Re: [Gta04-owner] [PATCH 0/4] UART slave device support - version 4

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Jan 15, 2016 at 04:05:45PM +0100, H. Nikolaus Schaller wrote:
> Hi Mark,
> 
> Am 15.01.2016 um 12:01 schrieb Mark Rutland <mark.rutland@arm.com>:
> 
> > On Fri, Jan 15, 2016 at 10:34:51AM +0100, H. Nikolaus Schaller wrote:
> >> Hi Mark,
> >> 
> >> Am 13.01.2016 um 20:15 schrieb Mark Rutland <mark.rutland@arm.com>:
> >> 
> >>> On Tue, Jan 12, 2016 at 02:28:00PM +0100, H. Nikolaus Schaller wrote:
> >>>> There is one point still to be solved: the exact style of the DT bindings.
> >>>> 
> >>>> We have an idea how a driver can implement two different styles (child node AND phandle)
> >>>> so that it is up to the DTS developer to use the one that best fits into the existing DTS.
> >>> 
> >>> From my perspective as a binding maintainer, and as I stated before, the
> >>> child node approach made the most sense and was most consistent with the
> >> 
> >>> way we handle other devices.
> >> 
> >> I simply don't see that this is the most common way other devices are handled.
> >> 
> >> I find many counter-examples which use phandles:
> >> * gpios
> >> * regulators
> >> * iio channels used by other drivers (e.g. iio-hwmon)
> >> * phy devices
> >> * timers
> >> * pwms
> >> * interrupts
> >> * dma
> > 
> > As was previously described to you, in these cases phandles are used
> > when these are _resources_ used by another device, not for the main
> > programmer-visible interface to the device.
> 
> Ah, I think I finally begin to understand the rule you are following:
> 
> 	If a device's data interface can be seen in user space, this interface
> 	is sort of a "main interface" and must be modelled in DT by a
> 	parent-child relationship.

No, you have misunderstood. This has _nothing_ to do with userspace.

This has everything to do with the "programmer's interface" as you would
find documented in a TRM -- effectively where a driver would communicate
with the device (e.g. MMIO/SPI/I2C registers).

[...]

> Now I also think I better understand what you meant by "main interface"
> a while ago.
> 
> For me, when looking into a chip data sheet, the main interface is a sometimes
> arbitrary. Or when viewing it through device driver implementors glasses
> I may end up with a different main interface.
> 
> From the hardware schematics, I can't read which interface is "main". I can
> only read which components and signals are connected. So the information
> what is "main" must come from somewhere else, but not the hardware.
> 
> You appear to have a definition based on Linux user space interfaces
> which is distinct from mine and at least explains why the discussion takes
> so long and we don't come to a common view.

This is nothing to do with userspace.

Admittedly, on a device which has multiple slave interfaces, "main" is
arbitrary. If I've followed correctly, that's not the cae here.

> > Conceptually, A UART slave is far closer to SPI or I2C, where the slave
> > is represented as a sub-node.
> 
> Only if you have the goal to describe the data/command path ("main interface")
> in DT.

This is the way devices are described in DT -- walking from the root to
a leaf you follow the path from the CPU to a slave interface.

Some things don't have slave interfaces (e.g. fixed-clocks), but still
need to be referred to, so those still get placed in the DT. There are
arguments to be had w.r.t. their placement, but that's another
discussion entirely.

> I mentioned it several times: USB-PHYs use the phandle approach to attach
> a single PHY to the usb controller, although there is usually some ULPI-"bus"
> interface (12 parallel wires) between. And the PHY is clearly more "slave" than
> the usb controller, isn't it?

Some PHYs have additional interfaces, so their CPU-visible slave
interface is described. This necessitates the phandle reference in the
general case.

> But with phandle, the usb controller is a _resource_ for the PHY. So would
> you say this is wrong?

Not entirely.

> This is the design pattern (for DT and drivers) we have copied for our tty-slave
> proposal.

We have plenty of other ways of describing relationships today. The
existence of one style (and its continued support for ABI reasons) does
not mean that it's a style we want to proliferate.

> 
> > I wasn't aware of any instances of timers being referred to by phandle
> > by other devices -- that seems distinctly odd. Where do you see that
> > happening.
> 
> I found it in connection with dmtimer / pwm on OMAP3. May be a rare exception
> and that may be a special type of OMAP timers.

I took a quick look and couldn't spot where that happened. I take it
the timer was a "ti,omap3430-timer"? Or have I misunderstood?

What referenced it by phandle? In which dt?

> >> * mcbsp (see e.g. http://lxr.free-electrons.com/source/arch/arm/boot/dts/omap3-n900.dts#L127)
> > 
> > Subsystem type bindings are more of a special case, and regardless the
> > components have nodes in the relevant portions of the DT.
> 
> > 
> >> * mmc-pwr-seq-simple (which does not even describe a physical piece of hardware)
> > 
> > If this is so different, how is it relevant?
> 
> It could as well be subnode of the affected mmc interface or mmc-slave, but obviously it
> isn't grouped there, and uses a phandle to refer to its &mmc "master".
> 
> Its function is quite similar what we need for our GPS chip: control power sequences
> of a remote device.

That may fit your one particular use-case, but for UART slaves generally
we should have a kernel driver (which can do more than a trivial
power-up), at which point you need a node, and the separate sequence
node is useless.

> >> All of them define the provider in one node. And refer to it by a phandle in another node
> >> where they are used.
> >> 
> >> So I see a lot of provider-consumer relationships modeled by phandles but not by child nodes.
> > 
> > I agree that provider-consumer type relationships are typically
> > described in this manner.
> 
> Ok.
> 
> > 
> > However, master-slave relationships are not.
> 
> It looks as if you see a significant difference between provider-consumer and master-slave
> relationship which I was not sure of which applies to what and where you make the distinction.
> 
> By the way: what exactly makes the UART on the SoC side "master" and the UART on the connected
> device a "slave" (except the user-space view)?

Typically a "slave" accepts commands, and won't send commands of its own
accord. This case is admittedly fuzzier given that a UART slave could
send a stream of bytes at any point, but that's data.

The distinction is really that the UART slave won't control other
devices -- you can think of the entire path of the CPU to the UART as
the master.

> UARTs per se have no master-slave roles and are symmetrical (contrary to SPI and I2C where it
> is well defined). Rather, both are formally DTE connected by a null-modem.
> 
> This is another substantial difference between UART and I2C/SPI besides addressability.

Sure, except for the fact that the device attached to the UART logically
follows a "slave" role. The rest of the system would be usable without
it, and it assert no control over anything else.

Thanks,
Mark.