FAILED: patch "[PATCH] spi: bcm2835: Fix 3-wire mode if DMA is enabled" failed to apply to 5.2-stable tree

The patch below does not apply to the 5.2-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 8d8bef50365847134b51c1ec46786bc2873e4e47 Mon Sep 17 00:00:00 2001
From: Lukas Wunner <[email protected]>
Date: Wed, 3 Jul 2019 12:29:31 +0200
Subject: [PATCH] spi: bcm2835: Fix 3-wire mode if DMA is enabled
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Commit 6935224da248 ("spi: bcm2835: enable support of 3-wire mode") added 3-wire support to the BCM2835 SPI driver by setting the REN bit (Read Enable) in the CS register when receiving data. The REN bit puts the transmitter in high-impedance state. The driver recognizes that data is to be received by checking whether the rx_buf of a transfer is non-NULL.

Commit 3ecd37edaa2a ("spi: bcm2835: enable dma modes for transfers meeting certain conditions") subsequently broke 3-wire support because it set the SPI_MASTER_MUST_RX flag which causes spi_map_msg() to replace rx_buf with a dummy buffer if it is NULL. As a result, rx_buf is *always* non-NULL if DMA is enabled.

Reinstate 3-wire support by not only checking whether rx_buf is non-NULL, but also checking that it is not the dummy buffer.

index 6f243a90c844..840b1b8ff3dc 100644
@@ -834,7 +834,8 @@ static int bcm2835_spi_transfer_one(struct spi_controller *ctlr,
 	bcm2835_wr(bs, BCM2835_SPI_CLK, cdiv);
 
 	/* handle all the 3-wire mode */
-	if ((spi->mode & SPI_3WIRE) && (tfr->rx_buf))
+	if (spi->mode & SPI_3WIRE && tfr->rx_buf &&
+	    tfr->rx_buf != ctlr->dummy_rx)
 		cs |= BCM2835_SPI_CS_REN;
 	else
 		cs &= ~BCM2835_SPI_CS_REN;

That's odd, it works for me:

$ git fetch linux-stable
$ git checkout linux-stable/linux-5.2.y
Checking out files: 100% (43562/43562), done.
HEAD is now at 5697a9d... Linux 5.2.7
$ git am /tmp/pt
Applying: spi: bcm2835: Fix 3-wire mode if DMA is enabled

Could you be a little more specific why it couldn't be applied when you tried it?

Thanks,

Lukas

Sorry, that's the problem of form letters, sometimes they are wrong :(

The patch applies everywhere, but breaks the build in all trees. So it needs to be modified to build properly. I need to write up a different error message for this type of failure one of these days, luckily it is pretty rare.

thanks,

greg k-h

Ugh, yes you are right, my apologies.

The reason is that v5.3 converted spi-bcm2835.c to use the "spi_controller" nomenclature instead of "spi_master" with commit 5f336ea53b6b ("spi: bcm2835: Replace spi_master by spi_controller").

The replacement patch below should hopefully not break the build. It's the same as upstream commit 8d8bef503658, except one occurrence of "ctlr" is replaced by "master".

Thanks!

-- >8 --
From 8d8bef50365847134b51c1ec46786bc2873e4e47 Mon Sep 17 00:00:00 2001
From: Lukas Wunner <[email protected]>
Date: Wed, 3 Jul 2019 12:29:31 +0200
Subject: [PATCH] spi: bcm2835: Fix 3-wire mode if DMA is enabled
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Commit 6935224da248 ("spi: bcm2835: enable support of 3-wire mode") added 3-wire support to the BCM2835 SPI driver by setting the REN bit (Read Enable) in the CS register when receiving data. The REN bit puts the transmitter in high-impedance state. The driver recognizes that data is to be received by checking whether the rx_buf of a transfer is non-NULL.

Commit 3ecd37edaa2a ("spi: bcm2835: enable dma modes for transfers meeting certain conditions") subsequently broke 3-wire support because it set the SPI_MASTER_MUST_RX flag which causes spi_map_msg() to replace rx_buf with a dummy buffer if it is NULL. As a result, rx_buf is *always* non-NULL if DMA is enabled.

Reinstate 3-wire support by not only checking whether rx_buf is non-NULL, but also checking that it is not the dummy buffer.

index 6f243a90c844..840b1b8ff3dc 100644
@@ -834,7 +834,8 @@ static int bcm2835_spi_transfer_one(struct spi_master *master,
 	bcm2835_wr(bs, BCM2835_SPI_CLK, cdiv);
 
 	/* handle all the 3-wire mode */
-	if ((spi->mode & SPI_3WIRE) && (tfr->rx_buf))
+	if (spi->mode & SPI_3WIRE && tfr->rx_buf &&
+	    tfr->rx_buf != master->dummy_rx)
 		cs |= BCM2835_SPI_CS_REN;
 	else
 		cs &= ~BCM2835_SPI_CS_REN;

Thanks for this, now queued up.

greg k-h