diff mbox series

[v1] Input: ads7846: do not overwrite spi->mode flags set by spi framework

Message ID 20201021090434.16387-1-o.rempel@pengutronix.de
State New
Headers show
Series [v1] Input: ads7846: do not overwrite spi->mode flags set by spi framework | expand

Commit Message

Oleksij Rempel Oct. 21, 2020, 9:04 a.m. UTC
Do not overwrite spi->mode flags set by spi framework, otherwise the
chip select polarity will get lost.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 drivers/input/touchscreen/ads7846.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Ardelean, Alexandru Oct. 21, 2020, 9:29 a.m. UTC | #1
> -----Original Message-----
> From: Oleksij Rempel <o.rempel@pengutronix.de>
> Sent: Wednesday, October 21, 2020 12:05 PM
> To: Dmitry Torokhov <dmitry.torokhov@gmail.com>; Ardelean, Alexandru
> <alexandru.Ardelean@analog.com>
> Cc: Oleksij Rempel <o.rempel@pengutronix.de>; kernel@pengutronix.de; linux-
> kernel@vger.kernel.org; linux-input@vger.kernel.org; David Jander
> <david@protonic.nl>
> Subject: [PATCH v1] Input: ads7846: do not overwrite spi->mode flags set by spi
> framework
> 
> [External]
> 
> Do not overwrite spi->mode flags set by spi framework, otherwise the chip
> select polarity will get lost.
> 
> Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
> ---
>  drivers/input/touchscreen/ads7846.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/input/touchscreen/ads7846.c
> b/drivers/input/touchscreen/ads7846.c
> index 8fd7fc39c4fd..ea31956f3a90 100644
> --- a/drivers/input/touchscreen/ads7846.c
> +++ b/drivers/input/touchscreen/ads7846.c
> @@ -1288,7 +1288,7 @@ static int ads7846_probe(struct spi_device *spi)
>  	 * may not.  So we stick to very-portable 8 bit words, both RX and TX.
>  	 */
>  	spi->bits_per_word = 8;
> -	spi->mode = SPI_MODE_0;

I think the patch is incorrect.
The initial version is correct; assuming that the datasheet says that this driver operates in mode 0.
If the initial mode is incorrect, maybe we need to change that.

What is unfortunate, is that you cannot [yet] override the mode parameters [polarity & phase] from the device-tree, in case there are some things in-between the SPI controller & SPI chip [level inverters for example].
I was planning to do something for this.

> +	spi->mode |= SPI_MODE_0;
>  	err = spi_setup(spi);
>  	if (err < 0)
>  		return err;
> --
> 2.28.0
Oleksij Rempel Oct. 21, 2020, 10:56 a.m. UTC | #2
On Wed, Oct 21, 2020 at 09:29:35AM +0000, Ardelean, Alexandru wrote:
> 
> 
> > -----Original Message-----
> > From: Oleksij Rempel <o.rempel@pengutronix.de>
> > Sent: Wednesday, October 21, 2020 12:05 PM
> > To: Dmitry Torokhov <dmitry.torokhov@gmail.com>; Ardelean, Alexandru
> > <alexandru.Ardelean@analog.com>
> > Cc: Oleksij Rempel <o.rempel@pengutronix.de>; kernel@pengutronix.de; linux-
> > kernel@vger.kernel.org; linux-input@vger.kernel.org; David Jander
> > <david@protonic.nl>
> > Subject: [PATCH v1] Input: ads7846: do not overwrite spi->mode flags set by spi
> > framework
> > 
> > [External]
> > 
> > Do not overwrite spi->mode flags set by spi framework, otherwise the chip
> > select polarity will get lost.
> > 
> > Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
> > ---
> >  drivers/input/touchscreen/ads7846.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/input/touchscreen/ads7846.c
> > b/drivers/input/touchscreen/ads7846.c
> > index 8fd7fc39c4fd..ea31956f3a90 100644
> > --- a/drivers/input/touchscreen/ads7846.c
> > +++ b/drivers/input/touchscreen/ads7846.c
> > @@ -1288,7 +1288,7 @@ static int ads7846_probe(struct spi_device *spi)
> >  	 * may not.  So we stick to very-portable 8 bit words, both RX and TX.
> >  	 */
> >  	spi->bits_per_word = 8;
> > -	spi->mode = SPI_MODE_0;
> 
> I think the patch is incorrect.
> The initial version is correct; assuming that the datasheet says that this driver operates in mode 0.
> If the initial mode is incorrect, maybe we need to change that.
> 
> What is unfortunate, is that you cannot [yet] override the mode parameters [polarity & phase] from the device-tree, in case there are some things in-between the SPI controller & SPI chip [level inverters for example].
> I was planning to do something for this.

