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sfc: Reduce RX scatter buffer size, and reduce alignment if appropriate
efx_start_datapath() asserts that we can fit 2 RX scatter buffers plus a software structure, each appropriately aligned, into a single page. Where L1_CACHE_BYTES == 256 and PAGE_SIZE == 4096, which is the case on s390, this assertion fails. The current scatter buffer size is also not a multiple of 64 or 128, which are more common cache line sizes. If we can make both the start and end of a scatter buffer cache-aligned, this will reduce the need for read-modify-write operations on inter- processor links. Fix the alignment by reducing EFX_RX_USR_BUF_SIZE to 2048 - 256 == 1792. (We could use 2048 - L1_CACHE_BYTES, but EFX_RX_USR_BUF_SIZE also affects user-level networking where a larger amount of housekeeping data may be needed. Although this version of the driver does not support user-level networking, I prefer to keep scattering behaviour consistent with the out-of-tree version.) This still doesn't fix the s390 build because like most architectures it has NET_IP_ALIGN == 2. When NET_IP_ALIGN != 0 we cannot achieve cache line alignment at either the start or end of a scatter buffer, so there is actually no point in padding the buffers to a multiple of the cache line size. All we need is 4-byte alignment of the network header, so do that. Adjust the assertions accordingly. Reported-by:Geert Uytterhoeven <geert@linux-m68k.org> Reported-by: