Message-ID: <30210c25d4ee878a3399caf758de9342d1f83f20.camel@gmail.com>
Date: Fri, 06 Sep 2024 07:54:26 +0200
From: Nuno Sá <noname.nuno@...il.com>
To: Angelo Dureghello <adureghello@...libre.com>, Jonathan Cameron
	 <jic23@...nel.org>
Cc: Lars-Peter Clausen <lars@...afoo.de>, Michael Hennerich
 <Michael.Hennerich@...log.com>, Nuno Sá
 <nuno.sa@...log.com>,  Rob Herring <robh@...nel.org>, Krzysztof Kozlowski
 <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>,  Olivier Moysan
 <olivier.moysan@...s.st.com>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org,  linux-kernel@...r.kernel.org,
 dlechner@...libre.com
Subject: Re: [PATCH RFC 3/8] iio: backend adi-axi-dac: backend features

On Thu, 2024-09-05 at 13:58 +0200, Angelo Dureghello wrote:
> Hi,
> 
> On 05/09/24 12:49 PM, Nuno Sá wrote:
> > On Tue, 2024-09-03 at 20:16 +0100, Jonathan Cameron wrote:
> > > On Mon, 2 Sep 2024 18:04:51 +0200
> > > Angelo Dureghello <adureghello@...libre.com> wrote:
> > > 
> > > > On 31/08/24 1:34 PM, Jonathan Cameron wrote:
> > > > > On Thu, 29 Aug 2024 14:32:01 +0200
> > > > > Angelo Dureghello <adureghello@...libre.com> wrote:
> > > > >   
> > > > > > From: Angelo Dureghello <adureghello@...libre.com>
> > > > > > 
> > > > > > Extend DAC backend with new features required for the AXI driver
> > > > > > version for the a3552r DAC.
> > > > > > 
> > > > > > Signed-off-by: Angelo Dureghello <adureghello@...libre.com>
> > > > > Hi Angelo
> > > > > Minor comments inline.
> > > > > >    
> > > > > >    static int axi_dac_enable(struct iio_backend *back)
> > > > > > @@ -460,7 +493,13 @@ static int axi_dac_data_source_set(struct
> > > > > > iio_backend *back, unsigned int chan,
> > > > > >    	case IIO_BACKEND_EXTERNAL:
> > > > > >    		return regmap_update_bits(st->regmap,
> > > > > >    					
> > > > > > AXI_DAC_REG_CHAN_CNTRL_7(chan),
> > > > > > -					  AXI_DAC_DATA_SEL,
> > > > > > AXI_DAC_DATA_DMA);
> > > > > > +					  AXI_DAC_DATA_SEL,
> > > > > > +					  AXI_DAC_DATA_DMA);
> > > > > Unrelated change.   If you want to change this, separate patch.
> > > > Thanks, fixed.
> > > > >   
> > > > > > +	case IIO_BACKEND_INTERNAL_RAMP_16:
> > > > > > +		return regmap_update_bits(st->regmap,
> > > > > > +					
> > > > > > AXI_DAC_REG_CHAN_CNTRL_7(chan),
> > > > > > +					  AXI_DAC_DATA_SEL,
> > > > > > +					
> > > > > > AXI_DAC_DATA_INTERNAL_RAMP_16);
> > > > > >    	default:
> > > > > >    		return -EINVAL;
> > > > > >    	}
> > > > > > @@ -518,9 +557,204 @@ static int axi_dac_reg_access(struct iio_backend
> > > > > > *back, unsigned int reg,
> > > > > >    	return regmap_write(st->regmap, reg, writeval);
> > > > > >    }
> > > > > >    
> > > > > > +
> > > > > > +static int axi_dac_bus_reg_write(struct iio_backend *back,
> > > > > > +				 u32 reg, void *val, size_t size)
> > > > > Maybe just pass an unsigned int for val?
> > > > > So follow what regmap does? You will still need the size, but it
> > > > > will just be configuration related rather than affecting the type
> > > > > of val.
