Re: [PATCH RFC 1/3] spi: Allow SPI controller override device buswidth

Geert Uytterhoeven
2020-03-02T16:12:05Z

Hi John,

Thanks for your patch!

On Fri, Feb 28, 2020 at 4:23 PM John Garry <john.garry@huawei.com> wrote:
> Currently ACPI firmware description for a SPI device does not have any
> method to describe the data buswidth on the board.
>
> So even through the controller and device may support higher modes than
> standard SPI, it cannot be assumed that the board does - as such, that
> device is limited to standard SPI in such a circumstance.

Indeed.

> As a workaround, allow the controller driver supply buswidth override bits,
> which are used inform the core code that the controller driver knows the
> buswidth supported on that board for that device.

I feel this is a bit dangerous, and might bite us one day.

> A host controller driver might know this info from DMI tables, for example.

Can't acpi_register_spi_device() obtain that info from DMI tables,
to avoid contaminating the generic code?

> Signed-off-by: John Garry <john.garry@huawei.com>

> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -510,6 +510,7 @@ struct spi_device *spi_alloc_device(struct spi_controller *ctlr)
> spi->dev.bus = &spi_bus_type;
> spi->dev.release = spidev_release;
> spi->cs_gpio = -ENOENT;
> + spi->mode = ctlr->buswidth_override_bits;

This could just be moved to acpi_register_spi_device(), right?

>
> spin_lock_init(&spi->statistics.lock);
>
> @@ -2181,9 +2182,10 @@ static acpi_status acpi_register_spi_device(struct spi_controller *ctlr,
> return AE_NO_MEMORY;
> }
>
> +
> ACPI_COMPANION_SET(&spi->dev, adev);
> spi->max_speed_hz = lookup.max_speed_hz;
> - spi->mode = lookup.mode;
> + spi->mode |= lookup.mode;
> spi->irq = lookup.irq;
> spi->bits_per_word = lookup.bits_per_word;
> spi->chip_select = lookup.chip_select;
> diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> index 6d16ba01ff5a..600e3793303e 100644
> --- a/include/linux/spi/spi.h
> +++ b/include/linux/spi/spi.h
> @@ -481,6 +481,9 @@ struct spi_controller {
> /* spi_device.mode flags understood by this controller driver */
> u32 mode_bits;
>
> + /* spi_device.mode flags override flags for this controller */
> + u32 buswidth_override_bits;

And I'd be happy if we could avoid adding this here ;-)

> +
> /* bitmask of supported bits_per_word for transfers */
> u32 bits_per_word_mask;
> #define SPI_BPW_MASK(bits) BIT((bits) - 1)

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds