linux-kernel - Re: [RFC net-next 15/15] net: lora: Add Semtech SX1301

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Message-ID: <20180703145022.GA13744@sirena.org.uk>
Date:   Tue, 3 Jul 2018 15:50:22 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Andreas Färber <afaerber@...e.de>
Cc:     netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Jian-Hong Pan <starnight@...cu.edu.tw>,
        Jiri Pirko <jiri@...nulli.us>,
        Marcel Holtmann <marcel@...tmann.org>,
        "David S . Miller" <davem@...emloft.net>,
        Matthias Brugger <mbrugger@...e.com>,
        Janus Piwek <jpiwek@...oweurope.com>,
        Michael Röder <michael.roeder@...et.eu>,
        Dollar Chen <dollar.chen@...ec.com>,
        Ken Yu <ken.yu@...wireless.com>,
        Ben Whitten <ben.whitten@...rdtech.com>,
        Steve deRosier <derosier@...il.com>, linux-spi@...r.kernel.org,
        LoRa_Community_Support@...tech.com
Subject: Re: [RFC net-next 15/15] net: lora: Add Semtech SX1301

On Mon, Jul 02, 2018 at 07:34:21PM +0200, Andreas Färber wrote:
> Hi Mark,
> 
> This driver is still evolving, there's newer code on my lora-next branch
> already: https://github.com/afaerber/linux/commits/lora-next

Please don't top post, reply in line with needed context.  This allows
readers to readily follow the flow of conversation and understand what
you are talking about and also helps ensure that everything in the
discussion is being addressed.

> 2) This SPI device is in turn exposing the two SPI masters that you
> already found below, and I didn't see a sane way to split that code out
> into drivers/spi/, so it's in drivers/net/lora/ here - has there been
> any precedence either way?

A MFD?

> Am 02.07.2018 um 18:12 schrieb Mark Brown:
> > On Sun, Jul 01, 2018 at 01:08:04PM +0200, Andreas Färber wrote:

> >> +static void sx1301_radio_spi_set_cs(struct spi_device *spi, bool enable)
> >> +{
> >> +	int ret;
> >> +
> >> +	dev_dbg(&spi->dev, "setting SPI CS to %s\n", enable ? "1" : "0");
> >> +
> >> +	if (enable)
> >> +		return;

> > So we never disable chip select?

> Not here, I instead did that in transfer_one below.

That's obviously at best going to be fragile, you're implementing half
the operation here and half somewhere else which is most likely going to
break at some point when the framework changes.

> >> +		if (ret) {
> >> +			dev_err(&spi->dev, "SPI radio address write failed\n");
> >> +			return ret;
> >> +		}
> >> +
> >> +		ret = sx1301_write(ssx->parent, ssx->regs + REG_RADIO_X_DATA, (tx_buf && xfr->len >= 2) ? tx_buf[1] : 0);
> >> +		if (ret) {
> >> +			dev_err(&spi->dev, "SPI radio data write failed\n");
> >> +			return ret;
> >> +		}

> > This looks awfully like you're coming in at the wrong abstraction layer
> > and the hardware actually implements a register abstraction rather than
> > a SPI one so you should be using regmap as the abstraction.

> I don't understand. Ben has suggested using regmap for the SPI _device_
> that we're talking to, which may be a good idea. But this SX1301 device
> in turn has two SPI _masters_ talking to an SX125x slave each. I don't
> see how using regmap instead of my wrappers avoids this spi_controller?

It seems obvious from the code that this isn't actually interacting with
a SPI controller, you're writing an address and a value to a register
map rather than dealing with a byte stream.  There may be a SPI bus
somewhere behind some other hardware but you don't seem to be
interacting with it as such.

> The whole point of this spi_controller is to abstract and separate the
> SX1255 vs. SX1257 vs. whatever-radio-attached into a separate driver,
> instead of mixing it into the SX1301 driver - to me that looks cleaner
> and more extensible. It also has the side-effect that we could configure
> the two radios via DT (frequencies, clk output, etc.).

A register map would work just as well here, we already have plenty of
devices that abstract at this level (most obviously the I2C/SPI devices
that use it to offer both interfaces with a single core driver).

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