Re: [PATCH] uio: User IRQ Mode

[Magnus Damm]

On Fri, Jul 4, 2008 at 10:26 PM, Hans J. Koch <hjk@xxxxxxxxxxxxx> wrote:
> On Fri, Jul 04, 2008 at 01:03:21PM +0900, Magnus Damm wrote:
>> On Thu, Jul 3, 2008 at 9:45 PM, Hans J. Koch <hjk@xxxxxxxxxxxxx> wrote:
>> > On Thu, Jul 03, 2008 at 09:10:19AM +0200, Uwe Kleine-K?nig wrote:
>> >> Hans J. Koch wrote:
>> >> > On Wed, Jul 02, 2008 at 07:59:51PM +0900, Magnus Damm wrote:
>> >> > > From: Uwe Kleine-K?nig <Uwe.Kleine-Koenig@xxxxxxxx>
>> >> > >
>> >> > > This patch adds a "User IRQ Mode" to UIO. In this mode the user space driver
>> >> > > is responsible for acknowledging and re-enabling the interrupt.
>> >> >
>> >> > This can easily be done without your patch.
>> >
>> > BTW, the above wording "the user space driver is responsible for
>> > acknowledging and re-enabling the interrupt" is misleading. The kernel
>> > always has to acknowledge/disable/mask the interrupt. Userspace can only
>> > reenable it, ideally by writing to a chip register. In some cornercases
>> > for broken hardware we need the newly introduced write function.
>>
>> You seem to be mixing up masking/acknowledging the interrupt
>> controller and masking/acknowledging the actual hardware device. In
>> User IRQ Mode, the only thing the user space driver is accessing is
>> the hardware device, with the exception of write() to re-enable
>> interrupts which results in a enable_irq() that touches the interrupt
>> controller.
>>
>> I'm not sure what kind of hardware devices you are talking about, but
>> I have hardware devices here on my desk that need to be acknowledged
>> by the device-specific driver to deassert the irq. And acknowledging
>> from user space works just fine.
>
> Yes, I know such devices. If you _need_ to do it this way, it's simply
> broken hardware design. You frequently find that in FPGAs designed by
> industry amateurs. It can easily be handled by UIO as it is.

This is crap. I'm sure you're having all sorts of problems with broken
FPGAs, but that's outside the scope of this discussion. If a device
needs to be acknowledged or not to deassert the interrupt line is
totally device specific and it has nothing to do with broken hardware
design.

> If you do it without need, then it's simply broken software design
> because you use a kernel function which shows different behaviour on
> different archs and machines where a simple hardware register access
> would suffice.

As much as I enjoy arguing with you, we are doing this because there
is need for it. =)

>> >> > > Shared interrupts are not supported by this mode.
>> >> > >
>> >> > > Signed-off-by: Uwe Kleine-K?nig <Uwe.Kleine-Koenig@xxxxxxxx>
>> >> > > Signed-off-by: Magnus Damm <damm@xxxxxxxxxx>
>> >> > > ---
>> >> > >
>> >> > >  Similar code has been posted some time ago as:
>> >> > >  "[PATCH] uio_pdrv: Unique IRQ Mode"
>> >> > >  "[PATCH 00/03][RFC] Reusable UIO Platform Driver".
>> >> >
>> >> > Yes, and in that thread I gave detailed explanations why I won't accept
>> >> > that.
>> >> I think for all of your concers one of the following is true:
>> >>
>> >>   - they are not valid any more in this version; or
>> >
>> > I question the whole concept as such. The concept of having a generic
>> > irq handler using disable_irq_nosync() makes no sense at all.
>> >
>> > Reasons:
>> >
>> > - We do not introduce new possibilities. Everything can be done without
>> >  that patch.
>>
>> Isn't it possible to use the same argument against UIO? There is no
>> point in having it since it is possible to do it all in kernel space
>> instead. But for various reasons people want to write their drivers in
>> user space.
>
> That's why we have UIO, mainly to allow clean interrupt handling.

