This adds support for memory-mapped ("DIRMAP") reads (called XIP by the datasheet). In theory, these have better performance than regular reads. In practice, the CPU is already fast enough to max out the performance of this peripheral.
The real end-goal is to be able to boot a XIP kernel. This SoC has just barely enough SRAM to boot Linux, and using a XIP kernel would decrease the memory pressure to something tolerable. It would also probably absolutely *tank* the performance, since not only is SPI flash ~8x slower than regular memory, there is no cache to mitigate that latency. So it remains to be seen whether this is worth it at all.
This series is RFC because it currently reads all 1s on one board I have. On the other board I have, it reads and writes fine. I will try and investigate this, but I expect any changes to occur in "spi: dw: Add support for DIRMAP".
Sean Anderson (13): linux err: Synchronize with Linux 5.10 spi-mem: Add dirmap API from Linux mtd: spi-nor: use spi-mem dirmap API core: ofnode: Fix inconsistent returns of *_read_u32_array mux: Inline mux functions when CONFIG_MUX is disabled mux: Define a stub for mux_get_by_index if CONFIG_MUX is disabled mux: mmio: Only complain about idle-states if it is malformed spi: dw: Define XIP registers spi: dw: Add XIP and XIP_CONCURRENT caps spi: dw: Use a mux to access registers spi: dw: Add support for DIRMAP riscv: k210: Increase SPI3 bus clock to CPU speed riscv: k210: Add bindings for SPI XIP
arch/riscv/dts/k210.dtsi | 25 ++- drivers/core/ofnode.c | 15 +- drivers/mtd/spi/sf_probe.c | 79 ++++++++ drivers/mtd/spi/spi-nor-core.c | 45 +++-- drivers/mux/mmio.c | 5 +- drivers/spi/Kconfig | 10 + drivers/spi/designware_spi.c | 337 +++++++++++++++++++++++++++++++-- drivers/spi/spi-mem.c | 270 ++++++++++++++++++++++++++ include/linux/err.h | 21 +- include/linux/mtd/spi-nor.h | 6 + include/mux.h | 25 ++- include/spi-mem.h | 99 ++++++++++ 12 files changed, 889 insertions(+), 48 deletions(-)