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

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Message-ID: <20190628190022.vya4h2lihm6x2xpb@wunner.de>
Date:   Fri, 28 Jun 2019 21:00:22 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Stefan Wahren <wahrenst@....net>
Cc:     Nuno Sá <nuno.sa@...log.com>,
        broonie@...nel.org, eric@...olt.net,
        Martin Sperl <kernel@...tin.sperl.org>, f.fainelli@...il.com,
        rjui@...adcom.com, sbranden@...adcom.com,
        linux-kernel@...r.kernel.org,
        bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-spi@...r.kernel.org
Subject: Re: [PATCH] spi: spi-bcm2835.c: Fix 3-wire mode

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

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

Thanks,

Lukas