linux-kernel - Re: [PATCH 2/5] spi: move splitting transfers to spi_optimize

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Date: Tue, 13 Feb 2024 17:35:45 +0000
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: David Lechner <dlechner@...libre.com>
CC: Mark Brown <broonie@...nel.org>, Martin Sperl <kernel@...tin.sperl.org>,
	David Jander <david@...tonic.nl>, Jonathan Cameron <jic23@...nel.org>,
	Michael Hennerich <michael.hennerich@...log.com>, Nuno Sá <nuno.sa@...log.com>, Alain Volmat
	<alain.volmat@...s.st.com>, Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>, <linux-spi@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-stm32@...md-mailman.stormreply.com>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-iio@...r.kernel.org>
Subject: Re: [PATCH 2/5] spi: move splitting transfers to
 spi_optimize_message()

On Mon, 12 Feb 2024 17:26:42 -0600
David Lechner <dlechner@...libre.com> wrote:

> Splitting transfers is an expensive operation so we can potentially
> optimize it by doing it only once per optimization of the message
> instead of repeating each time the message is transferred.
> 
> The transfer splitting functions are currently the only user of
> spi_res_alloc() so spi_res_release() can be safely moved at this time
> from spi_finalize_current_message() to spi_unoptimize_message().
> 
> The doc comments of the public functions for splitting transfers are
> also updated so that callers will know when it is safe to call them
> to ensure proper resource management.
> 
> Signed-off-by: David Lechner <dlechner@...libre.com>
> ---
Trivial thing (which applies equally to the original code).
Otherwise LGTM.
FWIW
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>

> +/**
> + * spi_split_transfers - generic handling of transfer splitting
> + * @msg: the message to split
> + *
> + * Under certain conditions, a SPI controller may not support arbitrary
> + * transfer sizes or other features required by a peripheral. This function
> + * will split the transfers in the message into smaller transfers that are
> + * supported by the controller.
> + *
> + * Controllers with special requirements not covered here can also split
> + * transfers in the optimize_message() callback.
> + *
> + * Context: can sleep
> + * Return: zero on success, else a negative error code
> + */
> +static int spi_split_transfers(struct spi_message *msg)
> +{
> +	struct spi_controller *ctlr = msg->spi->controller;
> +	struct spi_transfer *xfer;
> +	int ret;
> +
> +	/*
> +	 * If an SPI controller does not support toggling the CS line on each
> +	 * transfer (indicated by the SPI_CS_WORD flag) or we are using a GPIO
> +	 * for the CS line, we can emulate the CS-per-word hardware function by
> +	 * splitting transfers into one-word transfers and ensuring that
> +	 * cs_change is set for each transfer.
> +	 */
> +	if ((msg->spi->mode & SPI_CS_WORD) && (!(ctlr->mode_bits & SPI_CS_WORD) ||
> +					       spi_is_csgpiod(msg->spi))) {
	if ((msg->spi->mode & SPI_CS_WORD) &&
	    (!(ctlr->mode_bits & SPI_CS_WORD) || spi_is_csgpiod(msg->spi))) {

Seems easier to read to me. I appreciate you are just moving it though so
don't mind that much if you leave it in the original form.



> +		ret = spi_split_transfers_maxwords(ctlr, msg, 1);
> +		if (ret)
> +			return ret;
> +
> +		list_for_each_entry(xfer, &msg->transfers, transfer_list) {
> +			/* Don't change cs_change on the last entry in the list */
> +			if (list_is_last(&xfer->transfer_list, &msg->transfers))
> +				break;
> +
> +			xfer->cs_change = 1;
> +		}
> +	} else {
> +		ret = spi_split_transfers_maxsize(ctlr, msg,
> +						  spi_max_transfer_size(msg->spi));
> +		if (ret)
> +			return ret;
> +	}
> +
> +	return 0;
> +}