linux-kernel - Re: [PATCH 2/3] spi: of: allow instantiating slaves without a driver

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Message-ID: <20160626112133.GJ28202@sirena.org.uk>
Date:	Sun, 26 Jun 2016 12:21:33 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Michal Suchanek <hramrach@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dan Williams <dan.j.williams@...el.com>,
	Tejun Heo <tj@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Adrien Schildknecht <adrien+dev@...ischi.me>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-spi <linux-spi@...r.kernel.org>
Subject: Re: [PATCH 2/3] spi: of: allow instantiating slaves without a driver

On Sun, Jun 26, 2016 at 04:23:41AM +0200, Michal Suchanek wrote:
> On 26 June 2016 at 03:15, Mark Brown <broonie@...nel.org> wrote:

> > I can't relate this hunk to the changelog and there's a coding style
> > problem, if there's { } on one side of an if statement it should be on
> > both sides.  Why are we making this change?

> The intention is that you can specify that your SPI master controller
> has a CS available without setting up the slave

This is completely unrelated to the rest of the change and needs
describing in the changelog.

> Then you can amend the slave node with an overlay or bind a driver
> that can deal with the node having no configuration.

You can just add the entire slave node in the overlay, it's not clear
that this buys us anything useful (and typically you'd want to as the
maximum speed here is more a function of the slave device than the
master), and this needs to be joined up with the work going on to allow
expansion connector overlays to be loaded in the first place.

> The check here isn't very effective anyway since slaves with zero
> speed somehow creep into the kernel. I have seen people reporting
> division by zero in SPI master driver as a result. The proper way to
> fix this is to specify the master minimum and maximum speed limit so
> the SPI core can reject transfers with speed outside of the allowed
> range.

That's a separate argument which is again unrelated to the changelog.
If the check isn't working it would be better to have an analysis of why
it's not working.

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