Re: [PATCH 05/10] spi: spi-dw-mmio: Spin off MSCC platforms into spi-dw-mchp

[Serge Semin <Sergey.Semin@baikalelectronics.ru>]

On Tue, May 19, 2020 at 02:05:19PM +0200, Lars Povlsen wrote:
> On 13/05/20 16:18, Mark Brown wrote:
> > Date: Wed, 13 May 2020 16:18:11 +0100
> > From: Mark Brown <broonie@kernel.org>
> > To: Lars Povlsen <lars.povlsen@microchip.com>
> > Cc: SoC Team <soc@kernel.org>, Microchip Linux Driver Support
> >  <UNGLinuxDriver@microchip.com>, linux-spi@vger.kernel.org,
> >  devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
> >  linux-arm-kernel@lists.infradead.org, Alexandre Belloni
> >  <alexandre.belloni@bootlin.com>
> > Subject: Re: [PATCH 05/10] spi: spi-dw-mmio: Spin off MSCC platforms into
> >  spi-dw-mchp
> > User-Agent: Mutt/1.10.1 (2018-07-13)
> > 
> > On Wed, May 13, 2020 at 04:00:26PM +0200, Lars Povlsen wrote:
> > 
> > > +config SPI_DW_MCHP
> > > +	tristate "Memory-mapped io interface driver using DW SPI core of MSCC SoCs"
> > > +	default y if ARCH_SPARX5
> > > +	default y if SOC_VCOREIII
> > 
> > Why the default ys?
> 
> The SoC will typically boot from SPI... But its not a requirement per
> se. I will remove it.
> 
> > 
> > > +++ b/drivers/spi/Makefile
> > > @@ -37,6 +37,7 @@ obj-$(CONFIG_SPI_DAVINCI)		+= spi-davinci.o
> > >  obj-$(CONFIG_SPI_DLN2)			+= spi-dln2.o
> > >  obj-$(CONFIG_SPI_DESIGNWARE)		+= spi-dw.o
> > >  obj-$(CONFIG_SPI_DW_MMIO)		+= spi-dw-mmio.o
> > > +obj-$(CONFIG_SPI_DW_MCHP)		+= spi-dw-mchp.o
> > >  obj-$(CONFIG_SPI_DW_PCI)		+= spi-dw-midpci.o
> > >  spi-dw-midpci-objs			:= spi-dw-pci.o spi-dw-mid.o
> > >  obj-$(CONFIG_SPI_EFM32)			+= spi-efm32.o
> > 
> > Please keep the file alphabetically sorted.
> > 
> 
> Noted.
> 
> > > +++ b/drivers/spi/spi-dw-mchp.c
> > > @@ -0,0 +1,232 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> > > +/*
> > > + * Memory-mapped interface driver for MSCC SoCs
> > > + *
> > 
> > Please make the entire comment a C++ one so things look more
> > intentional.
> 
> Sure, I can do that. The presented form matches that of the other
> spi-dw-* drivers, but I can see other using // blocks. Ack.
> 
> > 
> > > +#define MAX_CS		4
> > 
> > This should be namespaced.
> 
> Ack.
> 

> > 
> > > +	rx_sample_dly = 0;
> > > +	device_property_read_u32(&pdev->dev, "spi-rx-delay-us", &rx_sample_dly);
> > > +	dws->rx_sample_dly = DIV_ROUND_UP(rx_sample_dly,
> > > +					  (dws->max_freq / 1000000));

Perhaps 100000 is better to be replace with macro USEC_PER_SEC...

Moreover are you sure the formulae is correct?
dws->rx_sample_dly - a number of ssi_clk periods/cycles to delay the Rx-data sample,
dws->max_freq - ssi_clk frequency (not period).

In real math the formulae would look like:
S = d * P [s], where d - number of delay cycles, P - ssi_clk period in seconds,
S - requested delay in seconds.
In the driver notation: d = dws->rx_sample_dly, P = 1 / dws->max_freq,
S = rx_sample_dly ("spi-rx-delay-us" property value).

dws->rx_sample_dly * (1 / dws->max_freq) = rx_sample_dly <=>
dws->rx_sample_dly = rx_sample_dly * dws->max_freq.

Though that's represented in seconds, so if rx_sample_dly is specified in usec,
then you'd need to scale it down dividing by USEC_PER_SEC.

For example, imagine we need a delay of 1 usec with ssi_clk of 50MHz.
By your formulae we'd have: 1 / (50000000 / 1000000) = 0 cycles (actually 1 due
to DIV_ROUND_UP, but incorrect anyway),
By mine: 1 * (500000000 / 1000000) = 50 cycles. Seems closer to reality.

Am I missing something?

> > 
> > If this is a standard feature of the DesignWare IP why parse it here and
> > not in the generic code?
> 
> This is a standard feature of the DesignWare IP, so good suggestion. I
> will arrange with Serge.

Regarding "spi-rx-delay-us" and the sampling delay the IP supports. Here is what
documentation says regarding the register, which is then initialized with this
parameter "This register controls the number of ssi_clk cycles that are
delayed from the default sample time before the actual sample of the rxd input
signal occurs." While the "spi-rx-delay-us" property is described as: "Delay, in
microseconds, after a read transfer." I may misunderstand something, but IMO
these descriptions don't refer to the same values. The only real use of the
"spi-rx-delay-us" property I've found in "./drivers/input/rmi4/rmi_spi.c".
That driver gets the value of the property and just sets the delay_usecs
of some transfers, which isn't even close to the functionality the RX_SAMPLE_DLY
register provides. 

To be clear the RX_SAMPLE_DLY register can be used to delay the RX-bits sample
with respect to the normal Rx sampling timing. The delay is measured in the 
numbers of the ssi_clk periods. (Note also that the maximum delay is limited
with a constant parameter pre-initialized at the IP-core synthesis stage. It can
be defined within a range [4, 255]. In our IP it's limited with just 4 periods.)

As I see it, a better way would be to either define a new vendor-specific
property like "snps,rx-sample-delay-ns" (note NS here, since normally the
ssi_clk is much higher than 1MHz), or define a new generic SPI property.
Mark, Andy?

-Sergey

> 
> Thank you for your comments!
> 
> ---Lars
> 
> 
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