Re: [RFC PATCH v2 3/3] SPI: Add virtio SPI driver (V10 draft specification).

[Haixu Cui]

On 1/4/2024 9:01 PM, Harald Mommer wrote:
> +static int virtio_spi_one_transfer(struct virtio_spi_req *spi_req,
> +				   struct spi_controller *ctrl,
> +				   struct spi_message *msg,
> +				   struct spi_transfer *xfer)
> +{
> +	struct virtio_spi_priv *priv = spi_controller_get_devdata(ctrl);
> +	struct device *dev = &priv->vdev->dev;
> +	struct spi_device *spi = msg->spi;
> +	struct spi_transfer_head *th;
> +	struct scatterlist sg_out_head, sg_out_payload;
> +	struct scatterlist sg_in_result, sg_in_payload;
> +	struct scatterlist *sgs[4];
> +	unsigned int outcnt = 0u;
> +	unsigned int incnt = 0u;
> +	int ret;
> +
> +	th = &spi_req->transfer_head;
> +
> +	/* Fill struct spi_transfer_head */
> +	th->chip_select_id = spi_get_chipselect(spi, 0);
> +	th->bits_per_word = spi->bits_per_word;
> +	/*
> +	 * Got comment: "The virtio spec for cs_change is*not*  what the Linux
> +	 * cs_change field does, this will not do the right thing."
> +	 * TODO: Understand/discuss this, still unclear what may be wrong here
> +	 */
> +	th->cs_change = xfer->cs_change;
> +	th->tx_nbits = xfer->tx_nbits;
> +	th->rx_nbits = xfer->rx_nbits;
> +	th->reserved[0] = 0;
> +	th->reserved[1] = 0;
> +	th->reserved[2] = 0;
> +
> +	BUILD_BUG_ON(VIRTIO_SPI_CPHA != SPI_CPHA);
> +	BUILD_BUG_ON(VIRTIO_SPI_CPOL != SPI_CPOL);
> +	BUILD_BUG_ON(VIRTIO_SPI_CS_HIGH != SPI_CS_HIGH);
> +	BUILD_BUG_ON(VIRTIO_SPI_MODE_LSB_FIRST != SPI_LSB_FIRST);
> +
> +	th->mode = cpu_to_le32(spi->mode & (SPI_LSB_FIRST | SPI_CS_HIGH |
> +					    SPI_CPOL | SPI_CPHA));
> +	if ((spi->mode & SPI_LOOP) != 0)
> +		th->mode |= cpu_to_le32(VIRTIO_SPI_MODE_LOOP);
> +
> +	th->freq = cpu_to_le32(xfer->speed_hz);
> +
> +	ret = spi_delay_to_ns(&xfer->word_delay, xfer);
> +	if (ret < 0) {
> +		dev_warn(dev, "Cannot convert word_delay\n");
> +		goto msg_done;
> +	}
> +	th->word_delay_ns = cpu_to_le32((u32)ret);
> +
> +	ret = spi_delay_to_ns(&xfer->delay, xfer);
> +	if (ret < 0) {
> +		dev_warn(dev, "Cannot convert delay\n");
> +		goto msg_done;
> +	}
> +	th->cs_setup_ns = cpu_to_le32((u32)ret);
> +	th->cs_delay_hold_ns = cpu_to_le32((u32)ret);
> +
> +	/* This is the "off" time when CS has to be deasserted for a moment */
> +	ret = spi_delay_to_ns(&xfer->cs_change_delay, xfer);
> +	if (ret < 0) {
> +		dev_warn(dev, "Cannot convert cs_change_delay\n");
> +		goto msg_done;
> +	}
> +	th->cs_change_delay_inactive_ns = cpu_to_le32((u32)ret);

Hi Harald,

    spi_device structure has three cs timing members, which will also 
affect the cs timing.

    struct spi_device {
        ...
        struct spi_delay        cs_setup;
        struct spi_delay        cs_hold;
        struct spi_delay        cs_inactive;
        ...
    }

    These three values should also be passed to the backend, for the 
controller to control cs lines.

    spi_device->cs_setup is the delay before cs asserted, 
spi_device->cs_hold with spi_transfer->delay work before cs deasserted, 
and spi_device->cs_inactive with spi_transfer->cs_change_delay take 
effect at the stage after cs deasserted.

Here is the diagram
      .   .      .    .    .   .   .   .   .   .
Delay + A +      + B  +    + C + D + E + F + A +
      .   .      .    .    .   .   .   .   .   .
   ___.   .      .    .    .   .   .___.___.   .
CS#   |___.______.____.____.___.___|   .   |___._____________
      .   .      .    .    .   .   .   .   .   .
      .   .      .    .    .   .   .   .   .   .
SCLK__.___.___NNN_____NNN__.___.___.___.___.___.___NNN_______


NOTE: 1st transfer has two words, the delay betweent these two words are 
'B' in the diagram.

A => struct spi_device -> cs_setup
B => max{struct spi_transfer -> word_delay,
         struct spi_device -> word_delay}
    Note: spi_device and spi_transfer both have word_delay, Linux
         choose the bigger one, refer to _spi_xfer_word_delay_update
         function
C => struct spi_transfer -> delay
D => struct spi_device -> cs_hold
E => struct spi_device -> cs_inactive
F => struct spi_transfer -> cs_change_delay

So the corresponding relationship:
A <===> cs_setup_ns (after CS asserted)
B <===> word_delay_ns (no matter with CS)
C+D <===> cs_delay_hold_ns (before CS deasserted)
E+F <===> cs_change_delay_inactive_ns (after CS deasserted, these two 
values also recommend in Linux driver to be added up)

Best Regards
Haixu


> +
> +	/* Set buffers */
> +	spi_req->tx_buf = xfer->tx_buf;
> +	spi_req->rx_buf = xfer->rx_buf;
> +
> +	/* Prepare sending of virtio message */
> +	init_completion(&spi_req->completion);
> +
> +	sg_init_one(&sg_out_head, &spi_req->transfer_head,
> +		    sizeof(struct spi_transfer_head));
> +	sgs[outcnt] = &sg_out_head;
> +	outcnt++;
> +
> +	if (spi_req->tx_buf) {
> +		sg_init_one(&sg_out_payload, spi_req->tx_buf, xfer->len);
> +		sgs[outcnt] = &sg_out_payload;
> +		outcnt++;
> +	}
> +
> +	if (spi_req->rx_buf) {
> +		sg_init_one(&sg_in_payload, spi_req->rx_buf, xfer->len);
> +		sgs[outcnt + incnt] = &sg_in_payload;
> +		incnt++;
> +	}
> +
> +	sg_init_one(&sg_in_result, &spi_req->result,
> +		    sizeof(struct spi_transfer_result));
> +	sgs[outcnt + incnt] = &sg_in_result;
> +	incnt++;
> +
> +	ret = virtqueue_add_sgs(priv->vq, sgs, outcnt, incnt, spi_req,
> +				GFP_KERNEL);
> +
> +msg_done:
> +	if (ret)
> +		msg->status = ret;
> +
> +	return ret;
> +}