spi: spi-bcm2835.c: Fix 3-wire mode
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Message ID 20190628123023.4696-1-nuno.sa@analog.com
State New
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Series
  • spi: spi-bcm2835.c: Fix 3-wire mode
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Commit Message

Sa, Nuno June 28, 2019, 12:30 p.m. UTC
As stated in
https://www.raspberrypi.org/documentation/hardware/raspberrypi/spi/README.md,
one of rx or tx buffer's must be null. However, if DMA is enabled, the
driver sets the SPI_CONTROLLER_MUST_RX | SPI_CONTROLLER_MUST_TX on the
controller flags. Hence, the spi core will provide dummy buffers even if
one of the buffers was set to null by the device driver. Thus, the
communication with the 3-wire device fails.

This patch uses the prepare_message callback to look for the device mode
and sets/clears the SPI_CONTROLLER_MUST_RX | SPI_CONTROLLER_MUST_TX on a
per spi message basis. It also assumes that DMA is not supported on
half-duplex devices.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
---
 drivers/spi/spi-bcm2835.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

Comments

Stefan Wahren June 28, 2019, 3:23 p.m. UTC | #1
Hi Nuno,

Am 28.06.19 um 14:30 schrieb Nuno Sá:
> As stated in
> https://www.raspberrypi.org/documentation/hardware/raspberrypi/spi/README.md,
> one of rx or tx buffer's must be null. However, if DMA is enabled, the
> driver sets the SPI_CONTROLLER_MUST_RX | SPI_CONTROLLER_MUST_TX on the
> controller flags. Hence, the spi core will provide dummy buffers even if
> one of the buffers was set to null by the device driver. Thus, the
> communication with the 3-wire device fails.
>
> This patch uses the prepare_message callback to look for the device mode
> and sets/clears the SPI_CONTROLLER_MUST_RX | SPI_CONTROLLER_MUST_TX on a
> per spi message basis. It also assumes that DMA is not supported on
> half-duplex devices.
>
> Signed-off-by: Nuno Sá <nuno.sa@analog.com>

i never tested the 3-wire mode. Could you please describe your test setup?

@Martin, @Lukas Are you fine with this patch?
Sa, Nuno July 1, 2019, 7:24 a.m. UTC | #2
Hi Lukas and Stefan,

On Fri, 2019-06-28 at 21:00 +0200, Lukas Wunner wrote:
> 
> On Fri, Jun 28, 2019 at 05:23:54PM +0200, Stefan Wahren wrote:
> > Am 28.06.19 um 14:30 schrieb Nuno Sá:
> > > As stated in
> > > https://www.raspberrypi.org/documentation/hardware/raspberrypi/spi/README.md,
> > > one of rx or tx buffer's must be null. However, if DMA is
> > > enabled, the
> > > driver sets the SPI_CONTROLLER_MUST_RX | SPI_CONTROLLER_MUST_TX
> > > on the
> > > controller flags. Hence, the spi core will provide dummy buffers
> > > even if
> > > one of the buffers was set to null by the device driver. Thus,
> > > the
> > > communication with the 3-wire device fails.
> > > 
> > > This patch uses the prepare_message callback to look for the
> > > device mode
> > > and sets/clears the SPI_CONTROLLER_MUST_RX |
> > > SPI_CONTROLLER_MUST_TX on a
> > > per spi message basis. It also assumes that DMA is not supported
> > > on
> > > half-duplex devices.
> > > 
> > > Signed-off-by: Nuno Sá <nuno.sa@analog.com>
> > 
> > i never tested the 3-wire mode. Could you please describe your test
> > setup?

I'm working on a rpi cape which uses this device 
https://github.com/analogdevicesinc/linux/blob/master/drivers/iio/frequency/adf4371.c
which is connected in 3-WIRE mode. I could confirm that spi was not
working.

> 
> __spi_validate() returns -EINVAL if 3-wire mode is used and both
> buffers
> are non-NULL, I guess that's the problem.

In my case, __spi_validate() was ok because my device driver is passing
one of rx or tx as NULL. 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...

> 
> 
> > @Martin, @Lukas Are you fine with this patch?
> 
> I have a patch set in the pipeline to drop SPI_CONTROLLER_MUST_RX
> and SPI_CONTROLLER_MUST_TX from spi-bcm2835.c.
> 
> Latest snapshot is available here (top-most 10 commits):
> https://github.com/l1k/linux/commits/revpi_staging
> 
> @Nuno, could you give this branch a spin and see if it fixes the
> issue for you?  If so, this might be a better solution.  Your patch
> is fine in principle since it works around the problem, but the
> patch set on the above-linked branch fixes it at the root.
> It also provides a nice welcome speedup and reduces resource
> consumption.
> 
> I've been working on this on-and-off for about half a year,
> I think the patch set is in pretty good shape now so I was
> planning to submit it probably in 2 weeks or so.
> 

This looks great. I will try to give this a try still today.

> Thanks,
> 
> Lukas

Regards,
Nuno Sá
Sa, Nuno July 1, 2019, 2:21 p.m. UTC | #3
On Mon, 2019-07-01 at 13:55 +0200, Lukas Wunner wrote:
> 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;
> 

This worked fine. Also, I did a quick backport of the state of your
driver's (both spi-bcm2835 and bcm2835-dma) in revpi_staging and it
also worked fine with my device.
So, as far as I understand, the above suggestion (or my patch) is not
intended to be upstreamed, right? It is just a temporary fix that I can
use while your patchset gets upstream.

Thanks for your help!
Nuno Sá

Patch
diff mbox series

diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index 6f243a90c844..8993a15a4684 100644
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -674,6 +674,10 @@  static bool bcm2835_spi_can_dma(struct spi_controller *ctlr,
 	if (tfr->len < BCM2835_SPI_DMA_MIN_LENGTH)
 		return false;
 
+	/* Do not DMA on half-duplex devices */
+	if (spi->mode & SPI_3WIRE)
+		return false;
+
 	/* return OK */
 	return true;
 }
@@ -902,6 +906,15 @@  static int bcm2835_spi_prepare_message(struct spi_controller *ctlr,
 		cs |= BCM2835_SPI_CS_CPOL;
 	if (spi->mode & SPI_CPHA)
 		cs |= BCM2835_SPI_CS_CPHA;
+	/*
+	 * For half-duplex devices, one of tx or rx buffers must be null
+	 * for one spi transfer. Hence, we need to clear the spi controller
+	 * flags so that we don't get dummy buffers...
+	 */
+	if (spi->mode & SPI_3WIRE)
+		master->flags &= ~(SPI_MASTER_MUST_RX | SPI_MASTER_MUST_TX);
+	else if (master->can_dma)
+		master->flags |= (SPI_MASTER_MUST_RX | SPI_MASTER_MUST_TX);
 
 	bcm2835_wr(bs, BCM2835_SPI_CS, cs);