Re: [PATCH] iio: adc: ti-ads7950: Ensure CS is deasserted after reading channels

Re: [PATCH] iio: adc: ti-ads7950: Ensure CS is deasserted after reading channels

[Uwe Kleine-König]

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Hello,

Cc += Mark + linux-spi

On Fri, Jul 09, 2021 at 11:39:48AM -0500, David Lechner wrote:
> On 7/9/21 5:11 AM, Uwe Kleine-König wrote:
> > The ADS7950 requires that CS is deasserted after each SPI word. Before
> > commit e2540da86ef8 ("iio: adc: ti-ads7950: use SPI_CS_WORD to reduce
> > CPU usage") the driver used a message with one spi transfer per channel
> > where each but the last one had .cs_change set to enforce a CS toggle.
> > This was wrongly translated into a message with a single transfer and
> > .cs_change set which results in a CS toggle after each word but the
> > last which corrupts the first adc conversion of all readouts after the
> > first readout.
> > 
> > Fixes: e2540da86ef8 ("iio: adc: ti-ads7950: use SPI_CS_WORD to reduce CPU usage")
> > Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> > ---
> >   drivers/iio/adc/ti-ads7950.c | 1 -
> >   1 file changed, 1 deletion(-)
> > 
> > diff --git a/drivers/iio/adc/ti-ads7950.c b/drivers/iio/adc/ti-ads7950.c
> > index 2383eacada87..a2b83f0bd526 100644
> > --- a/drivers/iio/adc/ti-ads7950.c
> > +++ b/drivers/iio/adc/ti-ads7950.c
> > @@ -568,7 +568,6 @@ static int ti_ads7950_probe(struct spi_device *spi)
> >   	st->ring_xfer.tx_buf = &st->tx_buf[0];
> >   	st->ring_xfer.rx_buf = &st->rx_buf[0];
> >   	/* len will be set later */
> > -	st->ring_xfer.cs_change = true;
> >   	spi_message_add_tail(&st->ring_xfer, &st->ring_msg);
> > 
> 
> Yes, it seems like the SPI_CS_WORD flag should have replaced this (it's
> been too long, I can't remember if it was intentional). And removing it
> doesn't seem to break anything for me.

If it's not broken for you without my patch, your spi bus driver doesn't
honor .cs_change in the last transfer. Out of interest: Which bus are
you using? I wonder if the driver should refuse the request if it cannot
honer .cs_change?! (spi-imx does honor it only if gpios are used as chip
select, the native chip selects cannot do that.)

> Reviewed-by: David Lechner <david@lechnology.com>
> Tested-by: David Lechner <david@lechnology.com>

Thanks

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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Re: [PATCH] iio: adc: ti-ads7950: Ensure CS is deasserted after reading channels

[David Lechner]

On 7/10/21 7:18 AM, Uwe Kleine-König wrote:
> Hello,
> 
> Cc += Mark + linux-spi
> 
> On Fri, Jul 09, 2021 at 11:39:48AM -0500, David Lechner wrote:
>> On 7/9/21 5:11 AM, Uwe Kleine-König wrote:
>>> The ADS7950 requires that CS is deasserted after each SPI word. Before
>>> commit e2540da86ef8 ("iio: adc: ti-ads7950: use SPI_CS_WORD to reduce
>>> CPU usage") the driver used a message with one spi transfer per channel
>>> where each but the last one had .cs_change set to enforce a CS toggle.
>>> This was wrongly translated into a message with a single transfer and
>>> .cs_change set which results in a CS toggle after each word but the
>>> last which corrupts the first adc conversion of all readouts after the
>>> first readout.
>>>
>>> Fixes: e2540da86ef8 ("iio: adc: ti-ads7950: use SPI_CS_WORD to reduce CPU usage")
>>> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
>>> ---
>>>    drivers/iio/adc/ti-ads7950.c | 1 -
>>>    1 file changed, 1 deletion(-)
>>>
>>> diff --git a/drivers/iio/adc/ti-ads7950.c b/drivers/iio/adc/ti-ads7950.c
>>> index 2383eacada87..a2b83f0bd526 100644
>>> --- a/drivers/iio/adc/ti-ads7950.c
>>> +++ b/drivers/iio/adc/ti-ads7950.c
>>> @@ -568,7 +568,6 @@ static int ti_ads7950_probe(struct spi_device *spi)
>>>    	st->ring_xfer.tx_buf = &st->tx_buf[0];
>>>    	st->ring_xfer.rx_buf = &st->rx_buf[0];
>>>    	/* len will be set later */
>>> -	st->ring_xfer.cs_change = true;
>>>    	spi_message_add_tail(&st->ring_xfer, &st->ring_msg);
>>>
>>
>> Yes, it seems like the SPI_CS_WORD flag should have replaced this (it's
>> been too long, I can't remember if it was intentional). And removing it
>> doesn't seem to break anything for me.
> 
> If it's not broken for you without my patch, your spi bus driver doesn't
> honor .cs_change in the last transfer. Out of interest: Which bus are
> you using? I wonder if the driver should refuse the request if it cannot
> honer .cs_change?! (spi-imx does honor it only if gpios are used as chip
> select, the native chip selects cannot do that.)
> 


I'm using spi-davinci. It uses the standard spi_transfer_one_message()
which handles cs_change. But I suspect when the SPI_CS_WORD flag is set,
and the message is big enough to use DMA, the hardware is probably
automatically toggling CS after the last transfer before the cs_change
logic asserts it again.

So unless there is a valid use case where we need both SPI_CS_WORD
and cs_change, I don't think we need to fix spi-davinci.

Re: [PATCH] iio: adc: ti-ads7950: Ensure CS is deasserted after reading channels

[Mark Brown]

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On Sat, Jul 10, 2021 at 10:32:46AM -0500, David Lechner wrote:
> On 7/10/21 7:18 AM, Uwe Kleine-König wrote:

> > If it's not broken for you without my patch, your spi bus driver doesn't
> > honor .cs_change in the last transfer. Out of interest: Which bus are
> > you using? I wonder if the driver should refuse the request if it cannot
> > honer .cs_change?! (spi-imx does honor it only if gpios are used as chip
> > select, the native chip selects cannot do that.)

> I'm using spi-davinci. It uses the standard spi_transfer_one_message()
> which handles cs_change. But I suspect when the SPI_CS_WORD flag is set,
> and the message is big enough to use DMA, the hardware is probably
> automatically toggling CS after the last transfer before the cs_change
> logic asserts it again.

> So unless there is a valid use case where we need both SPI_CS_WORD
> and cs_change, I don't think we need to fix spi-davinci.

In theory it does mean something, in practice the number of controllers
that can implement it must be approaching zero.

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