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

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Message-ID: <7b85e098-b9a9-dd14-203f-100cdf2e703e@pengutronix.de>
Date:   Wed, 26 Feb 2020 11:19:26 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Tim Harvey <tharvey@...eworks.com>
Cc:     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>
Subject: Re: [PATCH] can: mcp251x: convert to half-duplex SPI

On 2/26/20 8:37 AM, Marc Kleine-Budde wrote:
>> Your right... there is the mcp251x_hw_rx_frame() call that also uses
>> spi_rx_buf after a synchronous transfer (I didn't see any others).
>> I'll look at this again.
> 
> Have you hardware to test your changes? I think the SPI framework would
> return an -EINVAL in that case....though the return value is sometimes
> not checked by the driver :/

See https://elixir.bootlin.com/linux/v5.5.6/source/drivers/spi/spi.c#L3413

If you have really have HW with SPI_CONTROLLER_HALF_DUPLEX (a.k.a
SPI_MASTER_HALF_DUPLEX) restrictions, you need to convert _every_
mcp251x_spi_trans() call in the driver, as _always_ both rx_buf _and_
tx_buf are used.

Marc

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