching
algorithm would use these invalid slots first instead of replacing
valid ones.
- Invalidating is very cheap. Actually, flush and clean have the same
performance in our machine.
So, how about:
diff --git a/arch/riscv/mm/dma-noncoherent.c b/arch/riscv/mm/dma-noncoherent.c
index d919efab6eba..2c52fbc15064 100644
--- a/arch/riscv/mm/dma-noncoherent.c
+++ b/arch/riscv/mm/dma-noncoherent.c
@@ -22,8 +22,6 @@ void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
ALT_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);
break;
case DMA_FROM_DEVICE:
- ALT_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);
- break;
case DMA_BIDIRECTIONAL:
ALT_CMO_OP(flush, vaddr, size, riscv_cbom_block_size);
break;
@@ -42,7 +40,7 @@ void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size,
break;
case DMA_FROM_DEVICE:
case DMA_BIDIRECTIONAL:
/* I'm not sure all drivers have guaranteed cacheline
alignment. If not, this inval would cause problems */
- ALT_CMO_OP(flush, vaddr, size, riscv_cbom_block_size);
+ ALT_CMO_OP(inval, vaddr, size, riscv_cbom_block_size);
break;
default:
break;
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
arch/riscv/mm/dma-noncoherent.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/riscv/mm/dma-noncoherent.c b/arch/riscv/mm/dma-noncoherent.c
index 640f4c496d26..69c80b2155a1 100644
--- a/arch/riscv/mm/dma-noncoherent.c
+++ b/arch/riscv/mm/dma-noncoherent.c
@@ -25,7 +25,7 @@ void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
ALT_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);
break;
case DMA_BIDIRECTIONAL:
- ALT_CMO_OP(flush, vaddr, size, riscv_cbom_block_size);
+ ALT_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);
break;
default:
break;
--
2.39.2
--
Best Regards
Guo Ren
