linux-kernel - Re: [patch/rfc 2.6.19-rc5] arch-neutral GPIO calls

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Message-ID: <45628A1A.8060101@billgatliff.com>
Date:	Mon, 20 Nov 2006 23:09:46 -0600
From:	Bill Gatliff <bgat@...lgatliff.com>
To:	David Brownell <david-b@...bell.net>
CC:	Paul Mundt <lethal@...ux-sh.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Andrew Victor <andrew@...people.com>,
	Haavard Skinnemoen <hskinnemoen@...el.com>, jamey.hicks@...com,
	Kevin Hilman <khilman@...sta.com>,
	Nicolas Pitre <nico@....org>,
	Russell King <rmk@....linux.org.uk>,
	Tony Lindgren <tony@...mide.com>
Subject: Re: [patch/rfc 2.6.19-rc5] arch-neutral GPIO calls

David Brownell wrote:

>On Monday 20 November 2006 7:44 pm, Bill Gatliff wrote:
>  
>
>>So, you're saying that if GPIOA1 can come out on pins ZZ1 and BB6, then 
>>there would be two unique "GPIO numbers", one for each possibility?
>>    
>>
>
>No; one number, since it's controlled by the same set of bits in the GPIO
>controller (e.g. bit 12 in the registers of bank 3) regardless of how the
>signals are routed out through pins.  That's my point:  GPIO number need
>not imply a particular pin, and vice versa.
>  
>

Ok, thanks for the OMAP stuff.  I think I can understand now.

Why not have GPIO numbers refer to unique combinations of GPIO+pin?  If 
the GPIO line is tied to a piece of external hardware, that connection 
is surely through a specific pin.  So it seems like you'd need GPIO+pin 
every time there was an option.

>So regardless of whether GPIO_62 is routed to ball M7 or G20, it's still
>going to use number 62, which is bit 14 in various registers of the GPIO4
>module, starting at 0xfffbb400 ... and for good fun, the muxing API will
>refer to the balls on the (smaller) ZZG package even if your board uses
>the larger ZDY package (so your schematics might say J5 not M7, that table
>is very handy in such cases).
>  
>

Yikes!  :)

Explain to me why having GPIO enumerations map to unique GPIO+pin 
combinations would be a bad idea in this case?  It seems like the point 
here is to help a driver find and assert their GPIO _pin_ so that the 
driver can can talk to the attached external hardware.  Having an 
enumeration "GPIO62M7" would be a handy way to do that.  Maybe the 
enumeration is actually defined as ((0x400 << 16) | (14 << 8) | 4), or 
some other encoding that makes it easy in the implementation of 
gpio_XXX() to find the right registers and get the routing set up 
correctly.  It's just an opaque, magic number to the driver after all.

And since GPIOs are arch/mach/board-specific anyway, who would care if 
OMAP was the only system that had an enumeration called "GPIO62M7"?  
When other boards set up their platform_struct, they'd use the 
enumerations available for that platform.  "GPIOA1" in systems where 
that GPIO line could only go to one pin per the datasheet, for example.

b.g.

-- 
Bill Gatliff
bgat@...lgatliff.com

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