Re: [PATCH v1] ntb_transport: replace wmb() with dma_wmb() in TX paths

[Dave Jiang]

On 4/24/26 2:23 AM, Xingbang Liu wrote:
> The TX paths currently use wmb() after memcpy_toio() to ensure that
> writes to the peer memory window are visible before subsequent
> notification.
> 
> The actual ordering requirement is to ensure that data written to the
> peer memory is observed before the associated notification or status
> update. This matches the semantics of dma_wmb(), which orders memory
> writes with respect to external observers.
> 
> The peer memory window is mapped with ioremap_wc(), allowing
> write-combining. On weakly ordered architectures such as arm64,
> dma_wmb() ensures that prior writes are committed from write-combining
> buffers and become visible to the peer before subsequent operations.
> 
> This aligns with common patterns in PCIe-based drivers, where it is
> used to order descriptor or payload writes before notification, while
> avoiding the stronger ordering semantics of wmb().
> 
> The ordering with respect to peer observation is completed by the
> subsequent notification mechanism.
> 
> No functional change is intended.
> 
> Signed-off-by: Xingbang Liu <liu.airalert@xxxxxxxxx>

NAK. dma_wmb() is used for shared memory between CPU and a device. For example:

memcpy(dma_buffer, data, size);
dma_wmb(); /* Ensure CPU writes visible to device DMA */
writel(DOORBELL, device->reg + DB_OFFSET); /* trigger device DMA from dma_buffer */

In this case we are copying data to MMIO and therefore wmb() is needed to ensure all MMIO writes have happened before we proceed.

DJ

> ---
>  drivers/ntb/ntb_transport.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
> index 7cabc82305d6..c8ef46ddcc57 100644
> --- a/drivers/ntb/ntb_transport.c
> +++ b/drivers/ntb/ntb_transport.c
> @@ -1791,7 +1791,7 @@ static void ntb_memcpy_tx_on_stack(struct ntb_queue_entry *entry, void __iomem *
>  #endif
>  
>  	/* Ensure that the data is fully copied out before setting the flags */
> -	wmb();
> +	dma_wmb();
>  
>  	ntb_tx_copy_callback(entry, NULL);
>  }