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

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Message-ID: <20180703153137.GB13744@sirena.org.uk>
Date:   Tue, 3 Jul 2018 16:31:37 +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 Tue, Jul 03, 2018 at 05:09:38PM +0200, Andreas Färber wrote:
> Am 03.07.2018 um 16:50 schrieb Mark Brown:

> >> 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?

> I know of mfd, but how would the the the net vs. spi pieces interact
> then? Some functions would need to be exported then or is there an
> easier way without needing to set a cross-module API in stone?

It's an in-kernel ABI it's not exactly set in stone but yeah, you'll
need some interface.  A lot of devices work by having the children know
that they're part of a MFD and fish things out of the parent device,
either the pdata or (in the common case where the MFD bit mostly just
instantiates subdevices and holds a regmap) with dev_get_regmap().

> > 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).

> The address and data registers together form a two-byte SPI message!

> It is transmitted by writing to the CS register.

> The received data is afterwards available in another register.

Right, but it seems from the code that the hardware understands that
it's formatting register I/O and not just shifting in and out a byte
stream which is what a SPI controller does.  I'd not be surprised to
learn that the register you're calling a chip select register is a
strobe that initiates the transfer (and that this may be some of the
difficulty you're having with handling it in the way the framework
expects), the pattern with writing 1 followed immediately by 0 is a bit
of a flag here.

I've seen such before hardware where I know it was intentionally
designed that way so it wouldn't be totally surprising.

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