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