Current kernel (v5.9) is doing following work:

  of_register_spi_device()
    of_spi_parse_dt()
      /* this will parse dt and set different flags in spi->mode
       * all of this flags are dropped by this driver
       */

        ...... and here we parse gpio properties ......

	/*
	 * For descriptors associated with the device, polarity inversion is
	 * handled in the gpiolib, so all gpio chip selects are "active high"
	 * in the logical sense, the gpiolib will invert the line if need be.
	 */
	if ((ctlr->use_gpio_descriptors) && ctlr->cs_gpiods &&
	    ctlr->cs_gpiods[spi->chip_select])
		spi->mode |= SPI_CS_HIGH;
        -------->  ^^^^^^^^
	as you can see, if we use gpio as chip select, then the SPI_CS_HIGH
	flag is set. And gpiolib make all needed level conversation.

Since this diver is removing SPI_CS_HIGH flag, we have fallowing call
sequence:

spi_set_cs(, enable)                                        <---- 1
  ....
  if (spi->cs_gpiod || gpio_is_valid(spi->cs_gpio)) {
     ....
     gpiod_set_value_cansleep(, !enable);                   <---- 0
       gpiod_set_value_nocheck()
         if (test_bit(FLAG_ACTIVE_LOW, &desc->flags))
	   value = !value;                                  <---- 1


So, at the end, we set GPIO to 1, even if DTS configured it as ACTIVE_LOW.
You may probably suggest to set gpio in DTS to active ACTIVE_HIGH. In
this case we would run it to following snippet:
of_get_named_gpiod_flags()
  of_gpio_flags_quirks()
    if (IS_ENABLED(CONFIG_SPI_MASTER) && !strcmp(propname, "cs-gpios")..
	/*
	 * SPI children have active low chip selects
	 * by default. This can be specified negatively
	 * by just omitting "spi-cs-high" in the
	 * device node, or actively by tagging on
	 * GPIO_ACTIVE_LOW as flag in the device
	 * tree. If the line is simultaneously
	 * tagged as active low in the device tree
	 * and has the "spi-cs-high" set, we get a
	 * conflict and the "spi-cs-high" flag will
	 * take precedence.
	 */
	if (of_property_read_bool(child, "spi-cs-high")) {
		if (*flags & OF_GPIO_ACTIVE_LOW) {
			pr_warn("%s GPIO handle specifies active low - ignored\n",
				of_node_full_name(child));
			*flags &= ~OF_GPIO_ACTIVE_LOW;
		}
	} else {
		if (!(*flags & OF_GPIO_ACTIVE_LOW))
			pr_info("%s enforce active low on chipselect handle\n",
				of_node_full_name(child));
		*flags |= OF_GPIO_ACTIVE_LOW;
	}

As you can see, I would need to configure my dts with spi-cs-high flag,
even if the hardware is actually ACTIVE_LOW. If I will go this way, I
would risk a regression as soon as this issue is fixed.

Since the spi framework is already parsing devicetree and set all needed
flags, I assume it is wrong to blindly drop all this flags in the
driver.

> > +	spi->mode |= SPI_MODE_0;
> >  	err = spi_setup(spi);
> >  	if (err < 0)
> >  		return err;
> > --
> > 2.28.0

Regards,
Oleksij
Dmitry Torokhov Oct. 21, 2020, 6:27 p.m. UTC | #3
On Wed, Oct 21, 2020 at 12:56:14PM +0200, Oleksij Rempel wrote:
> 
> As you can see, I would need to configure my dts with spi-cs-high flag,
> even if the hardware is actually ACTIVE_LOW. If I will go this way, I
> would risk a regression as soon as this issue is fixed.
> 
> Since the spi framework is already parsing devicetree and set all needed
> flags, I assume it is wrong to blindly drop all this flags in the
> driver.

Yes, but I wonder if the devices can only work in mode 0 we should be
doing:

	spi->mode &= ~SPI_MODE_MASK; // to be defined as 0x03 in spi.h
	spi->mode |= SPI_MODE_0;

as we can't simply "or" mode value as is (well, mode 0 is kind of
working, but just on accident).

Thanks.
Oleksij Rempel Oct. 22, 2020, 6:54 a.m. UTC | #4
On Wed, Oct 21, 2020 at 11:27:57AM -0700, Dmitry Torokhov wrote:
> On Wed, Oct 21, 2020 at 12:56:14PM +0200, Oleksij Rempel wrote:
> > 
> > As you can see, I would need to configure my dts with spi-cs-high flag,
> > even if the hardware is actually ACTIVE_LOW. If I will go this way, I
> > would risk a regression as soon as this issue is fixed.
> > 
> > Since the spi framework is already parsing devicetree and set all needed
> > flags, I assume it is wrong to blindly drop all this flags in the
> > driver.
> 
> Yes, but I wonder if the devices can only work in mode 0 we should be
> doing:
> 
> 	spi->mode &= ~SPI_MODE_MASK; // to be defined as 0x03 in spi.h
> 	spi->mode |= SPI_MODE_0;
> 
> as we can't simply "or" mode value as is

Why not? This values are taken from device tree. If some developer
decided to add them, then driver should take it over. Even if this
values will break the functionality.