> > > > >   
> > > > void * was used since data size in the future may vary depending
> > > > on the bus physical interface.
> > > > 
> > > I doubt it will get bigger than u64.  Passing void * is always
> > > nasty if we can do something else and this is a register writing
> > > operation.  I'm yet to meet an ADC or similar with > 64 bit registers
> > > (or even one with 64 bit ones!)
> > I think the original thinking was to support thinks like appending crc to the
> > register read/write. But even in that case, u32 for val might be enough. Not
> > sure. Anyways, as you often say with the backend stuff, this is all in the
> > kernel so I guess we can change it to unsigned int and change it in the future
> > if we need to.
> > 
> > Since you mentioned regmap, I also want to bring something that was discussed
> > before the RFC. Basically we talked about having the backend registering it's
> > own regmap_bus. Then we would either:
> > 
> > 1) Have a specific get_regmap_bus() callback for the frontend to initialize a
> > regmap on;
> > 2) Pass this bus into the core and have a new frontend API like
> > devm_iio_backend_regmap_init().
> > 
> > Then, on top of the API already provided by regmap (like _update_bit()), the
> > frontend could just use regmap independent of having a backend or not.
> > 
> > The current API is likely more generic but tbh (and David and Angelo are aware
> > of it) my preferred approach it to use the regmap_bus stuff. I just don't feel
> > that strong about it :)
> > 
> > > > Actually, a reg bus write involves several AXI regmap operations.
> > > > >   
> > > > > > +{
> > > > > > +	struct axi_dac_state *st = iio_backend_get_priv(back);
> > > > > > +
> > > > > > +	if (!st->bus_type)
> > > > > > +		return -EOPNOTSUPP;
> > > > > > +
> > > > > > +	if (st->bus_type == AXI_DAC_BUS_TYPE_QSPI) {
> > > > > As below, I'd use a switch and factor out this block as a separate
> > > > > bus specific function.
> > > > Ok, changed.
> > > > >   
> > > > > > +		int ret;
> > > > > > +		u32 ival;
> > > > > > +
> > > > > > +		if (size != 1 && size != 2)
> > > > > > +			return -EINVAL;
> > > > > > +
> > > > > > +		switch (size) {
> > > > > > +		case 1:
> > > > > > +			ival = FIELD_PREP(AXI_DAC_DATA_WR_8, *(u8
> > > > > > *)val);
> > > > > > +			break;
> > > > > > +		case 2:
> > > > > > +			ival =  FIELD_PREP(AXI_DAC_DATA_WR_16, *(u16
> > > > > > *)val);
> > > > > > +			break;
> > > > > > +		default:
> > > > > > +			return  -EINVAL;
> > > > > Hopefully compiler won't need this and the above. I'd drop the size != 1..
> > > > > check in favour of just doing it in this switch.
> > > > >   
> > > > sure, done.
> > > > 
> > > > 
> > > > > > +		}
> > > > > > +
> > > > > > +		ret = regmap_write(st->regmap, AXI_DAC_CNTRL_DATA_WR,
> > > > > > ival);
> > > > > > +		if (ret)
> > > > > > +			return ret;
> > > > > > +
> > > > > > +		/*
> > > > > > +		 * Both REG_CNTRL_2 and AXI_DAC_CNTRL_DATA_WR need to
> > > > > > know
> > > > > > +		 * the data size. So keeping data size control here
> > > > > > only,
> > > > > > +		 * since data size is mandatory for to the current
> > > > > > transfer.
> > > > > > +		 * DDR state handled separately by specific backend
> > > > > > calls,
> > > > > > +		 * generally all raw register writes are SDR.
