Re: [PATCH 3/8] spi: dw: Add support for Pensando Elba SoC SPI

[Brad Larson <brad@pensando.io>]

On Thu, Mar 4, 2021 at 12:48 AM Linus Walleij <linus.walleij@linaro.org> wrote:

> On Thu, Mar 4, 2021 at 4:42 AM Brad Larson <brad@pensando.io> wrote:
>
> > 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 <brad@pensando.io>
>
> 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