Yes, and that's fine.

>> And for various reasons I'd like to acknowledge interrupts in user
>> space. And I do that by disabling the interrupt in kernel space,
>> notify the user space driver and acknowledge and re-enable the
>> interrupt from there.
>
> This is all possible with UIO as it is.

It's not a matter if it's possible or not. It's a matter of abstraction.

>> The reason behind it is that I want to provide
>> user space device driver people with consistent interface to the
>> device without the need for any device specific code in kernel space.
>
> And if it were a solution that was machine- and platform-independent and
> worked with all kinds of interrupts, there wouldn't be much objections.

Exactly what do you mean by "so much objections"? You're the only one
blocking this, and it looks to me like you're just pushing your own
policy without thinking outside your little device driver box.

I'm not doing this because i want to change how UIO is working in
general or push some generic solution and force everyone to use it. I
_only_ want to export _our_ devices. If other people want to export
their devices the same way, then that's great. Uwe seems to want to do
that. Why do you feel you have to stop us? It's pretty natural for you
to have no insight in we are doing on the SuperH side, and because of
that you should just take our word for what we need and work with us
to resolve technical issues.

> BTW, you still need to touch kernel code to setup the structs for
> uio_pdrv, or did I misunderstand something? So where's the advantage?

The advantage is that we don't need to be aware of device-specific
details in the kernel.

You seem to be happy with the usual UIO way of splitting the driver
code in a kernel stub and a user space driver. Good for you, but we
want to skip the device specific kernel stub.

For our needs - on-chip memory mapped hardware blocks - we'd like to
replace the kernel stub with a single reusable implementation, and
that way we end up with a single per-hardware block specific driver -
in user space. This way we can let people with RTOS experience do
their thing in user space, without worrying about them overwriting our
precious kernel data structures.

You are right that we still need to feed the platform driver
information about the device. We only want to pass the base address of
the hardware block and the interrupt number. There is no point for us
to be aware about the inner workings of the hardware block - we only
want to export the thing to user space and be done with it. And in
some rare cases we don't even have documentation for a certain block,
so acknowledging interrupts from inside the kernel is out of the
question.

>> > - We offer users an irq handler that shouldn't be used. It is seldom the
>> >  best solution to call disable_irq_nosync() to disable an interrupt. In
>> >  almost all cases you should use the irq mask register of the chip your
>> >  driver handles. What do you want to write into the docs? Here's an irq
>> >  handler, but please use it only if you're really desperate?
>>
>> What is wrong with using disable_irq_nosync() compared to using the
>> device specific irq mask register? It works just fine, and it provides
>> a nice abstraction in my mind.
>
> It's the difference between one write to a chip register compared to a
> call to an arch specific kernel function. The performance and latency
> issues might be negligible on some systems, but there's no guarantee. A
> register access is one write on all platforms.

Wrong. It's not always that easy. The way to acknowledge interrupts is
very device specific. A good in-kernel example of not so easy
acknowledging would be drivers/input/touchscreen/ads7846.c. Look at
the interrupt handler code ads7846_irq() - what does it do? Exactly
what the User IRQ Mode is doing - using disable_irq(). The reason for
this is slow SPI bus speed. It takes a long time to write to the touch
screen controller over the serial bus, and the interrupt line will
remain asserted until the device has been acknowledged properly.

>> > - The only argument in favor of that concept was that it saves a few
>> >  lines of irq handler code. Given the fact that all the handler has to
>> >  do is toggle one bit in a hardware register, this is really not much.
>> >  And you tempt people to delete 5 lines of good code and replace them
>> >  with a sub-optimal generic irq handler.
>>
>> What is suboptimal about it? And since when is a device specific
>> solution better than a generic one?
>
> Drivers are always device specific. And disabling or masking an
> interrupt in the way the chip designer wanted you to do it is the
> fastest, most elegant, and most natural thing for a driver to do.

