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

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hi Rafał,

On Mon, Dec 8, 2014 at 5:31 PM, Rafał Miłecki <zajec5@xxxxxxxxx> 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:
>>> > On Mon, Dec 8, 2014 at 3:47 PM, Rafał Miłecki <zajec5@xxxxxxxxx> wrote:
>>> >> On 8 December 2014 at 15:41, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
>>> >>> On Mon, Dec 8, 2014 at 3:27 PM, Rafał Miłecki <zajec5@xxxxxxxxx> wrote:
>>> >>>> SN54HC595 and SN74HC595 are devices based on shift registers controlled
>>> >>>> with 5 input signals (serial-in) and providing 8 outputs (parallel-out).
>>> >>>>
>>> >>>> They are present on some Broadcom home router boards where manufacturer
>>> >>>> needed few extra GPIOs.
>>> >>>>
>>> >>>> This driver simply uses specified GPIOs to control shift registers and
>>> >>>> registers another GPIO chip. So you can call it a GPIO extender.
>>> >>>>
>>> >>>> Due to the hardware design only output direction is supported. Reading
>>> >>>> values is handled using tiny internal cache.
>>> >>>>
>>> >>>> Signed-off-by: Rafał Miłecki <zajec5@xxxxxxxxx>
>>> >>>> ---
>>> >>>>  .../devicetree/bindings/gpio/gpio-sn54hc595.txt    |  35 ++++
>>> >>>>  drivers/gpio/Kconfig                               |  11 ++
>>> >>>>  drivers/gpio/Makefile                              |   1 +
>>> >>>>  drivers/gpio/gpio-sn54hc595.c                      | 219 +++++++++++++++++++++
>>> >>>
>>> >>> The '595 is already handled by drivers/gpio/gpio-74x164.c.
>>> >>
>>> >> 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

There is: you implement SPI using bitbang on GPIOs.

> 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.

It should. Plenty of people have connected '595s to SPI buses (I did so
myself, too). Hint: connect chip select to the register clock.

> I think that implementing support for this extra SPI layer will
> actually require more code/tricks than a separated driver.

Yes, it will require more code, as spi-gpio is more generic than your simple
implementation.  But the end result is more flexible and reusable.

The only thing missing is the programmable OE and reset pins,
which are assumed hardwired by the gpio-74x164 driver.
These could be implemented using new gpio-oe and gpio-reset
properties.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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