Other properties of this driver will break the functionality too of this
driver too, so why should we silently filter only set of this bits?

> (well, mode 0 is kind of working, but just on accident).

Good question, will be probably a good reason to measure it.

> Thanks.
> 
> -- 
> Dmitry
> 
> 

Regards,
Oleksij
Dmitry Torokhov Oct. 27, 2020, 3:53 a.m. UTC | #5
On Thu, Oct 22, 2020 at 08:54:02AM +0200, Oleksij Rempel wrote:
> On Wed, Oct 21, 2020 at 11:27:57AM -0700, Dmitry Torokhov wrote:
> > On Wed, Oct 21, 2020 at 12:56:14PM +0200, Oleksij Rempel wrote:
> > > 
> > > As you can see, I would need to configure my dts with spi-cs-high flag,
> > > even if the hardware is actually ACTIVE_LOW. If I will go this way, I
> > > would risk a regression as soon as this issue is fixed.
> > > 
> > > Since the spi framework is already parsing devicetree and set all needed
> > > flags, I assume it is wrong to blindly drop all this flags in the
> > > driver.
> > 
> > Yes, but I wonder if the devices can only work in mode 0 we should be
> > doing:
> > 
> > 	spi->mode &= ~SPI_MODE_MASK; // to be defined as 0x03 in spi.h
> > 	spi->mode |= SPI_MODE_0;
> > 
> > as we can't simply "or" mode value as is
> 
> Why not? This values are taken from device tree. If some developer
> decided to add them, then driver should take it over. Even if this
> values will break the functionality.
> 
> Other properties of this driver will break the functionality too of this
> driver too, so why should we silently filter only set of this bits?

What I was trying to say is that if driver wants to set mode to
particular value it should not "or" the value, as it will not reset the
relevant bits. I.e. if there some undesirable data in spi->mode mode
bits it will not get set properly by essentially doing "spi->mode |= 0".
That is why I said the driver needs to clear mode bits and set them to
the desired mode.

Thanks.
Oleksij Rempel Oct. 27, 2020, 8:07 a.m. UTC | #6
On Mon, Oct 26, 2020 at 08:53:09PM -0700, Dmitry Torokhov wrote:
> On Thu, Oct 22, 2020 at 08:54:02AM +0200, Oleksij Rempel wrote:
> > On Wed, Oct 21, 2020 at 11:27:57AM -0700, Dmitry Torokhov wrote:
> > > On Wed, Oct 21, 2020 at 12:56:14PM +0200, Oleksij Rempel wrote:
> > > > 
> > > > As you can see, I would need to configure my dts with spi-cs-high flag,
> > > > even if the hardware is actually ACTIVE_LOW. If I will go this way, I
> > > > would risk a regression as soon as this issue is fixed.
> > > > 
> > > > Since the spi framework is already parsing devicetree and set all needed
> > > > flags, I assume it is wrong to blindly drop all this flags in the
> > > > driver.
> > > 
> > > Yes, but I wonder if the devices can only work in mode 0 we should be
> > > doing:
> > > 
> > > 	spi->mode &= ~SPI_MODE_MASK; // to be defined as 0x03 in spi.h
> > > 	spi->mode |= SPI_MODE_0;
> > > 
> > > as we can't simply "or" mode value as is
> > 
> > Why not? This values are taken from device tree. If some developer
> > decided to add them, then driver should take it over. Even if this
> > values will break the functionality.
> > 
> > Other properties of this driver will break the functionality too of this
> > driver too, so why should we silently filter only set of this bits?
> 
> What I was trying to say is that if driver wants to set mode to
> particular value it should not "or" the value, as it will not reset the
> relevant bits. I.e. if there some undesirable data in spi->mode mode
> bits it will not get set properly by essentially doing "spi->mode |= 0".
> That is why I said the driver needs to clear mode bits and set them to
> the desired mode.

Ok, i'll update this patch as you suggested.

Regards,
Oleksij
diff mbox series

Patch

diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c
index 8fd7fc39c4fd..ea31956f3a90 100644
--- a/drivers/input/touchscreen/ads7846.c
+++ b/drivers/input/touchscreen/ads7846.c
@@ -1288,7 +1288,7 @@  static int ads7846_probe(struct spi_device *spi)
 	 * may not.  So we stick to very-portable 8 bit words, both RX and TX.
 	 */
 	spi->bits_per_word = 8;
-	spi->mode = SPI_MODE_0;
+	spi->mode |= SPI_MODE_0;
 	err = spi_setup(spi);
 	if (err < 0)
 		return err;