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    SubjectRe: [PATCH v2] misc: Driver for Silicon Labs Si570 and compatibles
    On Thu, Apr 21, 2011 at 01:11:25PM +0200, Arnd Bergmann wrote:
    > On Wednesday 20 April 2011, Guenter Roeck wrote:
    > > On Wed, Apr 20, 2011 at 12:44:30PM -0400, Arnd Bergmann wrote:
    > > > On Tuesday 19 April 2011, Guenter Roeck wrote:
    > > > > This driver adds support for Si570, Si571, Si598, and Si599
    > > > > programmable XO/VCXO.
    > > > >
    > > > > Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
    > > >
    > > > This needs some more explanation of what the hardware is there for,
    > > > and why it's unlike everything else that we support in Linux.
    > > >
    > > This is a generic configurable clock device. I'll be happy to add
    > > some text such as "The device can be used for any application requiring
    > > a static or a dynamically configurable clock, such as serdes clocks".
    > > Not sure if that would add much value, though.
    > >
    > > Regarding "unlike everything else", not sure if that is really correct.
    > > The DDS chips Jonathan mentioned do get pretty close, and there are
    > > other drivers providing support for clock chips, though typically more
    > > dedicated. ics932s401 in misc is one example, and then there are all
    > > the tuner chips in media/common/tuners/.
    >
    > Isn't that what you'd normally call a 'struct clk' then?
    >
    > > > If that is true, it should probably not have a user-visible
    > > > interface, but only an interface that can be used by other
    > > > kernel drivers.
    > > >
    > > Depends. In our case, turns out the devices consuming the clock
    > > have user mode drivers. Lots of history there, but the chip vendors
    > > provide those user mode drivers, and the teams responsible for
    > > integrating the drivers decided to not rewrite it to kernel mode drivers.
    > > Also, for special purposes such as margining, it is necessary to control
    > > the clock from userspace. So, for our use case, I need the user-visible
    > > interface. I _don't_ need the kernel interface, at least not right now,
    > > which is why I did not add it.
    > >
    > > Browsing through the web, it seems the chip is somewhat popular with
    > > Amateur Radio. No idea if it would ever be controlled for such a purpose
    > > from Linux, but if so, it would also require a user configurable frequency.
    > >
    > > If there is a better place for such a driver than misc, please let me know.
    >
    > When you say user mode driver, do you mean as in drivers/uio? (taking Hans
    > on Cc for these)

    No UIO driver like that ever reached me.

    >
    > Those already have generic support for memory and interrupt resources,
    > maybe we can just add a common way to associate a uio device with a struct clk
    > and provide a sysfs or ioctl interface to set a clock for a given device.

    I don't think the UIO framework is the right place for such a thing. If it's
    just this one driver that needs modification of a clock from userspace, then
    a sysfs attribute could be added to that driver. If there are several drivers
    that need this, then the clock framework should be extended.

    Thanks,
    Hans


    >
    > Arnd
    >


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