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Date:   Mon, 6 Sep 2021 19:34:13 +0200
From:   Miquel Raynal <miquel.raynal@bootlin.com>
To:     Jonathan Cameron <jic23@kernel.org>
Cc:     "Sa, Nuno" <Nuno.Sa@analog.com>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
        "linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 03/16] iio: adc: max1027: Push only the requested
 samples
Message-ID: <20210906193413.1892f0c8@xps13>
In-Reply-To: <20210906175657.62cb0bc6@jic23-huawei>
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Hi Jonathan,

jic23@kernel.org wrote on Mon, 6 Sep 2021 17:56:57 +0100:

> On Mon, 6 Sep 2021 08:59:55 +0000
> "Sa, Nuno" <Nuno.Sa@analog.com> wrote:
> 
> > > -----Original Message-----
> > > From: Miquel Raynal <miquel.raynal@bootlin.com>
> > > Sent: Wednesday, September 1, 2021 10:12 AM
> > > To: Sa, Nuno <Nuno.Sa@analog.com>
> > > Cc: Jonathan Cameron <jic23@kernel.org>; Lars-Peter Clausen
> > > <lars@metafoo.de>; Thomas Petazzoni
> > > <thomas.petazzoni@bootlin.com>; linux-iio@vger.kernel.org; linux-
> > > kernel@vger.kernel.org
> > > Subject: Re: [PATCH 03/16] iio: adc: max1027: Push only the requested
> > > samples
> > > 
> > > [External]
> > > 
> > > Hello,
> > > 
> > > "Sa, Nuno" <Nuno.Sa@analog.com> wrote on Mon, 30 Aug 2021
> > > 15:02:26
> > > +0000:
> > >     
> > > > > -----Original Message-----
> > > > > From: Jonathan Cameron <jic23@kernel.org>
> > > > > Sent: Monday, August 30, 2021 4:30 PM
> > > > > To: Sa, Nuno <Nuno.Sa@analog.com>
> > > > > Cc: Miquel Raynal <miquel.raynal@bootlin.com>; Lars-Peter    
> > > Clausen    
> > > > > <lars@metafoo.de>; Thomas Petazzoni
> > > > > <thomas.petazzoni@bootlin.com>; linux-iio@vger.kernel.org;    
> > > linux-    
> > > > > kernel@vger.kernel.org
> > > > > Subject: Re: [PATCH 03/16] iio: adc: max1027: Push only the    
> > > requested    
> > > > > samples
> > > > >
> > > > > [External]
> > > > >
> > > > > On Mon, 30 Aug 2021 10:49:50 +0000
> > > > > "Sa, Nuno" <Nuno.Sa@analog.com> wrote:
> > > > >    
> > > > > > > -----Original Message-----
> > > > > > > From: Jonathan Cameron <jic23@kernel.org>
> > > > > > > Sent: Monday, August 30, 2021 12:08 PM
> > > > > > > To: Sa, Nuno <Nuno.Sa@analog.com>
> > > > > > > Cc: Miquel Raynal <miquel.raynal@bootlin.com>; Lars-Peter    
> > > > > Clausen    
> > > > > > > <lars@metafoo.de>; Thomas Petazzoni
> > > > > > > <thomas.petazzoni@bootlin.com>; linux-iio@vger.kernel.org;    
> > > > > linux-    
> > > > > > > kernel@vger.kernel.org
> > > > > > > Subject: Re: [PATCH 03/16] iio: adc: max1027: Push only the    
> > > > > requested    
> > > > > > > samples
> > > > > > >
> > > > > > > [External]
> > > > > > >
> > > > > > > On Fri, 20 Aug 2021 07:10:48 +0000
> > > > > > > "Sa, Nuno" <Nuno.Sa@analog.com> wrote:
> > > > > > >    
> > > > > > > > > -----Original Message-----
> > > > > > > > > From: Miquel Raynal <miquel.raynal@bootlin.com>
> > > > > > > > > Sent: Wednesday, August 18, 2021 1:11 PM
> > > > > > > > > To: Jonathan Cameron <jic23@kernel.org>; Lars-Peter    
> > > Clausen    
> > > > > > > > > <lars@metafoo.de>
> > > > > > > > > Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>;    
> > > linux-    
> > > > > > > > > iio@vger.kernel.org; linux-kernel@vger.kernel.org; Miquel    
> > > > > Raynal    
> > > > > > > > > <miquel.raynal@bootlin.com>
> > > > > > > > > Subject: [PATCH 03/16] iio: adc: max1027: Push only the    
> > > > > requested    
> > > > > > > > > samples
> > > > > > > > >
> > > > > > > > > [External]
> > > > > > > > >
> > > > > > > > > When a triggered scan occurs, the identity of the desired    
> > > > > channels    
> > > > > > > is    
> > > > > > > > > known in indio_dev->active_scan_mask. Instead of reading    
> > > and    
> > > > > > > > > pushing to
> > > > > > > > > the IIO buffers all channels each time, scan the minimum    
> > > > > amount    
> > > > > > > of    
> > > > > > > > > channels (0 to maximum requested chan, to be exact) and    
> > > only    
> > > > > > > > > provide the
> > > > > > > > > samples requested by the user.
