Re: [PATCH] gpio: sn54hc595: new driver for GPIO shift registers chipsets

[Arnd Bergmann <arnd@xxxxxxxx>]

On Monday 08 December 2014 17:31:45 Rafał Miłecki wrote:
> On 8 December 2014 at 16:41, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > On Monday 08 December 2014 16:23:01 Rafał Miłecki wrote:
> >> On 8 December 2014 at 16:03, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> >> >> gpio-74x164.c seems to be tight closely to the SPI. In my case it's
> >> >> GPIO-controller '595.
> >> >
> >> > Right. gpio-74x164.c uses the SPI framework, so it will work with any SPI
> >> > master controller, while your driver contains a very simple variant (without any
> >> > timing constraints) of spi-gpio.c, and is limited to connecting to GPIO pins.
> >> >
> >> >> Do you have any other idea how we could handle this?
> >> >
> >> > Your driver does provide OE support, which gpio-74x164 doesn't support.
> >> > Perhaps that can be added to gpio-74x164 instead?
> >>
> >> It's not the missing OE support in gpio-74x164 that worries me, but
> >> the whole rest.
> >>
> >> I would need to modify gpio-74x164 to:
> >> 1) Use another OF table with different entry
> >> 2) Use different probe function that doesn't take spi_device parameter
> >> and doesn't do spi setup
> >> 3) Extend struct gen_74x164_chip to include GPIOs
> >> 4) Use totally different __gen_74x164_write_config
> >
> > I think the suggestion was to use the spi-gpio driver in combination
> > with gpio-74x164.
> 
> Oh, OK, I've obviously missed that.
> 
> I don't know... I'm somehow not really convinced to that, because
> AFAIK there isn't any SPI master device between GPIOs and 'HC595. I
> don't know if pretending there is some extra hardware (just to use
> another Linux driver/layer) is a good idea. This would:
> 1) Not match hardware design
> 2) Involve some extra code.
> 
> I'm not even sure if this would work out-of-the box (without some
> extra changes) at all. From early look at spi-gpio.c I can't see it
> implementing everything I need for GPIO-connected 'HC595.
> I think that implementing support for this extra SPI layer will
> actually require more code/tricks than a separated driver.

The purpose of the spi-gpio driver is to avoid having to write
two drivers for every spi slave device if someone wants to drive
it using bit-banged gpios.

The existing users of the driver are in fact all using spi-gpio,
see:
arch/arm/boot/dts/armada-xp-lenovo-ix4-300d.dts
arch/arm/boot/dts/imx28-cfa10049.dts

and in openwrt:
target/linux/brcm63xx/patches-3.14/501-board-NB4.patch
target/linux/brcm63xx/patches-3.14/526-board_CT6373-1.patch

so there shouldn't be too much missing, and extending spi-gpio
for any missing features would help other people trying to attach
random spi devices using gpio as well.

	Arnd
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