Re: SPI Slave with handshaking

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hi Lubo,

On Tue, Aug 21, 2018 at 10:14 AM Lubomir Rintel <lkundrak@xxxxx> wrote:
> I'm trying to port the OPLC XO 1.75 Embedded Controler (EC) driver to the SPI
> slave framework. The EC is a SPI master that uses an extra pair of handshake
> signals to determine when is it safe to transmit. The original driver [1]
> included the SPI slave driver.
>
> [1] http://dev.laptop.org/git/olpc-kernel/tree/drivers/misc/olpc-ec-1-75.c?h=arm-3.0-wip
>
> To send the command to the EC, the protocol requires me to [2] signal to the
> master that the tx FIFO is filled and it can start the transaction. The problem
> is that I have no idea when that is. I'm wondering what my options are?

The SPI slave output FIFO has been filled after the SPI slave controller's
spi_controller.transfer_one() function has written all data to the controller's
FIFO.  Hence this is SPI controller specific.

After filling the FIFO, the SPI slave controller should assert the ACK line.

> [2] http://wiki.laptop.org/go/XO_1.75_HOST_to_EC_Protocol
>
> 1.) Add a ready() callback to the spi_message, that would fire after the tx
>     FIFO has been filled?
>
> 2.) Delay the ACK strobe a bit after spi_async() and just hope for the best
>     (Eek)
>
> 3.) Have the transfer with NULL rx_buf complete just after sending the data to
>     the controller, not waiting for actual transfer. And then strobe the ACK.
>
> 4.) Keep the SPI slave code in the EC driver. I suppose this wouldn't be
>     acceptable?
>
> Better ideas, opinions & advice appreciated!

Is this on a DT-based platform?
If yes, the GPIO (I assume it's a GPIO) used for the ACK signal will be
provided by DT, and the SPI slave controller driver can assert it from its
transfer_one() (or .transfer_one_message()) function after filling the FIFO.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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