Re: [PATCHv1 2/3] spi: Add spi driver for Socionext Synquacer platform

[Trent Piepho <tpiepho-cgc2CodaaHDQT0dZR+AlfA@public.gmane.org>]

On Tue, 2018-01-09 at 18:40 +0530, jassisinghbrar@gmail.com wrote:
> From: Jassi Brar <jaswinder.singh@linaro.org>
> 
> This patch adds support for controller found on synquacer platforms.
> 
> Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
> 
> +static int synquacer_spi_config(struct spi_master *master,
> +				  struct spi_device *spi,
> +				  struct spi_transfer *xfer)
> +{
> +	struct synquacer_spi *sspi = spi_master_get_devdata(master);
> +	unsigned int speed, mode, bpw, cs, bus_width;
> +	unsigned long rate, transf_mode;
> +	u32 val, div;
> +
> +	/* Full Duplex only on 1bit wide bus */
> +	if (xfer->rx_buf && xfer->tx_buf &&
> +			(xfer->rx_nbits != 1 || xfer->tx_nbits != 1)) {
> +		dev_err(sspi->dev, "RX and TX bus widths must match!\n");

This looks like the wrong error message is being printed.  It does not
match the comment nor the code.

> +	speed = xfer->speed_hz ? : spi->max_speed_hz;
> +	bpw = xfer->bits_per_word ? : spi->bits_per_word;

You shouldn't need to do this checking. __spi_validate() should have
already set these fields in xfer to their defaults if they weren't set,
checked xfer speed is less than controller max speed, checked bus width
is supported, etc.

> +	/* return if nothing to change */
> +	if (speed == sspi->speed &&
> +			bus_width == sspi->bus_width && bpw == sspi->bpw &&
> +			mode == sspi->mode && cs == sspi->cs &&
> +			transf_mode == sspi->old_transf_mode) {
> +		return 0;
> +	}

Good optimization to not reprogram on every xfer, since usually speed,
bits, etc. stays the same for a series of xfers.  However, often SPI
clients generate a number write, read pairs.  In this case transf_mode
will alternate and the optimization will fail to work, even through it
is very likely that nothing (especially bus speed, which can be slow to
change) else changed.

Consider allowing changing between read and write without unnecessarily
reprogramming everything else.

> +
> +	if (xfer->tx_buf)
> +		sspi->old_transf_mode = TXBIT;
> +	else
> +		sspi->old_transf_mode = RXBIT;
Why not just:

sspi->old_transf_mode = transf_mode;

Also, why is this "old_" when all the other ones, sspi->speed, etc.,
don't have "old_" in front.  Seems inconsistent unless there is
something special about transf_mode I have missed.

> 
> +
> +	if (xfer->tx_buf && xfer->rx_buf)
> +		val |= (DATA_TXRX << DATA_SHIFT);
> +	else if (xfer->rx_buf)
> +		val |= (DATA_RX << DATA_SHIFT);
> +	else
> +		val |= (DATA_TX << DATA_SHIFT);

Looks like one needs to set three different modes for bi, rx, and tx. 
But your transf_mode variable only has two settings, rx and tx.  It
won't detect change between tx and bi.

> 
> +
> +	val = readl_relaxed(sspi->regs + FIFOCFG);
> +	val |= RX_FLUSH;
> +	val |= TX_FLUSH;
> +	writel_relaxed(val, sspi->regs + FIFOCFG);

If this causes the master to pulse the chipselect it will mess up
messages that have more than one xfer.

> +
> +	/* See if we can transfer 4-bytes as 1 word even if not asked */
> +	bpw = xfer->bits_per_word ? : spi->bits_per_word;

Don't neeed this as xfer->bits_per_word is already normalized.

> +	if (bpw == 8 && !(xfer->len % 4) && !(spi->mode & SPI_LSB_FIRST)) {
> +		bpw = xfer->bits_per_word;
> +		xfer->bits_per_word = 32;
> +	} else {
> +		bpw = xfer->bits_per_word;
> +	}

What's the point of the bpw = xfer->bits_per_word above?  It would
already be set to that value.

> +
> +	ret = synquacer_spi_config(master, spi, xfer);
> +	if (ret) {
> +		xfer->bits_per_word = bpw;

Why not restore xfer->bits_per_word immediately after call to
synquacer_spi_config, before if statement.  It's not used again.  Then
you don't have to restore it also at the end of the function.

> 
> +
> +	spin_lock_irqsave(&sspi->lock, flags);

What's this lock for?  I see no other use than here.

> +	master->min_speed_hz = master->max_speed_hz / 254;

The configure function rejects divisors larger than 127.  Seems like
these should match.

I notice your transfer function does not use interrupts.  It will spin
in a tight loop polling DMSTATUS waiting for data to arrive.  Not very
friendly for a multitasking operating system.  It would be much better
if there was a way wait for in an interrupt at the start of your while
loop and have the controller trigger one when the tx and/or rx fifo has
 some amount of data in it (like half full).

You'll likely also find that in an interruptable kernel that your
transfer function will eventually stop running and another process will
run.  Multitasking.  When this happens, it might be many milliseconds
before it is scheduled to run again.  Hope that fifo is large...

Usually when a device has a fifo that must be emptied before it
overflows, it is either emptied from an interrupt handler or the
interrupt handler will queue a work function or tasklet that will empty
it.  Doing it from user context has a maximum latency that is too high.