linux-kernel - Re: [PATCH] spi: spi-bcm2835.c: Fix 3-wire mode

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Date:   Mon, 1 Jul 2019 13:55:06 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     "Sa, Nuno" <Nuno.Sa@...log.com>
Cc:     "wahrenst@....net" <wahrenst@....net>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "bcm-kernel-feedback-list@...adcom.com" 
        <bcm-kernel-feedback-list@...adcom.com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "kernel@...tin.sperl.org" <kernel@...tin.sperl.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "sbranden@...adcom.com" <sbranden@...adcom.com>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "rjui@...adcom.com" <rjui@...adcom.com>,
        "linux-rpi-kernel@...ts.infradead.org" 
        <linux-rpi-kernel@...ts.infradead.org>,
        "eric@...olt.net" <eric@...olt.net>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] spi: spi-bcm2835.c: Fix 3-wire mode

On Mon, Jul 01, 2019 at 07:24:23AM +0000, Sa, Nuno wrote:
> The problem is in spi_map_msg() which
> allocates dummy buffers (because of the ctrl flags). As a result, in
> bcm2835_spi_transfer_one() we set "cs |= BCM2835_SPI_CS_REN;" when we
> want to do tx only. I believe this was the actual problem...

I see.  In that case, try:

 	/* handle all the 3-wire mode */
-	if ((spi->mode & SPI_3WIRE) && (tfr->rx_buf))
+	if ((spi->mode & SPI_3WIRE) && (tfr->rx_buf != ctlr->dummy_rx))
 		cs |= BCM2835_SPI_CS_REN;
 	else
 		cs &= ~BCM2835_SPI_CS_REN;

Use master->dummy_rx instead of ctlr->dummy_rx prior to commit
5f336ea53b6b ("spi: bcm2835: Replace spi_master by spi_controller").

This could be something that users of your cape might use on older
kernels in lieu of my upcoming patch set to drop MUST_RX / MUST_TX.

Thanks,

Lukas