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From:   Colin Foster <colin.foster@in-advantage.com>
To:     linux-kernel@vger.kernel.org
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mark Brown <broonie@kernel.org>
Subject: [RFC v1 0/2] Add more detailed regmap formatting capabilities
Date:   Sun, 13 Mar 2022 15:45:22 -0700
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The Ocelot chips (specifically the VSC7512 I'm using) have a method of
accessing their registers internally via MMIO, or externally via SPI.
When accessing these registers externally, a 24-bit address is used and
downshifted by two. The manual references it as:

SI_ADDR = (REG_ADDR & 0x00FFFFFF) >> 2;

By adding this configurable downshift, and a configurable register base
address, the regmap definitions can be shared between MMIO and SPI
configurations.

This also allows regmap to be used in a bus configuration. My initial
testing shows that even at a much slower bus speed of 500KHz, I'm seeing
an improvement of 10ms (was 14... now 4) to perform bulk read operations.

The relevant MMIO code can be found in drivers/net/mscc/ocelot_io.c:

static struct regmap_config ocelot_regmap_config = {
        .reg_bits       = 32,
        .val_bits       = 32,
        .reg_stride     = 4,
};

struct regmap *ocelot_regmap_init(struct ocelot *ocelot, struct resource *res)
{
        void __iomem *regs;

        regs = devm_ioremap_resource(ocelot->dev, res);
        if (IS_ERR(regs))
                return ERR_CAST(regs);

        ocelot_regmap_config.name = res->name;

        return devm_regmap_init_mmio(ocelot->dev, regs, &ocelot_regmap_config);
}


And the SPI counterpart is slightly more complex:


static const struct regmap_config ocelot_spi_regmap_config = {
        .reg_bits = 24,
        .reg_stride = 4,
        .reg_downshift = 2,
        .val_bits = 32,

        .write_flag_mask = 0x80,

        .max_register = 0xffffffff,
        .use_single_write = true,
        .can_multi_write = false,

        .reg_format_endian = REGMAP_ENDIAN_BIG,
        .val_format_endian = REGMAP_ENDIAN_NATIVE,
};

static const struct regmap_bus ocelot_spi_regmap_bus = {
       .write = ocelot_spi_regmap_bus_write,
       .read = ocelot_spi_regmap_bus_read,
};

struct regmap *
ocelot_spi_devm_get_regmap(struct ocelot_core *core, struct device *child,
                           const struct resource *res)
{
        struct regmap_config regmap_config;

        memcpy(&regmap_config, &ocelot_spi_regmap_config,
               sizeof(ocelot_spi_regmap_config));

        regmap_config.name = res->name;
        regmap_config.max_register = res->end - res->start;
        regmap_config.reg_base = res->start;

        return devm_regmap_init(child, &ocelot_spi_regmap_bus, core,
                                &regmap_config);
}


If there's anything I missed, or if there's a different way to go about
this, please let me know. I can also drag this along with my VSC7512
development or I can send this patch (or whatever it might become) after
the merge window.


Colin Foster (2):
  regmap: add configurable downshift for addresses
  regmap: allow a defined reg_base to be added to every address

 drivers/base/regmap/internal.h |  2 ++
 drivers/base/regmap/regmap.c   | 11 +++++++++++
 include/linux/regmap.h         |  6 ++++++
 3 files changed, 19 insertions(+)

-- 
2.25.1