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Date:   Mon, 25 May 2020 12:31:06 +0100
From:   Mark Brown <broonie@kernel.org>
To:     Marc Kleine-Budde <mkl@pengutronix.de>
Cc:     Tim Harvey <tharvey@gateworks.com>,
        open list <linux-kernel@vger.kernel.org>,
        linux-can@vger.kernel.org, Wolfgang Grandegger <wg@grandegger.com>,
        Timo =?iso-8859-1?B?U2NobPzfbGVy?= <schluessler@krause.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        linux-spi@vger.kernel.org, Jan Glauber <jglauber@marvell.com>,
        Robert Richter <rrichter@marvell.com>
Subject: Re: [PATCH] can: mcp251x: convert to half-duplex SPI
Message-ID: <20200525113106.GB4544@sirena.org.uk>
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On Mon, May 25, 2020 at 01:17:01PM +0200, Marc Kleine-Budde wrote:
> On 5/21/20 10:19 PM, Tim Harvey wrote:

>=20
> > Should I be submitting this patch with logic that only does
> > half-duplex if the spi controller doesn't support it (if
> > (spi->controller->flags & SPI_CONTROLLER_HALF_DUPLEX)) or is it
> > acceptable to simply make the driver half-duplex like this for all
> > cases?

> Please make half duplex transfers depending on SPI_CONTROLLER_HALF_DUPLEX=
 as
> most drivers have a considerable overhead at the end of a transfer.

> Most of them wait for a transfer complete interrupt. Which might take lon=
ger
> than the actual SPI transfer. Splitting one full duplex read-register tra=
nsfer
> (which is a write followed by a read) into two half duplex transfers woul=
d kill
> performance on full duplex capable controllers.

This isn't something that every individual driver should be doing, such
rewriting should happen in the core so that everything sees the benefit.

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