linux-kernel - Re: [PATCH v2 1/4] spi: Add 'last' flag to spi

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Message-ID: <20141118140658.GI22111@sirena.org.uk>
Date:	Tue, 18 Nov 2014 14:06:58 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Beniamino Galvani <b.galvani@...il.com>
Cc:	linux-spi@...r.kernel.org, Carlo Caione <carlo@...one.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	devicetree@...r.kernel.org, Russell King <linux@....linux.org.uk>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Jerry Cao <jerry.cao@...ogic.com>,
	Victor Wan <victor.wan@...ogic.com>
Subject: Re: [PATCH v2 1/4] spi: Add 'last' flag to spi_transfer structure

On Mon, Nov 17, 2014 at 11:47:48PM +0100, Beniamino Galvani wrote:

> +	list_for_each_entry(xfer, &master->cur_msg->transfers, transfer_list) {
> +		xfer->last = list_is_last(&xfer->transfer_list,
> +					  &master->cur_msg->transfers);
> +	}

It's incredibly sad to iterate through the entire list in order to find
the last entry, especially given that it's a doubly linked list and this
is a bit of a hot path.  We should look at the previous entry for the
list head instead, or perhaps better yet by doing this as part of
spi_validate() which already itereates over the entire list and is where
we do other similar fixups.

Though looking at this I'm not sure that a flag is the best approach at
all - why not just have the driver call list_is_last() in the transfer
function or ideally provide an inline function that does that so that we
can change the implementation later?

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