Message-ID: <20110701073755.GR6069@pengutronix.de>
Date:	Fri, 1 Jul 2011 09:37:55 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Ryan Mallon <rmallon@...il.com>
Cc:	Bill Gatliff <bgat@...lgatliff.com>, Arnd Bergmann <arnd@...db.de>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	viresh kumar <viresh.kumar@...com>,
	Shawn Guo <shawn.guo@...aro.org>,
	Ryan Mallon <ryan@...ewatersys.com>
Subject: Re: [PATCH 1/3] PWM: add pwm framework support

On Fri, Jul 01, 2011 at 09:24:05AM +1000, Ryan Mallon wrote:
> On 01/07/11 03:02, Sascha Hauer wrote:
> >Bill,
> >
> >On Thu, Jun 30, 2011 at 11:17:54AM -0500, Bill Gatliff wrote:
> >>Guys:
> >>
> >>On Thu, Jun 30, 2011 at 7:41 AM, Arnd Bergmann<arnd@...db.de>  wrote:
> >>
> >>>A lot of people want to see a framework get merged, and I think it's
> >>>great that Sascha has volunteered to do the work to push that
> >>>through this time, especially since you have not been able to
> >>>finish your work.
> >>Sascha is wasting his time by reinventing the wheel.  He's traveling
> >>over exactly the same path I have already covered.  In fact, some of
> >>his reviewer comments are almost word-for-word the same as those I
> >>have received and addressed in the past.
> >>
> >>My patches were always kept current in this mailing list and others,
> >>and Sascha clearly has the skills necessary to make improvements and
> >>corrections should he have chosen to do so.
> >I think that you made the fundamental mistake to completly ignore the
> >existing pwm API and its users. With a competing api we are basically
> >stuck. We can't convert the existing hardware drivers to the new API
> >because leds-pwm.c, pwm_bl.c and others still depend on the old API and
> >boards using it would break.
> 
> I don't think this is really a blocker to Bill's patches. There are
> three (that I can see) pwm users currently:
> drivers/video/backlight/pwm_bl.c
> drivers/leds/leds-pwm.c
> drivers/input/misc/pwm-beeper.c
> 
> All of those drivers are trivial and good easily be updated to work
> with Bill's patches. Bill already provided a leds-pwm driver.

Yes, it is easy but that's not my point. The point is that you can't
convert the drivers without converting *all* hardware drivers in a
*single* step. If you choose to have two competing APIs in the tree
for one purpose you can't convert the drivers but instead you have
to copy them, either with cp or ifdefs. I have just looked at the
leds-pwm driver Bill provided. Applying it immediately breaks all
users.

> 
> There is also the interesting case of the Atmel pwm driver, which
> does not fit the current pwm api and has its own backlight and leds
> drivers. Bill's patches addressed this, Sasha's patches do not. If
> we merge Sasha's patches then we are going to be in the same
> position as Bill's patches at some point in that we need to change
> the pwm api (and all its users) to meet the needs of the Atmel (and
> any similar hardware) pwm device.

My patches are compatible to the current (user-) API on purpose. It
offers the possibility to convert each hardware driver independently
of the others. Once we have a framework we can change the driver API
independent of the user API.

> 
> The ep93xx pwm driver (drivers/misc/ep93xx-pwm.c) also does not fit
> the current api (though it could), but instead provides a sysfs
> interface to user-space. Again, this was addressed by Bill's
> patches.
> 
> >We can't convert the function drivers
> >either because again this would break boards for which only an old style
> >pwm driver exists.  So the logical thing to do is to put a step in
> >between: Consolidate the existing drivers and *then* change the API
> >atomically so that nothing breaks. Your patches don't do this, so I
> >don't think at all that what I did is duplication of work.
> 
> You have to modify the drivers anyway match the new pwm core.

Yes. But changing the user API *and* the driver API in a single patch
really is a bad idea.

I don't say at all that the end result Bill is aiming at is bad because
it isn't. We are not talking about the end result, but only the way to
get there. And getting from a to b in bisectable small steps is a well
established development model in the Kernel, or have I missed something?

> The
> Atmel and ep93xx drivers are going to be difficult to merge into the
> new api, and seeing as there are only about seven pwm drivers total
> in the kernel I think its a significant portion. Any pwm api
> patchset could easily convert all of the existing pwm drivers
> without becoming overly massive.
> 
> If we merge Sasha's api, then we can move most of the existing
> drivers and maybe add some new ones, but we will still have the
> unconsolidated outliers. When (if?) we try to fix those we will
> probably need to change the pwm api and therefore all of the drivers
> to. So its basically a case of do the effort now (Bill's patches) or
> do it later. Doing it later will probably require more effort.
> 
> >Given the current rush to move drivers out of arch/ it probably won't
> >take long until all pwm drivers are moved to drivers/pwm/ and converted
> >to use the framework, and then you have a good base to put your work onto.
> >So please don't complain too much: We are currently only doing the work
> >you didn't want to do.
> >
> 
> You can move all of the drivers out of arch now if you want. You
> just need to make sure you are only compiling one of them in.

Yes, we can. But this does not solve the migration problem I try
to describe in this and the previous mail.

Let's face it: Bill is working on his PWM patches for three years now,
still the merge of these patches is not in sight. Let's just split
them into smaller parts which are easier to swallow.

> The
> real job in consolidating means making sure that the api meets the
> needs of all of the drivers. The in kernel Atmel pwm driver at least
> is not going to convert easily to this api.

Then let's change the API. I have nothing against this.

> 
> Also, please not my change of email address for future emails.

ack

Sascha

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