[RFC PATCH 00/13] spi: dw: Add support for XIP mode

[Sean Anderson]

On 2/4/21 11:39 PM, Sean Anderson wrote:
> 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".

Forgot to mention, but this series depends on [1]. A git branch with
this complete series is available at [2].

[1] https://patchwork.ozlabs.org/project/uboot/list/?series=228123
[2] https://github.com/Forty-Bot/u-boot/tree/maix_xip

> 
> 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(-)
>