Re: [PATCH v3 3/3] i2c: gpio: support write-only sda

Peter Rosin
2023-01-16T23:26:34Z

Hi!

2023-01-16 at 22:59, Heiner Kallweit wrote:
> On 16.01.2023 08:18, Peter Rosin wrote:
>> Hi!
>>
>> 2023-01-15 at 11:18, Heiner Kallweit wrote:
>>> There are slave devices that understand I2C but have read-only
>>> SDA and SCL. Examples are FD650 7-segment LED controller and
>>> its derivatives. Typical board designs don't even have a
>>> pull-up for both pins. Therefore don't enforce open-drain
>>> if SDA and SCL both are unidirectional. This patch makes
>>> i2c-gpio usable with such devices, based on new DT property
>>> i2c-gpio,sda-output-only.
>>>
>>> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
>>> ---
>>> v3:
>>> - improve description of attribute sda_is_output_only
>>> ---
>>> drivers/i2c/busses/i2c-gpio.c | 14 +++++++++++---
>>> include/linux/platform_data/i2c-gpio.h | 3 +++
>>> 2 files changed, 14 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/i2c/busses/i2c-gpio.c b/drivers/i2c/busses/i2c-gpio.c
>>> index 0e4385a9b..ea108d7e4 100644
>>> --- a/drivers/i2c/busses/i2c-gpio.c
>>> +++ b/drivers/i2c/busses/i2c-gpio.c
>>> @@ -316,6 +316,8 @@ static void of_i2c_gpio_get_props(struct device_node *np,
>>> of_property_read_bool(np, "i2c-gpio,scl-open-drain");
>>> pdata->scl_is_output_only =
>>> of_property_read_bool(np, "i2c-gpio,scl-output-only");
>>> + pdata->sda_is_output_only =
>>> + of_property_read_bool(np, "i2c-gpio,sda-output-only");
>>> }
>>>
>>> static struct gpio_desc *i2c_gpio_get_desc(struct device *dev,
>>> @@ -363,6 +365,7 @@ static int i2c_gpio_probe(struct platform_device *pdev)
>>> struct device *dev = &pdev->dev;
>>> struct device_node *np = dev->of_node;
>>> enum gpiod_flags gflags;
>>> + bool sda_scl_output_only;
>>> int ret;
>>>
>>> priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
>>> @@ -391,8 +394,12 @@ static int i2c_gpio_probe(struct platform_device *pdev)
>>> * marking these lines to be handled as open drain, and we should just
>>> * handle them as we handle any other output. Else we enforce open
>>> * drain as this is required for an I2C bus.
>>> + * If SCL/SDA both are write-only, then this indicates I2C-like slaves
>>> + * with read-only SCL/SDA. Such slaves don't need open-drain, and partially
>>> + * don't even work with open-drain.
>>> */
>>> - if (pdata->sda_is_open_drain)
>>> + sda_scl_output_only = pdata->sda_is_output_only && pdata->scl_is_output_only;
>>> + if (pdata->sda_is_open_drain || sda_scl_output_only)
>>
>> I have not looked closely, but I see no strong reason to tie the SCL
>> output-only property to the flags of SDA?
>>
>
> Maybe I best start with explaining my use case. It's about a slave device that has
> read-only SDA/SCL and is connected to SDA/SCL GPIO's w/o pull-up resistor.
> If the GPIO's are configured as open-drain the slave device doesn't work.
> Maybe the slave device SDA/SCL pins have an internal pull-low, I don't know.
> So I have to ensure that GPIOD_OUT_HIGH is used for both GPIO's.
>
> I have some doubt that it's safe to use the following. The master may pull SCL
> high whilst a clock stretching capable slave pulls SCL low.
>
> if (pdata->scl_is_open_drain || pdata->scl_is_output_only)
> gflags = GPIOD_OUT_HIGH;
> else
> gflags = GPIOD_OUT_HIGH_OPEN_DRAIN;
>
> I could use the i2c-gpio,scl-open-drain property that does what I need,
> but just by chance and it would be misleading to (mis-)use it.
>
> Maybe add a property i2c-gpio,scl-no-pullup that de-facto is an alias for
> i2c-gpio,scl-open-drain?
>
> My patch was based on the assumption that if both, SDA and SCL, are write-only,
> then only slaves with read-only SDA/SCL are attached.
> I can't imagine a meaningful setup where this assumption isn't true.

