forked from cores/microwatt
dcache: Reduce latencies and improve timing
This implements various improvements to the dcache with the aim of making it go faster. - We can now execute operations that don't need to access main memory (cacheable loads that hit in the cache and TLB operations) as soon as any previous operation has completed, without waiting for the state machine to become idle. - Cache line refills start with the doubleword that is needed to satisfy the load that initiated them. - Cacheable loads that miss return their data and complete as soon as the requested doubleword comes back from memory; they don't wait for the refill to finish. - We now have per-doubleword valid bits for the cache line being refilled, meaning that if a load comes in for a line that is in the process of being refilled, we can return the data and complete it within a couple of cycles of the doubleword coming in from memory. - There is now a bypass path for data being written to the cache RAM so that we can do a store hit followed immediately by a load hit to the same doubleword. This also makes the data from a refill available to load hits one cycle earlier than it would be otherwise. - Stores complete in the cycle where their wishbone operation is initiated, without waiting for the wishbone cycle to complete. - During the wishbone cycle for a store, if another store comes in that is to the same page, and we don't have a stall from the wishbone, we can send out the write for the second store in the same wishbone cycle and without going through the IDLE state first. We limit it to 7 outstanding writes that have not yet been acknowledged. - The cache tag RAM is now read on a clock edge rather than being combinatorial for reading. Its width is rounded up to a multiple of 8 bits per way so that byte enables can be used for writing individual tags. - The cache tag RAM is now written a cycle later than previously, in order to ease timing. - Data for a store hit is now written one cycle later than previously. This eases timing since we don't have to get through the tag matching and on to the write enable within a single cycle. The 2-stage bypass path means we can still handle a load hit on either of the two cycles after the store and return the correct data. (A load hit 3 or more cycles later will get the correct data from the BRAM.) - Operations can sit in r0 while there is an uncompleted operation in r1. Once the operation in r1 is completed, the operation in r0 spends one cycle in r0 for TLB/cache tag lookup and then gets put into r1.req. This can happen before r1 gets to the IDLE state. Some operations can then be completed before r1 gets to the IDLE state - a load miss to the cache line being refilled, or a store to the same page as a previous store. Signed-off-by: Paul Mackerras <paulus@ozlabs.org>jtag-port
parent
65a36cc0fc
commit
b595963233