Re: [PATCH v2 2/3] ARM: dts: socfpga: fpga bridges bindings docs

[Steffen Trumtrar <s.trumtrar@pengutronix.de>]

On Mon, Oct 27, 2014 at 05:52:02PM +0200, Pantelis Antoniou wrote:
> Hi Steffen,
> 
> > On Oct 27, 2014, at 17:23 , Steffen Trumtrar <s.trumtrar@pengutronix.de> wrote:
> > 
> > Hi!
> > 
> > On Mon, Oct 27, 2014 at 01:54:20PM +0200, Pantelis Antoniou wrote:
> >> Hi Stefen,
> >> 
> >>> On Oct 25, 2014, at 17:42 , Steffen Trumtrar <s.trumtrar@pengutronix.de> wrote:
> >>> 
> >>> Hi Pantelis!
> >>> 
> >>> On Fri, Oct 24, 2014 at 12:20:53PM +0300, Pantelis Antoniou wrote:
> >>>> Hi Stefan, Allan,
> >>>> 
> >>>> Sorry, haven’t had a chance to review all this yet; will do so in the weekend.
> >>>> Just wanted to pop in and comment on a few things.
> >>>> 
> >>>>> On Oct 24, 2014, at 10:00 AM, Steffen Trumtrar <s.trumtrar@pengutronix.de> wrote:
> >>>>> 
> >>>>> Hi!
> >>>>> 
> >>>>> On Thu, Oct 23, 2014 at 06:51:06PM -0500, atull@opensource.altera.com wrote:
> >>>>>> From: Alan Tull <atull@opensource.altera.com>
> >>>>> 
> >>>>> (...)
> >>>>> 
> >>>>>> diff --git a/Documentation/devicetree/bindings/fpga/altera-hps2fpga-bridge.txt b/Documentation/devicetree/bindings/fpga/altera-hps2fpga-bridge.txt
> >>>>>> new file mode 100644
> >>>>>> index 0000000..bc24a2e
> >>>>>> --- /dev/null
> >>>>>> +++ b/Documentation/devicetree/bindings/fpga/altera-hps2fpga-bridge.txt
> >>>>>> @@ -0,0 +1,53 @@
> >>>>>> +Altera FPGA/HPS Bridge Driver
> >>>>>> +
> >>>>>> +This driver manages a bridge between a FPGA and a host processor system (HPS).
> >>>>>> +User space can enable or disable the bridge by writing a "1" or a "0",
> >>>>>> +respectively, to its enable file under bridge's entry in
> >>>>>> +/sys/class/fpga-bridge.  Typically, one disables the bridges before
> >>>>>> +reprogramming the FPGA.  Once the FPGA is reprogrammed, the bridges are
> >>>>>> +reenabled.
> >>>>>> +
> >>>>> 
> >>>>> NAK.
> >>>>> 
> >>>>> This is all linux specific and doesn't belong here.
> >>>>> 
> >>>> 
> >>>> We know. The DT spec hasn’t been updated for a while, and this is going to be
> >>>> part of what we want to do with the restarting of the ePAPR spec process.
> >>>> 
> >>> 
> >>> As we don't have a new spec yet, I go with the current state of the art.
> >>> And I don't see how "file under ... /sys/class/fpga-bridge" fits the
> >>> current spec.
> >>> 
> >>>> So absolutes like “DT is a hardware description only" might be too strong statements, that
> >>>> do not work in practice anymore.
> >>>> 
> >>>> There’s no such thing as pure hardware devices lately - there is always a configuration
> >>>> component; some of it OS specific, some of it not.
> >>>> 
> >>> 
> >>> If it really is necessary, I'm not against it, don't get me wrong.
> >>> But in the grand scheme I wouldn't say that this fits my experience.
> >>> There are some devices that need configuration and often when it is
> >>> done in the DT, it is just a shortcut or not thought through.
> >>> And can be also introspected dynamically. With some discussion on the
> >>> list these bindings often change.
> >>> 
> >> 
> >> Already answered and given a possible way this might work; admittedly this
> >> specific example is not a good fit for what I was trying to say, but whatever.
> >> Let’s get the ball rolling.
> >> 
> > 
> > I would be okay with chosen-node. Not for this driver; but in general.
> > 
> 
> Let’s figure out how to do it then…
> 
> >>> Regarding OS specifics: already there, e.g. console setup in the chosen node.
> >>> And other things I saw are ethernet aliases just for u-boot. Not a problem
> >>> of the spec, just a problem of u-boot and unneccessary "configuration".
> >>> Just to fix a bad bootloader. barebox can handle this without any
> >>> such stuff. Maybe we should integrate the DT "overlays" barebox uses into
> >>> the in-kernel DTs as well...but does it really help/interest someone who
> >>> decides to use u-boot where barebox stores its environment? I guess not.
> >>> 
> >> 
> >> Although I’m not against having DT overlays in the bootloader, I only see them
> >> as a method that helps a platform developer express things easier. I am completely
> >> against having the kernel tied to any particular bootloader.
> >> 
> >> We’ve all been through the hell where a kernel only boots if the bootloader has 
> >> setup the platform “correctly”. This “correctly” has a very loose definition and
> >> 99/100 times is extremely badly documented or not at all.
> >> 
> >> IMHO the bootloader should (as part of the normal boot sequence) only setup the
> >> absolutely bare minimum and then pass control to the kernel as soon as possible.
> >> 
> >> The full featured bootloader environment should only be presented when the user
> >> requests so.
> >> 
> > 
> > Totally agree. The kernel shouldn't be tied to any bootloader if at all possible
> > (the SoCFPGA is an especially bad example here again, as the pinmuxing can only
> > happen in the bootloader).
> 
> I’m not familiar with SoCFPGA. Why pinmuxing is only possible in the bootloader?
> Can’t the bootloader configure the minimum pinmux config and then have Linux setup
> the rest?
> 
> If we’re feeling particularly nasty, we might just require bootloaders to clean the
> hardware state before passing control to Linux (besides memory controller setup) and
> see what’s broken.
> 

Okay, offtopic, but:

a) datasheet explicitely states that dynamic pinmuxing is not supported.
b) last time I checked, no documentation at all. You get some magic values spit
   out by Altera's tools.
c) you don't have registers. Pinmuxing is done via JTAG scan chain; which you
   feed the magic values.

Apart from that, it seems to be the most sane idea, to at least write your
driver in a way, that doesn't make any assumptions on HW state.
But I think this is already common practice in mainline.

> > But should the DT include stuff than, that is only interesting for linux?
> > 
> 
> Like it or not it’s Linux that’s in the forefront of many hardware designs.
> 

I like it; pays my rent ;-)
But I also like freedom to choose...

> There are configuration settings in DT that are part of the way hardware is presented to
> Linux and user-space, and have been for quite some time. We’ve been pretending they don’t
> exist, but they are there…
> 

As far as I know, a lot of that are older bindings, where more
things just got applied, because it wasn't clear where DT will go.


Regards,
Steffen

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