linux-kernel - Re: [PATCH] mlx5: avoid 64-bit division

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Message-ID: <20190527181534.GA10029@ziepe.ca>
Date:   Mon, 27 May 2019 15:15:34 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Michal Kubecek <mkubecek@...e.cz>
Cc:     Leon Romanovsky <leon@...nel.org>,
        Doug Ledford <dledford@...hat.com>,
        Ariel Levkovich <lariel@...lanox.com>,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mlx5: avoid 64-bit division

On Mon, May 20, 2019 at 01:19:02PM +0200, Michal Kubecek wrote:
> Commit 25c13324d03d ("IB/mlx5: Add steering SW ICM device memory type")
> breaks i386 build by introducing three 64-bit divisions. As the divisor
> is MLX5_SW_ICM_BLOCK_SIZE() which is always a power of 2, we can replace
> the division with bit operations.
> 
> Fixes: 25c13324d03d ("IB/mlx5: Add steering SW ICM device memory type")
> Signed-off-by: Michal Kubecek <mkubecek@...e.cz>
>  drivers/infiniband/hw/mlx5/cmd.c  | 9 +++++++--
>  drivers/infiniband/hw/mlx5/main.c | 2 +-
>  2 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/infiniband/hw/mlx5/cmd.c b/drivers/infiniband/hw/mlx5/cmd.c
> index e3ec79b8f7f5..6c8645033102 100644
> +++ b/drivers/infiniband/hw/mlx5/cmd.c
> @@ -190,12 +190,12 @@ int mlx5_cmd_alloc_sw_icm(struct mlx5_dm *dm, int type, u64 length,
>  			  u16 uid, phys_addr_t *addr, u32 *obj_id)
>  {
>  	struct mlx5_core_dev *dev = dm->dev;
> -	u32 num_blocks = DIV_ROUND_UP(length, MLX5_SW_ICM_BLOCK_SIZE(dev));
>  	u32 out[MLX5_ST_SZ_DW(general_obj_out_cmd_hdr)] = {};
>  	u32 in[MLX5_ST_SZ_DW(create_sw_icm_in)] = {};
>  	unsigned long *block_map;
>  	u64 icm_start_addr;
>  	u32 log_icm_size;
> +	u32 num_blocks;
>  	u32 max_blocks;
>  	u64 block_idx;
>  	void *sw_icm;
> @@ -224,6 +224,8 @@ int mlx5_cmd_alloc_sw_icm(struct mlx5_dm *dm, int type, u64 length,
>  		return -EINVAL;
>  	}
>  
> +	num_blocks = (length + MLX5_SW_ICM_BLOCK_SIZE(dev) - 1) >>
> +		     MLX5_LOG_SW_ICM_BLOCK_SIZE(dev);
>  	max_blocks = BIT(log_icm_size - MLX5_LOG_SW_ICM_BLOCK_SIZE(dev));
>  	spin_lock(&dm->lock);
>  	block_idx = bitmap_find_next_zero_area(block_map,
> @@ -266,13 +268,16 @@ int mlx5_cmd_dealloc_sw_icm(struct mlx5_dm *dm, int type, u64 length,
>  			    u16 uid, phys_addr_t addr, u32 obj_id)
>  {
>  	struct mlx5_core_dev *dev = dm->dev;
> -	u32 num_blocks = DIV_ROUND_UP(length, MLX5_SW_ICM_BLOCK_SIZE(dev));
>  	u32 out[MLX5_ST_SZ_DW(general_obj_out_cmd_hdr)] = {};
>  	u32 in[MLX5_ST_SZ_DW(general_obj_in_cmd_hdr)] = {};
>  	unsigned long *block_map;
> +	u32 num_blocks;
>  	u64 start_idx;
>  	int err;
>  
> +	num_blocks = (length + MLX5_SW_ICM_BLOCK_SIZE(dev) - 1) >>
> +		     MLX5_LOG_SW_ICM_BLOCK_SIZE(dev);
> +
>  	switch (type) {
>  	case MLX5_IB_UAPI_DM_TYPE_STEERING_SW_ICM:
>  		start_idx =
> diff --git a/drivers/infiniband/hw/mlx5/main.c b/drivers/infiniband/hw/mlx5/main.c
> index abac70ad5c7c..340290b883fe 100644
> +++ b/drivers/infiniband/hw/mlx5/main.c
> @@ -2344,7 +2344,7 @@ static int handle_alloc_dm_sw_icm(struct ib_ucontext *ctx,
>  	/* Allocation size must a multiple of the basic block size
>  	 * and a power of 2.
>  	 */
> -	act_size = roundup(attr->length, MLX5_SW_ICM_BLOCK_SIZE(dm_db->dev));
> +	act_size = round_up(attr->length, MLX5_SW_ICM_BLOCK_SIZE(dm_db->dev));
>  	act_size = roundup_pow_of_two(act_size);

It is kind of weird that we have round_up and the bitshift
version.. None of this is performance critical so why not just use
round_up everywhere?

Ariel, it is true MLX5_SW_ICM_BLOCK_SIZE will always be a power of
two?

Jason