From 4293d5f528b5a0a1b0c1b0c6eb522366822a965a Mon Sep 17 00:00:00 2001 From: Mitch Williams Date: Tue, 8 Nov 2016 13:05:14 -0800 Subject: i40e: simplify txd use count calculation The i40e_txd_use_count function was fast but confusing. In the comments, it even admits that it's ugly. So replace it with a new function that is (very) slightly faster and has extensive commenting to help the thicker among us (including the author, who will forget in a week) understand how it works. Change-ID: Ifb533f13786a0bf39cb29f77969a5be2c83d9a87 Signed-off-by: Mitch Williams Signed-off-by: Alexander Duyck Tested-by: Andrew Bowers Signed-off-by: Jeff Kirsher --- drivers/net/ethernet/intel/i40e/i40e_txrx.h | 45 ++++++++++++++++++----------- 1 file changed, 28 insertions(+), 17 deletions(-) (limited to 'drivers/net/ethernet/intel/i40e/i40e_txrx.h') diff --git a/drivers/net/ethernet/intel/i40e/i40e_txrx.h b/drivers/net/ethernet/intel/i40e/i40e_txrx.h index de8550f4e3a4..e065321ce8ed 100644 --- a/drivers/net/ethernet/intel/i40e/i40e_txrx.h +++ b/drivers/net/ethernet/intel/i40e/i40e_txrx.h @@ -173,26 +173,37 @@ static inline bool i40e_test_staterr(union i40e_rx_desc *rx_desc, #define I40E_MAX_DATA_PER_TXD_ALIGNED \ (I40E_MAX_DATA_PER_TXD & ~(I40E_MAX_READ_REQ_SIZE - 1)) -/* This ugly bit of math is equivalent to DIV_ROUNDUP(size, X) where X is - * the value I40E_MAX_DATA_PER_TXD_ALIGNED. It is needed due to the fact - * that 12K is not a power of 2 and division is expensive. It is used to - * approximate the number of descriptors used per linear buffer. Note - * that this will overestimate in some cases as it doesn't account for the - * fact that we will add up to 4K - 1 in aligning the 12K buffer, however - * the error should not impact things much as large buffers usually mean - * we will use fewer descriptors then there are frags in an skb. +/** + * i40e_txd_use_count - estimate the number of descriptors needed for Tx + * @size: transmit request size in bytes + * + * Due to hardware alignment restrictions (4K alignment), we need to + * assume that we can have no more than 12K of data per descriptor, even + * though each descriptor can take up to 16K - 1 bytes of aligned memory. + * Thus, we need to divide by 12K. But division is slow! Instead, + * we decompose the operation into shifts and one relatively cheap + * multiply operation. + * + * To divide by 12K, we first divide by 4K, then divide by 3: + * To divide by 4K, shift right by 12 bits + * To divide by 3, multiply by 85, then divide by 256 + * (Divide by 256 is done by shifting right by 8 bits) + * Finally, we add one to round up. Because 256 isn't an exact multiple of + * 3, we'll underestimate near each multiple of 12K. This is actually more + * accurate as we have 4K - 1 of wiggle room that we can fit into the last + * segment. For our purposes this is accurate out to 1M which is orders of + * magnitude greater than our largest possible GSO size. + * + * This would then be implemented as: + * return (((size >> 12) * 85) >> 8) + 1; + * + * Since multiplication and division are commutative, we can reorder + * operations into: + * return ((size * 85) >> 20) + 1; */ static inline unsigned int i40e_txd_use_count(unsigned int size) { - const unsigned int max = I40E_MAX_DATA_PER_TXD_ALIGNED; - const unsigned int reciprocal = ((1ull << 32) - 1 + (max / 2)) / max; - unsigned int adjust = ~(u32)0; - - /* if we rounded up on the reciprocal pull down the adjustment */ - if ((max * reciprocal) > adjust) - adjust = ~(u32)(reciprocal - 1); - - return (u32)((((u64)size * reciprocal) + adjust) >> 32); + return ((size * 85) >> 20) + 1; } /* Tx Descriptors needed, worst case */ -- cgit v1.2.3-59-g8ed1b