From: "Steven A. Falco" <sfalco@harris.com>
To: avorontsov@ru.mvista.com
Cc: "linuxppc-dev@ozlabs.org" <linuxppc-dev@ozlabs.org>
Subject: Re: [RFC] Dummy GPIO driver for use with SPI
Date: Fri, 12 Dec 2008 11:59:13 -0500 [thread overview]
Message-ID: <49429861.9060305@harris.com> (raw)
In-Reply-To: <20081212150144.GA28147@oksana.dev.rtsoft.ru>
Anton Vorontsov wrote:
> On Fri, Dec 12, 2008 at 09:22:02AM -0500, Steven A. Falco wrote:
>> This patch adds a dummy GPIO driver, which is useful for SPI devices
>> that do not have a physical chip select.
>
> Hm. Then you don't need a chip-select, and SPI driver must understand
> this case. When SPI controller has no "gpios" property, it should just
> ignore any chip-select toggling operations.
>
> As an implementation example you can use this patch:
>
> http://patchwork.ozlabs.org/patch/12499/
>
> grep for "SPI w/o chip-select line."
>
My actual situation is a bit more complicated - serves me right for
trying to simplify it in my RFC.
We have three devices on the SPI bus. Two have well-behaved chip
selects - they are ST flash memory devices. The third device, the
Atmel chip does not have a chip select. It does have a RESET pin,
which is similar to a chip select, but the Atmel protocol requires
that that pin be low during the entire programming operation, and
I cannot chain all the tx/rx operations together into one atomic
SPI transaction, so I cannot use that pin as the SPI chip select.
Instead, I manage the RESET pin outside of the SPI driver, and hence
there is no chip select for that one device, so I use my dummy CS
driver to provide a fake chip select to satisfy the SPI driver.
This does have the limitation that I must be careful not to access
the flash parts at the same time as I access the Atmel, but that is
ok for my application. I guess I could use something like your
patch, but I'd maybe have to extend the flags to include a "do not
use" bit, which would bypass the gpio_is_valid and gpio_request
calls.
What do you think about having a mechanism to specify that some
SPI slaves have a chip select, while others don't have to have a
chip select managed by the SPI subsystem?
Steve
next prev parent reply other threads:[~2008-12-12 16:59 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-12-12 14:22 [RFC] Dummy GPIO driver for use with SPI Steven A. Falco
2008-12-12 15:01 ` Anton Vorontsov
2008-12-12 16:59 ` Steven A. Falco [this message]
2008-12-12 17:14 ` Anton Vorontsov
2008-12-12 17:33 ` Steven A. Falco
2008-12-12 21:39 ` Trent Piepho
2008-12-12 22:46 ` David Gibson
2008-12-16 16:34 ` Anton Vorontsov
2008-12-15 0:12 ` David Gibson
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