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SubjectRE: [PATCH] [v2] mlx5: stop warning for 64KB pages
Date
From: Arnd Bergmann
> Sent: 15 October 2021 16:21
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> When building with 64KB pages, clang points out that xsk->chunk_size
> can never be PAGE_SIZE:
>
> drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c:19:22: error: result of comparison of constant
> 65536 with expression of type 'u16' (aka 'unsigned short') is always false [-Werror,-Wtautological-
> constant-out-of-range-compare]
> if (xsk->chunk_size > PAGE_SIZE ||
> ~~~~~~~~~~~~~~~ ^ ~~~~~~~~~
>
> In older versions of this code, using PAGE_SIZE was the only
> possibility, so this would have never worked on 64KB page kernels,
> but the patch apparently did not address this case completely.
>
> As Maxim Mikityanskiy suggested, 64KB chunks are really not all that
> useful, so just shut up the warning by adding a cast.
>
> Fixes: 282c0c798f8e ("net/mlx5e: Allow XSK frames smaller than a page")
> Link: https://lore.kernel.org/netdev/20211013150232.2942146-1-arnd@kernel.org/
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c
> b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c
> index 538bc2419bd8..228257010f32 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c
> @@ -15,8 +15,10 @@ bool mlx5e_validate_xsk_param(struct mlx5e_params *params,
> struct mlx5e_xsk_param *xsk,
> struct mlx5_core_dev *mdev)
> {
> - /* AF_XDP doesn't support frames larger than PAGE_SIZE. */
> - if (xsk->chunk_size > PAGE_SIZE ||
> + /* AF_XDP doesn't support frames larger than PAGE_SIZE,
> + * and xsk->chunk_size is limited to 65535 bytes.
> + */
> + if ((size_t)xsk->chunk_size > PAGE_SIZE ||
> xsk->chunk_size < MLX5E_MIN_XSK_CHUNK_SIZE)
> return false;

How much smaller does the kernel get if you change 'chunk_size' from
_u16 to 'unsigned int'. ?
Especially for a non-x86 build?
Or is it a hardware constrained size??

David

-
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