Re: [PATCH 1/4] of: Add device tree bindings for Evatronix

[Ricard Wanderlof]

On Fri, 10 Jun 2016, Boris Brezillon wrote:

> > The use cases as I see are as follows:
> > 
> > a) Two identical chips sharing all but the CS lines, in order to implement 
> > a seemingly-larger address space. (e.g. two 256 Mbit chips implementing a 
> > 4 GB area). In this case, for certain operations, the controller does not 
> > have to wait for one device to complete before issuing a command to 
> > another. I'm not sure how the controller keeps track of the two devices 
> > though.
> 
> I think it's the chip-enable use case, isn't it?
> 
> > 
> > b) Two different chips with the same connection, which provide disjunct 
> > functions, e.g. one (small) flash for program storage and one (large) for 
> > data.
> 
> Then they can't share the CS line (or be actived at the same time), at
> least it doesn't make any sense to me.

Sorry if I haven't been clear enough, but in neither case are the CS lines 
shared between the devices. There is still a separate CS for each flash 
chip. The question is how the controller handles the CS lines.

Basically, in the general case, the controller can handle a matrix of nand 
flash chips. There can be a number of banks, each of which can have a 
number of individual CS lines. For the (in this case academic) case of 3 
banks and 4 chip selects per bank, there would be a total of 3 x 4 = 12 CS 
lines.

For the IP configuration the driver was written for, there are only 2 CS 
lines, and we can configure if they are to be viewed by the controller as 
2 CS lines within the same single bank, or 2 separate banks with one CS 
each. This is what the DT property is intended to express. It basically 
translates directly into a register write in the IP.

But, as you are driving at below, the bindings should really cover the 
more general case, for which a simple use-bank-select property doesn't 
really cut it. Since the driver only supports a single CS at the time 
being, I'm really proposing to drop it completely, alternatively we could 
have two: 'banks' and 'devices-per-bank', which reflect what the general 
IP case would be able to handle. For the version of the IP in use these 
would have the permitted values of 1 and 2, with some combinations being 
illegal. Unfortunately, the IP configuration cannot be read out (neither 
the version of it), so it's not possible for the driver to verify the 
DT settings against the actual IP configuration. I don't really know how 
to solve that.

> > > > I didn't really want to have the added flexibility (and complexity) of 
> > > > being able to use any R/B line for any connected flash chip. It seems an 
> > > > unnecessary complication for the driver without much gain.  
> > > 
> > > Not really, at least if you properly separate the chip and controller
> > > objects it's quite easy to deal with, and I'll ask you to do this clean
> > > separation anyway (even if you say you only have a single chip per
> > > controller) :P.  
> > 
> > Yes, the separation of chip and controller is needed, but the R/B line 
> > flexibility requires an additional mapping functionality within the 
> > driver. Not rocket science of course, but I can't see much point in it 
> > (other than to cover up a potential routing mistake done by a PCB 
> > designer).
> 
> You seem to argue on all the minor things I'm asking. Honestly, it
> should be hard or error-prone to do it. And let's say you don't support
> it in your driver, you should still think about that when designing
> your bindings.

Sorry, it's just that over the years I've seen too much code that 
introduces various flexibilities just because someone thought it might be 
'nice to have at some point' or 'because the hardware supports it', but 
which in reality is never used and still must be maintained. Sure, one 
small mapping function is certainly not going to break the bank, far from 
it, but over time the matrix of things that can be configured can grow to 
awkward proportions, and often something thought of as a minor issue can 
turn out to be complex to support in the end. And given a stable API rule, 
it's too late to simplify things once they have been implemented.

Of course, the code should not be written in a way to limit future 
expansion either.

> > I mean, an IP may be capable of a lot of things, and we don't necessarily 
> > want to implement them all in the driver to start with and have DT 
> > propertes for them all do we?
> 
> Hm, you should at least take the capabilities you know about into
> account when defining a new binding, and you already know that some
> SoCs might decide to expose 2 or more R/B pins, so it should already be
> designed this way.

Ok. I'll give it some more thought then.

> I'm opposed to the idea of putting information that can be
> automatically deduced in the DT. For this specific case, the main
> reason is that a board vendor can decide to use different NAND chips
> on the same design, and they might not all share the same
> capabilities (and you don't want to have a dts for each NAND).

Yes, that makes sense of course, but what if someone would want to 
override the automatic settings, for whatever reason, using an optional DT 
property? I can think of several reasons either way, that's why I'm 
asking.

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30