Re: [RFC: DMA_PMD 19/22] net/mlx5e: Use DMA_PMD memory for tx header bounce buffers

[Dragos Tatulea <dtatulea@nvidia.com>]

On 14.10.26 19:22, Dragos Tatulea wrote:
> 
> 
> On 03.10.26 23:22, Luigi Rizzo wrote:
>> When dev->dma_pmd_tx_hdrs is enabled, allocate a MAX_TCP_HEADER
>> bounce buffer per TX DMA FIFO slot out of the DMA_PMD arena. When the
>> linear portion of a transmitted packet is at most MAX_TCP_HEADER bytes,
>> copy it into the pre-mapped bounce buffer instead of calling
>> dma_map_single()/dma_unmap_single() on every packet.
>>
> What is the difference between using the DMA_PMD pool or one (or a few)
> coherent DMA (2MB) buffers allocated by the driver?
> 
The patch was actually using the coherent buffer. My bad, sorry.

>> Signed-off-by: Luigi Rizzo <lrizzo@google.com>
>> ---
>>  drivers/net/ethernet/mellanox/mlx5/core/en.h  |  2 ++
>>  .../net/ethernet/mellanox/mlx5/core/en/txrx.h |  4 ++-
>>  .../net/ethernet/mellanox/mlx5/core/en_main.c | 14 ++++++++++
>>  .../net/ethernet/mellanox/mlx5/core/en_tx.c   | 28 +++++++++++++++----
>>  4 files changed, 42 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en.h b/drivers/net/ethernet/mellanox/mlx5/core/en.h
>> index a3945195f454e..d762ad1c1ffaf 100644
>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en.h
>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en.h
>> @@ -442,6 +442,8 @@ struct mlx5e_txqsq {
>>  		struct mlx5e_sq_dma       *dma_fifo;
>>  		struct mlx5e_skb_fifo      skb_fifo;
>>  		struct mlx5e_tx_wqe_info  *wqe_info;
>> +		void                      *tx_hdr_bufs;
>> +		dma_addr_t                 tx_hdr_bufs_dma;
> Why are only headers considered for bouncing? Because they have the
> highest mapping overhead?

I meant to say headers and small packets.

> 
> Note that TSO/GSO packets will copy the header inline in the WQE
> already. But not for small packets.
> 
>>  	} db;
>>  	void __iomem              *uar_map;
>>  	struct netdev_queue       *txq;
>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h b/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
>> index f2a8453d8dce0..6de7a2a430d72 100644
>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
>> @@ -365,7 +365,9 @@ mlx5e_tx_dma_unmap(struct device *pdev, struct mlx5e_sq_dma *dma)
>>  {
>>  	switch (dma->type) {
>>  	case MLX5E_DMA_MAP_SINGLE:
>> -		dma_unmap_single(pdev, dma->addr, dma->size, DMA_TO_DEVICE);
>> +		if (dma->size)
>> +			dma_unmap_single(pdev, dma->addr, dma->size,
>> +					 DMA_TO_DEVICE);
>>  		break;
>>  	case MLX5E_DMA_MAP_PAGE:
>>  		netmem_dma_unmap_page_attrs(pdev, dma->addr, dma->size,
>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>> index 4c6060d54fbed..efd766aa43a02 100644
>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>> @@ -1666,6 +1666,13 @@ static void mlx5e_free_icosq(struct mlx5e_icosq *sq)
>>  
>>  void mlx5e_free_txqsq_db(struct mlx5e_txqsq *sq)
>>  {
>> +	if (sq->db.tx_hdr_bufs) {
>> +		int wq_sz = mlx5_wq_cyc_get_size(&sq->wq);
>> +		int df_sz = wq_sz * MLX5_SEND_WQEBB_NUM_DS;
>> +
>> +		dma_free_coherent(sq->pdev, df_sz * MAX_TCP_HEADER,
>> +				  sq->db.tx_hdr_bufs, sq->db.tx_hdr_bufs_dma);
>> +	}
>>  	kvfree(sq->db.wqe_info);
>>  	kvfree(sq->db.skb_fifo.fifo);
>>  	kvfree(sq->db.dma_fifo);
>> @@ -1690,6 +1697,13 @@ int mlx5e_alloc_txqsq_db(struct mlx5e_txqsq *sq, int numa)
>>  		return -ENOMEM;
>>  	}
>>  
>> +	if (sq->pdev->dma_pmd_tx_hdrs)
>> +		sq->db.tx_hdr_bufs =
>> +			dma_alloc_coherent(sq->pdev,
>> +					   df_sz * MAX_TCP_HEADER,
>> +					   &sq->db.tx_hdr_bufs_dma,
>> +					   GFP_KERNEL | __GFP_NOWARN);
>> +
>>  	sq->dma_fifo_mask = df_sz - 1;
>>  
>>  	sq->db.skb_fifo.pc   = &sq->skb_fifo_pc;
>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
>> index 0b5e600e4a6a9..3b6a7a8b0128e 100644
>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
>> @@ -177,6 +177,27 @@ mlx5e_tx_get_gso_ihs(struct mlx5e_txqsq *sq, struct sk_buff *skb)
>>  	return ihs;
>>  }
>>  
>> +static inline dma_addr_t
>> +mlx5e_tx_map_hdr(struct mlx5e_txqsq *sq, unsigned char *data, u32 len)
>> +{
>> +	dma_addr_t dma_addr;
>> +
>> +	if (sq->db.tx_hdr_bufs && len <= MAX_TCP_HEADER) {
>> +		u32 off = (sq->dma_fifo_pc & sq->dma_fifo_mask) *
>> +			  MAX_TCP_HEADER;
>> +
>> +		memcpy(sq->db.tx_hdr_bufs + off, data, len);
>> +		dma_addr = sq->db.tx_hdr_bufs_dma + off;
>> +		mlx5e_dma_push_single(sq, dma_addr, 0);
>> +		return dma_addr;
>> +	}
>> +
>> +	dma_addr = dma_map_single(sq->pdev, data, len, DMA_TO_DEVICE);
>> +	if (likely(!dma_mapping_error(sq->pdev, dma_addr)))
>> +		mlx5e_dma_push_single(sq, dma_addr, len);
>> +	return dma_addr;
>> +}
>> +
> This looks like a form of tx-copybreak.
> 
> Thanks,
> Dragos