Re: [PATCH mlx5-next 3/4] net/mlx5: Use query_special_contexts for mkeys

From: Saeed Mahameed <saeed@kernel.org>
To: Leon Romanovsky <leon@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>,
	Or Har-Toov <ohartoov@nvidia.com>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>,
	linux-rdma@vger.kernel.org,
	Michael Guralnik <michaelgur@nvidia.com>,
	netdev@vger.kernel.org, Paolo Abeni <pabeni@redhat.com>,
	Saeed Mahameed <saeedm@nvidia.com>
Subject: Re: [PATCH mlx5-next 3/4] net/mlx5: Use query_special_contexts for mkeys
Date: Mon, 9 Jan 2023 14:31:32 -0800	[thread overview]
Message-ID: <Y7yVxD1eNYwx3VT0@x130> (raw)
In-Reply-To: <Y7qb1aeuGowalgzs@unreal>

On 08 Jan 12:32, Leon Romanovsky wrote:
>On Thu, Jan 05, 2023 at 04:58:22PM -0800, Saeed Mahameed wrote:
>> On 04 Jan 10:11, Leon Romanovsky wrote:
>> > From: Or Har-Toov <ohartoov@nvidia.com>
>> >
>> > Using query_sepcial_contexts in order to get the correct value of
>> > terminate_scatter_list_mkey, as FW will change it in some configurations.
>> > This is done one time when the device is loading and the value is being
>> > saved in the device context.
>> >
>> > Signed-off-by: Or Har-Toov <ohartoov@nvidia.com>
>> > Reviewed-by: Michael Guralnik <michaelgur@nvidia.com>
>> > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
>> > ---
>> > .../net/ethernet/mellanox/mlx5/core/en_main.c |  2 +-
>> > .../net/ethernet/mellanox/mlx5/core/main.c    | 27 +++++++++++++++++++
>> > include/linux/mlx5/driver.h                   |  1 +
>> > 3 files changed, 29 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>> > index c76f15505a76..33d7a7095988 100644
>> > --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>> > @@ -826,7 +826,7 @@ static int mlx5e_alloc_rq(struct mlx5e_params *params,
>> > 			if (rq->wqe.info.num_frags < (1 << rq->wqe.info.log_num_frags)) {
>> > 				wqe->data[f].byte_count = 0;
>> > 				wqe->data[f].lkey =
>> > -					MLX5_TERMINATE_SCATTER_LIST_LKEY;
>> > +					mdev->terminate_scatter_list_mkey;
>> > 				wqe->data[f].addr = 0;
>> > 			}
>> > 		}
>> > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/main.c b/drivers/net/ethernet/mellanox/mlx5/core/main.c
>> > index 7f5db13e3550..d39d758744a0 100644
>> > --- a/drivers/net/ethernet/mellanox/mlx5/core/main.c
>> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/main.c
>> > @@ -1221,6 +1221,28 @@ static int mlx5_function_teardown(struct mlx5_core_dev *dev, bool boot)
>> > 	return 0;
>> > }
>> >
>> > +static int mlx5_get_terminate_scatter_list_mkey(struct mlx5_core_dev *dev)
>> > +{
>> > +	u32 out[MLX5_ST_SZ_DW(query_special_contexts_out)] = {};
>> > +	u32 in[MLX5_ST_SZ_DW(query_special_contexts_in)] = {};
>> > +	int err;
>> > +
>> > +	MLX5_SET(query_special_contexts_in, in, opcode,
>> > +		 MLX5_CMD_OP_QUERY_SPECIAL_CONTEXTS);
>> > +	err = mlx5_cmd_exec_inout(dev, query_special_contexts, in, out);
>> > +	if (err)
>> > +		return err;
>> > +
>> > +	if (MLX5_CAP_GEN(dev, terminate_scatter_list_mkey)) {
>>
>> Why did you execute the command unconditionally if the output is only read
>> conditionally?
>>
>> early exit before executing the command, older FW might fail and driver will
>> be unusable ..
>
>According to the documentation, this functionality was forever, but you
>are presenting valid concern.
>
>>
>> > +		dev->terminate_scatter_list_mkey =
>> > +			cpu_to_be32(MLX5_GET(query_special_contexts_out, out,
>> > +					     terminate_scatter_list_mkey));
>> > +		return 0;
>> > +	}
>> > +	dev->terminate_scatter_list_mkey = MLX5_TERMINATE_SCATTER_LIST_LKEY;
>>
>> Another concern, what happens with old driver that is using the hardcoded
>> value with newer fw ? will it fail ? will it be accepted ?
>
>It will be accepted and everything will work without glitches.
>FW keeps backward compatibility and use MLX5_TERMINATE_SCATTER_LIST_LKEY
>internally as a default.
>

Then please don't change mlx5e to use  mdev->terminate_scatter_list_mkey in
this patch, just keep it as is and have a separate patch to change mlx5e.