> > > > > > +		 */
> > > > > > +		if (size == 1)
> > > > > > +			ret = regmap_set_bits(st->regmap,
> > > > > > AXI_DAC_REG_CNTRL_2,
> > > > > > +					      AXI_DAC_SYMB_8B);
> > > > > > +		else
> > > > > > +			ret = regmap_clear_bits(st->regmap,
> > > > > > AXI_DAC_REG_CNTRL_2,
> > > > > > +						AXI_DAC_SYMB_8B);
> > > > > > +		if (ret)
> > > > > > +			return ret;
> > > > > > +
> > > > > > +		ret = regmap_update_bits(st->regmap,
> > > > > > AXI_DAC_REG_CUSTOM_CTRL,
> > > > > > +					 AXI_DAC_ADDRESS,
> > > > > > +					 FIELD_PREP(AXI_DAC_ADDRESS,
> > > > > > reg));
> > > > > > +		if (ret)
> > > > > > +			return ret;
> > > > > > +
> > > > > > +		ret = regmap_update_bits(st->regmap,
> > > > > > AXI_DAC_REG_CUSTOM_CTRL,
> > > > > > +					 AXI_DAC_TRANSFER_DATA,
> > > > > > +					 AXI_DAC_TRANSFER_DATA);
> > > > > > +		if (ret)
> > > > > > +			return ret;
> > > > > > +
> > > > > > +		ret = regmap_read_poll_timeout(st->regmap,
> > > > > > +					       AXI_DAC_REG_CUSTOM_CTRL,
> > > > > > ival,
> > > > > > +					       ival &
> > > > > > AXI_DAC_TRANSFER_DATA,
> > > > > > +					       10, 100 * KILO);
> > > > > > +		if (ret)
> > > > > > +			return ret;
> > > > > > +
> > > > > > +		return regmap_clear_bits(st->regmap,
> > > > > > AXI_DAC_REG_CUSTOM_CTRL,
> > > > > > +					  AXI_DAC_TRANSFER_DATA);
> > > > > > +	}
> > > > > > +
> > > > > > +	return -EINVAL;
> > > > > > +}
> > > > > > +
> > > > > > +static int axi_dac_bus_reg_read(struct iio_backend *back,
> > > > > > +				u32 reg, void *val, size_t size)
> > > > > As for write, I'd just use an unsigned int * for val like
> > > > > regmap does.
> > > > Ok, so initial choice was unsigned int, further thinking of
> > > > possible future busses drive the choice to void *.
> > > > 
> > > > Let me know, i can switch to unsigned int in case.
> > > I would just go with unsigned int or at a push u64 *
> > > 
> > > > 
> > > > >   
> > > > > > +{
> > > > > > +	struct axi_dac_state *st = iio_backend_get_priv(back);
> > > > > > +
> > > > > > +	if (!st->bus_type)
> > > > > > +		return -EOPNOTSUPP;
> > > > > > +
> > > > > > +	if (st->bus_type == AXI_DAC_BUS_TYPE_QSPI) {
> > > > > It got mentioned in binding review but if this isn't QSPI, even
> > > > > if similar don't call it that.
> > > > It's a bit difficult to find a different name, physically,
> > > > it is a QSPI, 4 lanes + clock + cs, and datasheet is naming it Quad SPI.
> > > > But looking the data protocol, it's a bit different.
> > > is QSPI actually defined anywhere? I assumed it would be like
> > > SPI for which everything is so flexible you can build whatever you like.
> > > 
> > > > QSPI has instruction, address and data.
> > > > Here we have just ADDR and DATA.
> > > > 
> > I'm not sure the instruction is really relevant for this. From a quick look, it
> > feels like something used for accessing external flash memory like spi-nors. So,
> > I would not be surprised if things are just like Jonathan said and this is just
> > flexible as spi (being that extra instruction field a protocol defined for flash
> > memory - where one typically sees this interface)
> 
> Ok, so QSPI is the hardware, and the protocol on it may vary for the target
> chip/application.
> 
> Looks like DDR makes the 33MUPS rate reachable, and not all the controllers
> have DDR mode. Also some controllers are supposed to work with a QSPI flash
> (so with instructions), and likely this reason driven the need to use a 
> custom IP.
> 

I do understand the custom IP, I just don't understand why not using the spi_engine
IP. Indeed maybe because of DDR (as that is already supported on axi-dac).

- Nuno Sá

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