Hello, I have a question regarding the SPI spi_write_then_read function.

2011/6/20 Hubert Feurstein <h.feurstein@gmail.com>

> This commit adds the following spi transfer functions:
>  - spi_write
>  - spi_read
>  - spi_write_then_read
>  - spi_w8r8
>
> The code has been ported from the linux kernel.
>
> Signed-off-by: Hubert Feurstein <h.feurstein@gmail.com>
> ---
>  drivers/spi/spi.c |   41 +++++++++++++++++++++++++++++
>  include/spi/spi.h |   74
> +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 115 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index 588c263..6456897 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -195,3 +195,44 @@ int spi_sync(struct spi_device *spi, struct
> spi_message *message)
>        return spi->master->transfer(spi, message);
>  }
>
> +/**
> + * spi_write_then_read - SPI synchronous write followed by read
> + * @spi: device with which data will be exchanged
> + * @txbuf: data to be written
> + * @n_tx: size of txbuf, in bytes
> + * @rxbuf: buffer into which data will be read
> + * @n_rx: size of rxbuf, in bytes
> + * Context: can sleep
> + *
> + * This performs a half duplex MicroWire style transaction with the
> + * device, sending txbuf and then reading rxbuf.  The return value
> + * is zero for success, else a negative errno status code.
> + * This call may only be used from a context that may sleep.
> + */
> +int spi_write_then_read(struct spi_device *spi,
> +               const void *txbuf, unsigned n_tx,
>

Could you explain me why const is used here ?


> +               void *rxbuf, unsigned n_rx)
> +{
> +       int                     status;
> +       struct spi_message      message;
> +       struct spi_transfer     x[2];
> +
> +       spi_message_init(&message);
> +       memset(x, 0, sizeof x);
>

Here we set everything to zero, including rx/tx buffer pointers.



> +       if (n_tx) {
> +               x[0].len = n_tx;
> +               spi_message_add_tail(&x[0], &message);
> +       }
> +       if (n_rx) {
> +               x[1].len = n_rx;
> +               spi_message_add_tail(&x[1], &message);
> +       }
> +
> +       x[0].tx_buf = txbuf;
> +       x[1].rx_buf = rxbuf;
>

We still have x[0].rx_buf = 0 and x[1].tx_buf = 0


> +
> +       /* do the i/o */
> +       status = spi_sync(spi, &message);
>

Then we call spi_sync then master->transfer. In this last function we have:

list_for_each_entry (t, &mesg->transfers, transfer_list) {

const u32 *txbuf = t->tx_buf;  <--- point to zero when x[1] right ?????
u32 *rxbuf = t->rx_buf;        <--- point to zero when x[0] right ?????

int i = 0;

while(i < t->len >> 2) {
rxbuf[i] = imx->xchg_single(imx, txbuf[i]);
i++;
}
}

What am I missing ???



> +       return status;
> +}
> +EXPORT_SYMBOL(spi_write_then_read);
> diff --git a/include/spi/spi.h b/include/spi/spi.h
> index c76f3b7..ac2013a 100644
> --- a/include/spi/spi.h
> +++ b/include/spi/spi.h
> @@ -349,6 +349,80 @@ static inline int spi_register_board_info(struct
> spi_board_info const *info,
>  }
>  #endif
>
> +/**
> + * spi_write - SPI synchronous write
> + * @spi: device to which data will be written
> + * @buf: data buffer
> + * @len: data buffer size
> + * Context: can sleep
> + *
> + * This writes the buffer and returns zero or a negative error code.
> + * Callable only from contexts that can sleep.
> + */
> +static inline int
> +spi_write(struct spi_device *spi, const void *buf, size_t len)
> +{
> +       struct spi_transfer     t = {
> +                       .tx_buf         = buf,
> +                       .len            = len,
> +               };
> +       struct spi_message      m;
> +
> +       spi_message_init(&m);
> +       spi_message_add_tail(&t, &m);
> +       return spi_sync(spi, &m);
> +}
> +
> +/**
> + * spi_read - SPI synchronous read
> + * @spi: device from which data will be read
> + * @buf: data buffer
> + * @len: data buffer size
> + * Context: can sleep
> + *
> + * This reads the buffer and returns zero or a negative error code.
> + * Callable only from contexts that can sleep.
> + */
> +static inline int
> +spi_read(struct spi_device *spi, void *buf, size_t len)
> +{
> +       struct spi_transfer     t = {
> +                       .rx_buf         = buf,
> +                       .len            = len,
> +               };
> +       struct spi_message      m;
> +
> +       spi_message_init(&m);
> +       spi_message_add_tail(&t, &m);
> +       return spi_sync(spi, &m);
> +}
> +
> +/* this copies txbuf and rxbuf data; for small transfers only! */
> +extern int spi_write_then_read(struct spi_device *spi,
> +               const void *txbuf, unsigned n_tx,
> +               void *rxbuf, unsigned n_rx);
> +
> +/**
> + * spi_w8r8 - SPI synchronous 8 bit write followed by 8 bit read
> + * @spi: device with which data will be exchanged
> + * @cmd: command to be written before data is read back
> + * Context: can sleep
> + *
> + * This returns the (unsigned) eight bit number returned by the
> + * device, or else a negative error code.  Callable only from
> + * contexts that can sleep.
> + */
> +static inline ssize_t spi_w8r8(struct spi_device *spi, u8 cmd)
> +{
> +       ssize_t                 status;
> +       u8                      result;
> +
> +       status = spi_write_then_read(spi, &cmd, 1, &result, 1);
> +
> +       /* return negative errno or unsigned value */
> +       return (status < 0) ? status : result;
> +}
> +
>  #endif /* DOXYGEN_SHOULD_SKIP_THIS */
>
>  #endif /* __INCLUDE_SPI_H */
> --
> 1.7.4.1
>
>
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Franck.