Date:	Tue, 30 Dec 2008 23:55:44 -0500
From:	Robin Getz <rgetz@...ckfin.uclinux.org>
To:	"David Brownell" <david-b@...bell.net>
Cc:	"Jaya Kumar" <jayakumar.lkml@...il.com>,
	"Eric Miao" <eric.y.miao@...il.com>,
	"Sam Ravnborg" <sam@...nborg.org>,
	"Eric Miao" <eric.miao@...vell.com>,
	"Haavard Skinnemoen" <hskinnemoen@...el.com>,
	"Philipp Zabel" <philipp.zabel@...il.com>,
	"Russell King" <rmk@....linux.org.uk>,
	"Ben Gardner" <bgardner@...tec.com>, "Greg KH" <greg@...ah.com>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-fbdev-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, linux-embedded@...r.kernel.org
Subject: Re: [RFC 2.6.27 1/1] gpiolib: add support for batch set of pins

On Mon 29 Dec 2008 14:56, David Brownell pondered:
> On Sunday 28 December 2008, Robin Getz wrote:
> > On Sat 27 Dec 2008 09:55, Jaya Kumar pondered:
> > 
> > I think that I would prefer 'group' or 'collection' to use some of 
> > the words that David described things as 'ganged' or 'bus' or 'bank' 
> > or 'adjacent'.  (but I think 'adjacent' or 'bank' really is an 
> > implementation convention).
> > 
> > I think I would also prefer to name things like gpio_bus_* as a 
> > rather than gpio_*_bus - so the operation always is on the end...
> 
> I'd avoid "bus"; the term has specific meanings, which aren't
> necessarily relevant in these contexts.  Agreed about "adjacent" and 
> "bank". 
> 
> If it weren't for its verb usage, I'd suggest "set".  :)

So, any suggestions? how about "gang" or "group"? or as you said 
below "multiple".


> > Shouldn't the write return an error code (if the data was not
> > written)?
> 
> For these operations, I think reads/writes should have that possibility.
> 
> The reason single-bit operations don't provide error paths is twofold.
> First, they started as wrappers for can't-fail register accessors.
> Second, it's extremely unrealisitic to expect much code to handle any
> kind of faults in the middle of bitbanging loops ... or even just in
> classic "set this bit and continue" configuration code.
> 
> Those reasons don't apply well to these multi-bit cases.

I'm just trying to think of what the error case might be. The only think I can 
really think of - is the case where you try to do a set/get on something that 
has been freed.

Otherwise - it should be pretty much the same. (basic bitbanging loops, which 
call multiple/basic can't-fail register accessors).

> > > While we are here, I was thinking about it, and its better if I give
> > > gpio_request/free/direction_batch a miss for now. Nothing prevents
> > > those features being added at a later point.
> > 
> > I don't think that request/free are optional.
> 
> Agreed.  If there's any difficulty implementing them, that would
> suggest there's a significant conceptual problem to address ...
>
> > For example - in most SoC implementations - gpios are implemented
> > as banks of 16 or 32. (a 16 or 32 bit register).
> > 
> > Are there facilities to span these registers? 
> >  - can you request 64 gpios as a 'bank'?
> >  - can you request gpio_8 -> gpio_40 as a 'bank' on a 32-bit system?
> > 
> > Are non-adjacent/non-contiguous gpios avaliable to be put into 
> > a 'bank/batch/bus'? can you use gpio_8 -> 11 &  28 -> 31 as a
> > 8-bit 'bus'? 
> > 
> > How do you know what is avaliable to be talked to as a bank/bus/batch
> > without the request/free operation?
> > 
> > 
> > I have seen various hardware designs (both at the PCB and SoC level)
> > require all of these options, and would like to see common 
> > infrastructure which handles this.
> 
> Right.  Get the interface right -- it should allow all those
> complexities! -- and maybe take shortcuts in the first versions
> of the implementation, by only talking to one gpio_chip at a
> time.  I'd expect any shortcuts would be resolved quickly, to
> the extent they cause problems.

Yeah, I hadn't thought about spanning more than one gpio_chip. That's a good 
point.

> > The issue is that on many SoC implementations - dedicated peripherals
> > can also be GPIOs - so it someone wants to use SPI (for example) GPIO's
> > 3->7 might be removed from the avaliable 'gpio' resources. This is
> > determined by the silicon designer - and even the PCB designer has 
> > little to no flexibility on this. It gets worse as multiple SPI or I2C
> > are used on the PCB (which can have lots of small (less than 5) 
> > dedicated pins in the middle of the larger gpio resources)....
> 
> Such pin-muxing is a different issue though.  Don't expect GPIO
> calls to resolve those problems.

I wasn't trying to make the point about pin muxing (I agree - different 
problem) - just that it is unlikely that the gang of gpios will be 
adjacent/contiguous in most cases.

> > I would think that a 'bank' / 'bus' (whatever) would be a collection
> > of random/multiple GPIOs (a struct of gpio_port_t) rather than a
> > start/length (as you described) - or better yet - the request function
> > takes a list (of individual GPIO's - defined in the platform data), 
> > and creates the struct itself.
> 
> That's why I pointed to <linux/bitmask.h> as one model:  it allows
> an arbitrary set of bits (GPIOs) to be specified.

I can see that being useful.


> I'll NAK any "gpio_port_t" name based entirely on kernel coding
> conventions though.  ;)
> 
> 
> > Something like the way gpio_keys are defined...
> > 
> > static struct gpio_bus bfin_gpio_bus_table[] = {
> >         {BIT_0, GPIO_PB8, 1, "gpio-bus: BIT0"},
> >         {BIT_1, GPIO_PB9, 1, "gpio-bus: BIT1"},
> >         {BIT_2, GPIO_PB10, 1, "gpio-bus: BIT2"},
> >         {BIT_3, GPIO_PB11, 1, "gpio-bus: BIT3"},
> > };
> > 
> > static struct gpio_bus_data bfin_gpio_bus_data = {
> >         .bits        = bfin_gpio_bus_table, 
> >         .width       = ARRAY_SIZE(bfin_gpio_keys_table),
> > };
> > 
> > static struct platform_device bfin_device_gpiobus = {
> >         .name      = "gpio-bus",
> >         .dev = {
> >                 .platform_data = &bfin_gpio_bus_data,
> >         },
> > };
> > 
> > The request function builds up the bus/masks/shifts from that...
> 
> That sort of thing might be made to work.  Whatever this
> multiple-GPIO-set abstraction is, I suspect you're right
> about wanting the request function to be able to build
> up some data structures behind it.  And that it would be
> simpler to require all the GPIOs to be listed up front,
> rather than support "add these to the set" requests.

Yeah, I would prefer the data structures to 16 calls to a request function to 
build up a 16-bit wide group...

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