linux-kernel - Re: [PATCH RESEND 1/5 v6] gpio: Add a block GPIO API to gpiolib

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Date:	Thu, 1 Nov 2012 15:44:07 +0100
From:	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Roland Stigge <stigge@...com.de>, gregkh@...uxfoundation.org,
	linus.walleij@...aro.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, w.sang@...gutronix.de,
	jbe@...gutronix.de, highguy@...il.com,
	broonie@...nsource.wolfsonmicro.com, daniel-gl@....net,
	rmallon@...il.com
Subject: Re: [PATCH RESEND 1/5 v6] gpio: Add a block GPIO API to gpiolib

On 19:59 Wed 31 Oct     , Grant Likely wrote:
> Hi Roland
> 
> On Wed, Oct 31, 2012 at 6:19 PM, Roland Stigge <stigge@...com.de> wrote:
> > On 10/31/2012 04:00 PM, Grant Likely wrote:
> >> For the API, I don't think it is a good idea at all to try and
> >> abstract away gpios on multiple controllers. I understand that it
> >> makes life a lot easier for userspace to abstract those details away,
> >> but the problem is that it hides very important information about how
> >> the system is actually constructed that is important to actually get
> >> things to work. For example, say you have a gpio-connected device with
> >> the constraint that GPIOA must change either before or at the same
> >> time as GPIOB, but never after. If those GPIOs are on separate
> >> controllers, then the order is completely undefined
> >
> > It is correct that it's not (yet) well documented and the API is also
> > not very explicit about it, but the actual approach of the manipulation
> > order is to let drivers handle gpios "as simultaneous as possible" and
> > when not possible, do it in the _order of bits specified_ (either
> > defined at the device tree level, or when created via
> > block_gpio_create() directly).
> 
> The documentation is actually fine. I do understand that the intent is
> "as simultaneous as possible", but I accept the point that the order
> of specification affects the behaviour*. However, it still remains
> that the method used by the ABI abstracts at the wrong level and that
> blocking arbitrary GPIO pins into a single virtual GPIO register is a
> bad idea.
> 
> *note that the current code doesn't implement that intended behaviour
> either since the gpios are processed in the order of the controllers,
> not the order of the bits.
> 
> > I'm not sure how far you tested the API in depth: You can already define
> > a block that maps onto a subset of gpios on a controller and internally
> > of course maps onto those set and clear operations. Whenever you need to
> > manipulate a different subset (whether disjoint or overlapping), you can
> > easily define _additional_ blocks. From my experience, this solves most
> > of the real world problems when n-bit busses are bit banged over GPIOs.
> > Doesn't this already solve this (in a different way, though)?
> 
> Blech! Requiring a new block for each possible combination of
> write-at-once bits is a horrible ABI. That just strengthens my opinion
> that the abstraction isn't right yet.
> 
> > Pin direction currently needs to be set up separately, analogous to
> > requesting gpios. Need to document this better, right. The assumption is
> > that I/O needs to be efficient primarily, before bloating the API with
> > direction functions. Or should I add functions for this?
> 
> Since this is a userspace facing ABI, once it is merged it cannot be
> changed in an incompatible way. I cannot merge it until there is at
> least a plan for how to handle all of the reasonable use cases. That
> means it must support set/clear or mask operations. Also, if it sticks
> with the design of grouping pins from multiple controllers, then it
> needs to handle explicitly constraining what order operations are
> performed in at the time of the operation. At the time of setup
> doesn't work since constraints between pins may not always be in the
> same order.
> 
> I really think you should consider implementing a command stream type
> of interface.
I agreed with Grant and I'm not also a fan of the sysfs ABI

as I already point out in the previous version and Linus too

Best Regards,
J.
> 
> g.
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