On Thu, Mar 4, 2021 at 12:48 AM Linus Walleij [off-list ref] wrote:

On Thu, Mar 4, 2021 at 4:42 AM Brad Larson [off-list ref] wrote:

quoted

The Pensando Elba SoC uses a GPIO based chip select
for two DW SPI busses with each bus having two
chip selects.

Signed-off-by: Brad Larson <redacted>

I agree with Serge's comments here: the existing cs callback should
work for your SoC, you should only need the new compatible string.

I see why you need the special GPIO driver for this now, as that
is obviously driven by totally different hardware.

Yours,
Linus Walleij

Thanks Serge and Linus for the review.

In the SPI driver, the reason we need our own set_cs function is that
our DW SPI controller only supports intrinsic 2 chip-select pins.

This is the standard DW set_cs function:

void dw_spi_set_cs(struct spi_device *spi, bool enable)
{
        struct dw_spi *dws = spi_controller_get_devdata(spi->controller);
        bool cs_high = !!(spi->mode & SPI_CS_HIGH);

        /*
         * DW SPI controller demands any native CS being set in order to
         * proceed with data transfer. So in order to activate the SPI
         * communications we must set a corresponding bit in the Slave
         * Enable register no matter whether the SPI core is configured to
         * support active-high or active-low CS level.
         */
        if (cs_high == enable)
                dw_writel(dws, DW_SPI_SER, BIT(spi->chip_select));
        else
                dw_writel(dws, DW_SPI_SER, 0);
}

The dw_writel function argument DW_SPI_SER, BIT(spi->chip_select)
works for chip-select 0 & 1, but not for 2 & 3, as the IP only
implements bits [1:0] in the DW_SPI_SER register.  In the Elba SoC we
require GPIO-style chip-selects, our own set_cs function, and we
always use bit 0 of DW_SPI_SER to start the serial machine, not as a
chip-select control.  In the dw_spi_set_cs() function the below else
clause is never taken and leads to confusion.

             } else {
                        /*
                         * Using the intrinsic DW chip-select; set the
                         * appropriate CS.
                         */
                        dw_writel(dws, DW_SPI_SER, BIT(spi->chip_select));
                }

This else clause will be removed in patch set V2.  I tried the generic
dw_spi_set_cs() thinking it would just start the serial machine while
the Elba spics drives the gpio chip select, that didn't work.  I will
take another look at it as I work on V2 of the patchset to see exactly
what breaks during spi init.

Best,
Brad