[PATCH 3/3] omap: add hwspinlock device

[ohad@wizery.com (Ohad Ben-Cohen)]

Hi Tony,

On Fri, Oct 22, 2010 at 6:56 PM, Tony Lindgren <tony@atomide.com> wrote:
> Guys, let's try to come up with a generic interface for this instead of
> polluting the device drivers with more omap specific interfaces.
>
> We really want to keep the drivers generic and platform independent.

I share this concern as well.

We basically have three options here:

1. Have the hwspinlock driver inside the omap folders and use pdata func ptrs
2. Have a generic hwspinlock framework, with a small omap-specific
implementation
3. Have a misc driver which is currently omap specific, but can very
easily be converted to a common framework

I don't like (1) so much; it's a driver that has very little that is
hardware specific (mainly just the two lines that actually access the
hardware - the lock and the unlock), and it's growing. E.g., we will
need to add it a user space interface (to allow userland IPC
implementations), we will need to add it a "virtual" locks layer that
would provide more locks than the hardware currently has, etc..

In addition, there seem to be a general discontent about drivers
piling up in the ARM folders, instead of having them visible in
drivers/ so they can become useful for other platforms as well.

Here's something Linus wrote about this awhile back:

http://www.spinics.net/lists/linux-arm-msm/msg00324.html

So initially I opted for (2). I have an implementation ready - it's a
common drivers/hwspinlock/core.c framework with a small omap-specific
part at drivers/hwspinlock/omap_hwspinlock.c. The core has all the
logic while the latter, omap-specific implementation, is really small
- it is basically just registering lock instances, that have a
trylock/unlock/relax ops struct, with the common framework.

But lack of additional users (besides the OMAP hardware), and lack of
generic drivers that would use it (we have syslink, which currently
tend to only need this from user space, i2c-omap and omap's upcoming
resource manager), made it look like an overkill for now.

So simplicity won, and (3) was eventually submitted. It exposes
exactly the same interface like (2) would (s/omap_hwspin/hwspin/), and
it's relatively easy to turn it back into a common framework in case
additional users show up.

But if you feel that (2) is justifiable/desirable, I would be more
than happy to submit that version.

> Unless somebody has better ideas, I suggest we pass a lock function
> in the platform_data to the drivers that need it, and do the omap
> specific nasty stuff in the platform code.

Do you mean (1) and then pass the whole bunch of APIs
(request/free/lock variants/unlock/get_id) via pdata ?

Or do you mean a variation of (2) with only the specific locking bits
coming from pdata func pointers ? I guess that in this case we just
might as well go with the full (2).

Thanks,
Ohad.