[RFC] Dummy GPIO driver for use with SPI

[Steven A. Falco]

Anton Vorontsov wrote:
> On Fri, Dec 12, 2008 at 11:59:13AM -0500, Steven A. Falco wrote:
>> 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?
> 
> Um.. do you know that you can pass '0' as a GPIO?

I did not know that. :-)  Ok, so I'll look at modifying spi_ppc4xx.c
based on your suggestions.

	Thanks!
	Steve

> 
> For example,
> 
> spi-controller {
> 	gpios = <&pio1 1 0	/* cs0 */
> 		 0		/* cs1, no GPIO */
> 		 &pio2 2 0>;	/* cs2 */
> 
> 	device at 0 {
> 		reg = <0>; /* spi device, cs 0: "&pio1 1 0" */
> 	}
> 
> 	device at 1 {
> 		reg = <1>; /* spi device, cs 1: no actual GPIO */
> 	}
> 
> 	device at 2 {
> 		reg = <2>; /* spi device, cs 2: "&pio2 2 0" */
> 	}
> };
> 
> With this patch
> http://patchwork.ozlabs.org/patch/12450/
> 
> of_get_gpio() will differentiate "end of gpios" and "no gpio" cases.
> So, in the SPI driver you can do something like this:
> 
> count = of_gpio_count(np);
> for (i = 0; i < count; i++) {
> 	int gpio;
> 
> 	gpio = of_get_gpio(np, i);
> 	if (gpio_is_valid(gpio)) {
> 		normal case;
> 	} else if (gpio == -EEXIST) {
> 		the special case;
> 	} else {
> 		error;
> 	}
> }
>