Re: clock driver

From: andrey
Date: Wed May 27 2015 - 20:29:18 EST




---- On Wed, 27 May 2015 17:10:06 -0700 Guenter Roeck<linux@xxxxxxxxxxxx> wrote ----
> On 05/27/2015 04:58 PM, andrey wrote:
> >
> >
> > ---- On Wed, 27 May 2015 16:08:12 -0700 Guenter Roeck<linux@xxxxxxxxxxxx> wrote ----
> >
> > > On 05/27/2015 12:44 PM, andrey wrote:
> > > > Hello all,
> > > > Let me add a comment on using sysfs to simplify user space access to the clock
> > > > features as opposed to controlling them from a driver that uses the clock chip driver.
> > > >
> > > > It is common to use such advanced clock chips with the FPGA devices (as me and
> > > > York do), and a lot of development (HDL code) is done before a fancy higher-level
> > > > driver is even started. And it is not just a temporary stage needed by a small minority
> > > > of developers - as HDL coding gets more to the the core of many new devices running
> > > > Linux kernel, it makes sense to create the chip drivers more developer-friendly, not
> > > > just for the final use in a higher level device driver - modification of the HDL code
> > > > (most modern FPGA are programmed at runtime) makes it a new device that may
> > > > need a new driver.
> > > > I'm sure that it is not just for me, when it starts with the chip driver that supports
> > > > low-level functionality exposing it to the user space, and then working on the HDL
> > > > code using Python scripts at that stage. And only later in the development designing
> > > > the higher level device drivers that may not need all of the chip functionality. And such
> > > > higher level driver will work for our systems, but other developers who work on their
> > > > embedded systems will again need access to low level chip functionality, and will have
> > > > to redo the same work all over again. This I believe is a rationale of exposing such
> > > > chip-specific hardware features (not all of them are probably easy to fit into a specific
> > > > standard model) to the user space scripts.
> > > >
> > > > I wrote the initial driver code for our system
> > > > ( https://github.com/Elphel/linux-elphel/blob/master/src/drivers/misc/si5338.c ) and
> > > > being very far from being a kernel developer myself (I'm more of a hardware guy)
> > > > I didn't even try to satisfy the required coding style and submit it, so I'm very thankful
> > > > to York who re-wrote the code and is trying to make it usable to others.
> > > >
> > >
> > > Line wraps at ~75 columns would make this a bet easier to read.
> >
> > Guenter, I'm sorry for using "rich text" email settings.
> >
> > >
> > > A more generic solution to your problem might be to implement a driver
> > > similar to i2c-dev, which exports raw i2c device information to user space.
> > > In your case, you would export information about the clocks in the system,
> > > possibly through sysfs (i2c-dev uses ioctl which is a bit old-fashioned).
> >
> > I was trying to make it safer to use low-level functionality of the particular
> > (and rather popular) clock chip and to avoid using SiLabs proprietary tools to
> > generate required settings offline. Using just raw i2c would require to have
> > large user space program to calculate valid settings for the device.
> >
> > I would consider this chip as both a generic clock device that can fit into
> > a standard framework and simultaneously a unique device that offers specific
> > functionality outside of the framework. I thought that sysfs (instead of
> > "old-fashioned" ioctl I used in such cases before) can offer
> > hardware developer-friendly solution as a supplement to in-framework
> > basic functionality.
> >
> > Device driver for this chip makes it possible to avoid proprietary configuration
> > software and calculate register settings at runtime, minimizing requirements to
> > the user space software and so the time developers of the new embedded
> > systems will need to (re-)implement these important chip-specific features.
> >
>
> I think we are in violent agreement ;-). Only question was how to implement
> sysfs (or user space access) support, where my preference would be a more
> generic solution.

Guenter,

I just considered this chip as a "frontier" device, not yet a member of an established
class of similar ones. It may be possible to generalize later, extracting common
functionality to a more abstract interface. But we just need this device support now,
and when this one will become a member of some generic class - "frontier" will again
move a step farther, new devices will emerge that stick out of the nice frameworks.

Andrey

>
> Thanks,
> Guenter
>
> > Andrey
> >
> > >
> > > This would be a driver independent solution, and work for all clock drivers.
> > > It might still not be accepted by Mike and Stephen, due to the risk, but it
> > > might be worth a try. After all, using i2c-dev to access i2c devices directly
> > > is just as risky.
> > >
> > > In my opinion, it is always better to have a driver in the upstream kernel,
> > > if possible one that uses a standard framework. That makes it much easier
> > > to support going forward.
> > >
> > > Guenter
> > >
> > >
> >
> >
> >
> >
>
>


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