Oh. I misread the patch, sorry for the confusion. But now that I see that
you avoid open-drain and use push-pull simply because *both* SCL and SDA
are output-only, I have to object to the mixing of these issues. It may
fit your case, but imagine a case where pull-ups are used to provide
a lower voltage level on the I2C bus than what is normal for the master.
That could be useful, even if something is also making SDA/SCL write-
only. Perhaps exactly that low bus level make reading buggy? That case
would perhaps not be possible if the master then pushed SCL and SDA
higher than intended by the pull-ups?

So. Yes, I do agree that i2c-gpio,scl-open-drain and i2c-gpio,sda-open-drain
look very wrong when they are used to get rid of open-drain and am personally
for an alias. i2c-gpio,scl-no-pullup is perhaps not the best name as it
does not describe the hardware, but instead the hardware that isn't there?

IDK, perhaps i2c-gpio,scl-push-pull ? Is that too specific? Are there other
methods to drive a GPIO pin?
Hmm, whatever, I'm ok with i2c-gpio,scl-no-pullup too...

Mixing i2c-gpio,scl-open-drain with whatever new alias should probably be
forbidden even if they trigger the same code in the driver, since it makes
no sense to have neither open-drain AND push-pull, nor open-drain AND
no-pullup.

Cheers,
Peter

>>> gflags = GPIOD_OUT_HIGH;
>>> else
>>> gflags = GPIOD_OUT_HIGH_OPEN_DRAIN;
>>> @@ -400,7 +407,7 @@ static int i2c_gpio_probe(struct platform_device *pdev)
>>> if (IS_ERR(priv->sda))
>>> return PTR_ERR(priv->sda);
>>>
>>> - if (pdata->scl_is_open_drain)
>>> + if (pdata->scl_is_open_drain || sda_scl_output_only)
>>
>> Same here, why tie the SDA output-only property to the flags of SCL?
>>
>> Cheers,
>> Peter
>>
>>> gflags = GPIOD_OUT_HIGH;
>>> else
>>> gflags = GPIOD_OUT_HIGH_OPEN_DRAIN;
>>> @@ -418,7 +425,8 @@ static int i2c_gpio_probe(struct platform_device *pdev)
>>>
>>> if (!pdata->scl_is_output_only)
>>> bit_data->getscl = i2c_gpio_getscl;
>>> - bit_data->getsda = i2c_gpio_getsda;
>>> + if (!pdata->sda_is_output_only)
>>> + bit_data->getsda = i2c_gpio_getsda;
>>>
>>> if (pdata->udelay)
>>> bit_data->udelay = pdata->udelay;
>>> diff --git a/include/linux/platform_data/i2c-gpio.h b/include/linux/platform_data/i2c-gpio.h
>>> index a907774fd..e9536c078 100644
>>> --- a/include/linux/platform_data/i2c-gpio.h
>>> +++ b/include/linux/platform_data/i2c-gpio.h
>>> @@ -16,6 +16,8 @@
>>> * isn't actively driven high when setting the output value high.
>>> * gpio_get_value() must return the actual pin state even if the
>>> * pin is configured as an output.
>>> + * @sda_is_output_only: SDA output drivers can't be turned off.
>>> + * This is for clients that can only read SDA/SCL.
>>> * @scl_is_open_drain: SCL is set up as open drain. Same requirements
>>> * as for sda_is_open_drain apply.
>>> * @scl_is_output_only: SCL output drivers cannot be turned off.
>>> @@ -24,6 +26,7 @@ struct i2c_gpio_platform_data {
>>> int udelay;
>>> int timeout;
>>> unsigned int sda_is_open_drain:1;
>>> + unsigned int sda_is_output_only:1;
>>> unsigned int scl_is_open_drain:1;
>>> unsigned int scl_is_output_only:1;
>>> };
>