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Subject: Re: MCP251x SPI CAN controller on Cavium ThunderX
To:     Mark Brown <broonie@kernel.org>, Tim Harvey <tharvey@gateworks.com>
Cc:     Jan Glauber <jan.glauber@caviumnetworks.com>,
        linux-spi@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Wolfgang Grandegger <wg@grandegger.com>,
        linux-can <linux-can@vger.kernel.org>
References: <CAJ+vNU3VK=NPKW+119t4MT7w5xs=FS2HWJTqbFb-HSeJhCvwWw@mail.gmail.com>
 <20171114120207.xbee2cgsai4qka46@sirena.org.uk>
From:   Marc Kleine-Budde <mkl@pengutronix.de>
Message-ID: <bfa971bb-1217-ed86-d1d8-e353ecc506aa@pengutronix.de>
Date:   Wed, 15 Nov 2017 11:54:20 +0100
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From: Marc Kleine-Budde <mkl@pengutronix.de>
To: Mark Brown <broonie@kernel.org>, Tim Harvey <tharvey@gateworks.com>
Cc: Jan Glauber <jan.glauber@caviumnetworks.com>, linux-spi@vger.kernel.org,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
 Wolfgang Grandegger <wg@grandegger.com>,
 linux-can <linux-can@vger.kernel.org>
Message-ID: <bfa971bb-1217-ed86-d1d8-e353ecc506aa@pengutronix.de>
Subject: Re: MCP251x SPI CAN controller on Cavium ThunderX
References: <CAJ+vNU3VK=NPKW+119t4MT7w5xs=FS2HWJTqbFb-HSeJhCvwWw@mail.gmail.com>
 <20171114120207.xbee2cgsai4qka46@sirena.org.uk>
In-Reply-To: <20171114120207.xbee2cgsai4qka46@sirena.org.uk>

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On 11/14/2017 01:02 PM, Mark Brown wrote:
> On Mon, Nov 13, 2017 at 01:17:42PM -0800, Tim Harvey wrote:
>=20
>> When a register is read from the mcp251x driver the
>> octeon_spi_do_transfer() gets a spi_message with a single spi_xfer of
>> len=3D3, a tx_buf, and an rx_buf which I believe is supposed to shift
>> out 3 bytes out MOSI and shift in 3 bytes from MISO where the last
>> byte shifted in would be the response.
>=20
> No, that will simultaneously transmit and recieve three bytes.

That's what the driver supposed to do.

> If you want to transmit two bytes and then recieve one byte you need
> two xfers, one with a len of 2 and a tx_buf, the other with a len of
> 1 and a rx_buf.
To read a register (mcp251x_read_reg()) the mcp251x does a 3 byte full
duplex transfer. The first byte send is the command (read register) the
second byte the register number the third byte is a dummy. The first 2
bytes received are ignored the 3rd byte is the register contents.

Marc

--=20
Pengutronix e.K.                  | Marc Kleine-Budde           |
Industrial Linux Solutions        | Phone: +49-231-2826-924     |
Vertretung West/Dortmund          | Fax:   +49-5121-206917-5555 |
Amtsgericht Hildesheim, HRA 2686  | http://www.pengutronix.de   |


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