[PATCH] [v2] mlx5: stop warning for 64KB pages

[PATCH] [v2] mlx5: stop warning for 64KB pages

[Arnd Bergmann]

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;
 
-- 
2.29.2

RE: [PATCH] [v2] mlx5: stop warning for 64KB pages

[David Laight]

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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