RE: Device tree BSP

[Stephen Neuendorffer <stephen.neuendorffer-gjFFaj9aHVfQT0dZR+AlfA@public.gmane.org>]

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My (limited) understanding from reading other posts is that there is already a GPIO subsystem for handling such misc-GPIO things.
To me the cost of gates for multiple GPIO cores is small compared to the system cost of having
a separate OF binding.  IMHO, this should be exposed the way any other GPIO with such capabilities
is on any other OF/device-tree enabled system.

Steve

-----Original Message-----
From: John Williams [mailto:john.williams-g5w7nrANp4BDPfheJLI6IQ@public.gmane.org]
Sent: Sun 7/5/2009 2:18 AM
To: Stephen Neuendorffer
Cc: michal.simek-g5w7nrANp4BDPfheJLI6IQ@public.gmane.org; John Linn; David DeBonis; devicetree-discuss@ozlabs.org
Subject: Re: Device tree BSP
 
On Sun, Jul 5, 2009 at 4:15 AM, Stephen Neuendorffer <
stephen.neuendorffer-gjFFaj9aHVfQT0dZR+AlfA@public.gmane.org> wrote:

>
>
> I'm not exactly sure what you're trying to do here, but it doesn't look
> right to me.
> In particular, why would I have to have a separate core for these
> functions?
> Furthermore, I'm sure there must be some generic mechanisms that can/should
> be used for these kind of things.
> In general, we shouldn't reinvent the wheel here..
>

The issue as I see it is that in a hardware design, the GPIO is a very
convenient core to drive a heartbeat LED, and aux input on proc_sys_reset
(for software-driven reset), or whatever.

Clearly you don't want these GPIO cores to enumerate as regular xps_gpio
devices, binding to the standard GPIO driver and appearing in /dev/gpioN

Is your concern the manual override of OF_ binding (the "compatible"
strings), or just the HW overhead of an extra core to drive these HW utility
functions?  Save us from the bad old days of "misc_logic"  IP cores that
drove this stuff in older Xilinx reference designs!

Would you propose to get tricky and allocate specific bits on existing GPIOs
for these "special" functions, and write the GPIO driver/framework to mask
off these bits etc?  Seems like a complicated software solution to a problem
trivially solved with a few extra gates.

Cheers,

John
-- 
John Williams, PhD, B.Eng, B.IT
PetaLogix - Linux Solutions for a Reconfigurable World
w: www.petalogix.com  p: +61-7-30090663  f: +61-7-30090663



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