> > > > > > > > >
> > > > > > > > > For example, if the user wants channels 1, 4 and 5, all    
> > > channels    
> > > > > > > from    
> > > > > > > > > 0 to 5 will be scanned but only the desired channels will be    
> > > > > pushed    
> > > > > > > to    
> > > > > > > > > the IIO buffers.
> > > > > > > > >
> > > > > > > > > Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
> > > > > > > > > ---
> > > > > > > > >  drivers/iio/adc/max1027.c | 25 +++++++++++++++++++++-    
> > > ---    
> > > > > > > > >  1 file changed, 21 insertions(+), 4 deletions(-)
> > > > > > > > >
> > > > > > > > > diff --git a/drivers/iio/adc/max1027.c    
> > > > > b/drivers/iio/adc/max1027.c    
> > > > > > > > > index b753658bb41e..8ab660f596b5 100644
> > > > > > > > > --- a/drivers/iio/adc/max1027.c
> > > > > > > > > +++ b/drivers/iio/adc/max1027.c
> > > > > > > > > @@ -360,6 +360,9 @@ static int    
> > > > > max1027_set_trigger_state(struct    
> > > > > > > > > iio_trigger *trig, bool state)
> > > > > > > > >  	struct max1027_state *st = iio_priv(indio_dev);
> > > > > > > > >  	int ret;
> > > > > > > > >
> > > > > > > > > +	if (bitmap_empty(indio_dev->active_scan_mask,    
> > > > > indio_dev-    
> > > > > > > > > >masklength))    
> > > > > > > > > +		return -EINVAL;
> > > > > > > > > +    
> > > > > > > >
> > > > > > > > I'm not sure this can actually happen. If you try to enable the    
> > > > > buffer    
> > > > > > > > with no scan element, it should give you an error before you    
> > > > > reach    
> > > > > > > > this point...
> > > > > > > >    
> > > > > > > > >  	if (state) {
> > > > > > > > >  		/* Start acquisition on cnvst */
> > > > > > > > >  		st->reg = MAX1027_SETUP_REG |
> > > > > > > > > MAX1027_CKS_MODE0 |
> > > > > > > > > @@ -368,9 +371,12 @@ static int    
> > > > > max1027_set_trigger_state(struct    
> > > > > > > > > iio_trigger *trig, bool state)
> > > > > > > > >  		if (ret < 0)
> > > > > > > > >  			return ret;
> > > > > > > > >
> > > > > > > > > -		/* Scan from 0 to max */
> > > > > > > > > -		st->reg = MAX1027_CONV_REG |    
> > > > > MAX1027_CHAN(0) |    
> > > > > > > > > -			  MAX1027_SCAN_N_M |    
> > > > > MAX1027_TEMP;    
> > > > > > > > > +		/*
> > > > > > > > > +		 * Scan from 0 to the highest requested    
> > > > > channel. The    
> > > > > > > > > temperature
> > > > > > > > > +		 * could be avoided but it simplifies a bit the    
> > > > > logic.    
