net/mlx5e: Simplify stride size calculation for linear RQ

net/mlx5e: Simplify stride size calculation for linear RQ

Linear RX buffers must be big enough to fit the MTU-sized packet along
with the headroom. On the other hand, they must be small enough to fit
into a page (or into an XSK frame). A straightforward way to check
whether the linear mode is possible would be comparing the required
buffer size to PAGE_SIZE or XSK frame size.

Stride size in the linear mode is defined by the following constraints:

1. A stride is at least as big as the buffer size, and it's a power of
two.

2. If non-XSK XDP is enabled, the stride size is PAGE_SIZE, because
mlx5e requires each packet to be in its own page when XDP is in use. The
previous constraint is automatically fulfilled, because buffer size
can't be bigger than PAGE_SIZE.

3. XSK uses stride size equal to PAGE_SIZE, but the following commits
will allow it to use roundup_pow_of_two(XSK frame size), by allowing the
NIC's MMU to use page sizes not equal to the CPU page size.

This commit puts the above requirements and constraints straight to the
code in an attempt to simplify it and to prepare it for changes made in
the next patches.

For the reference, the old code uses an equivalent, but trickier
calculation (high-level simplified pseudocode):

    if XDP or XSK:
        mlx5e_rx_get_linear_frag_sz := max(buffer size, PAGE_SIZE)
    else:
        mlx5e_rx_get_linear_frag_sz := buffer size
    mlx5e_rx_is_linear_skb := mlx5e_rx_get_linear_frag_sz <= PAGE_SIZE
    stride size := roundup_pow_of_two(mlx5e_rx_get_linear_frag_sz)

The new code effectively removes mlx5e_rx_get_linear_frag_sz that used
to return either buffer size or stride size, depending on the situation,
making it hard to work with and to make changes:

    if XDP or XSK:
        mlx5e_rx_get_linear_stride_sz := PAGE_SIZE
    else
        mlx5e_rx_get_linear_stride_sz := roundup_pow_of_two(buffer size)
    mlx5e_rx_is_linear_skb := buffer size <= (PAGE_SIZE or XSK frame sz)
    stride size := mlx5e_rx_get_linear_stride_sz

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>