linux-kernel - Re: [PATCH] can: mcp251x: convert to half-duplex SPI

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Message-ID: <20200525113106.GB4544@sirena.org.uk>
Date:   Mon, 25 May 2020 12:31:06 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     Tim Harvey <tharvey@...eworks.com>,
        open list <linux-kernel@...r.kernel.org>,
        linux-can@...r.kernel.org, Wolfgang Grandegger <wg@...ndegger.com>,
        Timo Schlüßler <schluessler@...use.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-spi@...r.kernel.org, Jan Glauber <jglauber@...vell.com>,
        Robert Richter <rrichter@...vell.com>
Subject: Re: [PATCH] can: mcp251x: convert to half-duplex SPI

On Mon, May 25, 2020 at 01:17:01PM +0200, Marc Kleine-Budde wrote:
> On 5/21/20 10:19 PM, Tim Harvey wrote:

> 
> > 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 longer
> than the actual SPI transfer. Splitting one full duplex read-register transfer
> (which is a write followed by a read) into two half duplex transfers would 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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