> > > > > > > > > +		 */
> > > > > > > > > +		st->reg = MAX1027_CONV_REG |
> > > > > > > > > MAX1027_SCAN_0_N | MAX1027_TEMP;
> > > > > > > > > +		st->reg |= MAX1027_CHAN(fls(*indio_dev-    
> > > > > > > > > >active_scan_mask) - 2);    
> > > > > > > > >  		ret = spi_write(st->spi, &st->reg, 1);
> > > > > > > > >  		if (ret < 0)
> > > > > > > > >  			return ret;
> > > > > > > > > @@ -391,11 +397,22 @@ static irqreturn_t
> > > > > > > > > max1027_trigger_handler(int irq, void *private)
> > > > > > > > >  	struct iio_poll_func *pf = private;
> > > > > > > > >  	struct iio_dev *indio_dev = pf->indio_dev;
> > > > > > > > >  	struct max1027_state *st = iio_priv(indio_dev);
> > > > > > > > > +	unsigned int scanned_chans = fls(*indio_dev-    
> > > > > > > > > >active_scan_mask);    
> > > > > > > > > +	u16 *buf = st->buffer;    
> > > > > > > >
> > > > > > > > I think sparse will complain here. buffer is a __be16 restricted
> > > > > > > > type so you should not mix those...    
> > > > > > > > > +	unsigned int bit;
> > > > > > > > >
> > > > > > > > >  	pr_debug("%s(irq=%d, private=0x%p)\n", __func__,    
> > > > > irq,    
> > > > > > > > >    
> > > > > > >    
> > > > >    
> > > private);in/20210818_miquel_raynal_bring_software_triggers_support    
> > > > > > > _to_max1027_like_adcs.mbx    
> > > > > > > > >
> > > > > > > > >  	/* fill buffer with all channel */
> > > > > > > > > -	spi_read(st->spi, st->buffer, indio_dev->masklength *    
> > > > > 2);    
> > > > > > > > > +	spi_read(st->spi, st->buffer, scanned_chans * 2);
> > > > > > > > > +
> > > > > > > > > +	/* Only keep the channels selected by the user */
> > > > > > > > > +	for_each_set_bit(bit, indio_dev->active_scan_mask,
> > > > > > > > > +			 indio_dev->masklength) {
> > > > > > > > > +		if (buf[0] != st->buffer[bit])
> > > > > > > > > +			buf[0] = st->buffer[bit];    
> > > > > > > >
> > > > > > > > Since we are here, when looking into the driver, I realized
> > > > > > > > that st->buffer is not DMA safe. In IIO, we kind of want to    
> > > > > enforce    
> > > > > > > > that all buffers that are passed to spi/i2c buses are safe...    
> > > Maybe    
> > > > > > > > this is something you can include in your series.    
> > > > > > >
> > > > > > > Why is it not?  st->buffer is result of a devm_kmalloc_array()    
> > > call    
> > > > > and    
> > > > > > > that should provide a DMA safe buffer as I understand it.
> > > > > > >    
> > > > > >
> > > > > > That's a good question. I'm not sure how I came to that    
> > > conclusion    
> > > > > which    
> > > > > > is clearly wrong. Though I think the buffer might share the line    
> > > with    
> > > > > the    
> > > > > > mutex...    
> > > > > Pointer shares a line.  The buffer it points to doesn't as allocated
> > > > > by separate heap allocation.
> > > > >    
> > > >
> > > > Ups, sure :facepalm:    
> > > 
> > > My understanding [1] was that devm_ allocations were generally not
> > > suitable for DMA and should not be used for this particular purpose
> > > because of the extra 16 bytes allocated for storing the devm magic
> > > somewhere, which shifts the entire buffer and prevents it to always
> > > be
> > > aligned on a cache line. I will propose a patch to switch to
> > > kmalloc_array() instead.    
> > 
> > I do not think this is a problem anymore [1]. Nowadays, 'devm_kmalloc'
> > should give you the same alignment guarantees as 'kmalloc'
> > 
> > [1]: https://elixir.bootlin.com/linux/latest/source/drivers/base/devres.c#L35  
> Great info. I remembered a discussion about fixing that, but couldn't find
> the patch.  For some reason I didn't just check the code :)

Nice! I didn't know about that, thanks a lot for sharing. So this patch
can be safely discarded then.

Thanks,
Miquèl