Again, exactly how to acknowledge an interrupt is device or hardware
block specific. And it's not always as easy as writing to a single
register. Of course we will acknowledge the device in the way the chip
designer want us to, but we will do it from user space.

>> > - You introduce the need for an irqcontrol function. This was not the
>> >  intention of that concept. Normal UIO devices don't need a write
>> >  function, this is only used for broken hardware. If you have normal
>> >  hardware, and implement a proper 5 lines irq handler, userspace can simply
>> >  reenable the irq by writing to a hardware register it has mapped
>> >  anyway. In your concept, it has to use write() on /dev/uioX, which
>> >  means you have to go all the way through libc, vfs, and the UIO core
>> >  to finally call your irqcontrol function, which in turn calls
>> >  enable_irq. As I said, there is broken hardware around where this is
>> >  the only way, but most chips allow you to do it properly.
>>
>> You are the one who posted the irqcontrol code because it solved some
>> issues you are having with broken hardware. I'm happy to use your code
>> in a generic way. Actually, my first version of this patch avoided the
>> extra write() call overhead by re-enabling things automatically, but
>> since your irqcontrol patch is cleaner I'd rather use that and live
>> with the small performance penalty.
>>
>> Performance in this case is really a non-issue.
>
> Your talking about the board you've got in front of you. I'm talking
> about the UIO framework and the 20+ platforms it supports.

I'm not forcing you to use my code on your platforms. Feel free to use
it if it does what you want to do, if not you should just keep on
using your regular code. How can this be my problem? Don't NAK just
because the code isn't suitable for all your needs. I'm not trying to
solve your needs. I'm posting this code because _we_ need it.

>> > Please, to make things simpler, let's only talk about the concept as
>> > such and not go into implementation details. I deliberately do not review
>> > that code (although I believe it has more bugs than the one Alan found),
>> > because as long as the concept doesn't make sense, I don't care how it
>> > is implemented.
>>
>> I'd say it makes sense to Paul, Uwe and me.
>
> Seems UIO gets tested intensively on SH at the moment :-)

No, you are wrong. It really doesn't get tested. Mainly because you
are refusing to accept my code. UIO _would_ be used and tested if we
can agree on these 50 lines of code and get on with our lives.

> Don't misunderstand me, even I can imagine cases where I _have_ to use
> similar solutions. But that's no reason to patch it into the generic UIO
> core. Because it should only be used if there's no other solution.
> There's no point in inviting people to do it that way.

You still haven't given any real technical reason why it's better to
acknowledge in kernel space over acknowledging in user space. Having
all hardware block / device specific code in a single place will keep
interfaces clean and make maintainance easier for us.

SuperH hardware just happens to use unique interrupt lines for most
internal hardware blocks, and we'd like to take advantage of that and
skip the entire UIO kernel stub thing.

>> >> And please, I try to work out the pros and cons in a constructive way
>> >> and hope there is nothing in it you will take personal.  It's really
>> >> that I consider the patch valuable and don't understand your concerns.
>> >
>> > You both keep telling me how valuable that patch is but never answered my
>> > question what the advantage would be. I cannot see it yet.
>>
>> Avoid having device specific interrupts handlers in kernel space?
>
> By replacing them with suboptimal generic handlers that work only in
> special cases?

There is nothing suboptimal about it.

>> > - Magnus turned in a patch that he never tested.
>>
>> Is that so?
>>
>> I've tested the patch using two different processors on three
>> different boards. I have written a user space uio driver that
>> interfaces to mplayer using vidix, providing hardware accelerated
>> color space conversion and stretching. So don't call my patch untested
>> please.
>
> OK, I apologize for that statement.

Thank you.

>> I plan on submitting the vidix driver upstream soon after the uio
>> interface gets stabilized, ie this patch gets merged.
>
> I'm sure you get your vidix driver to work without this patch.

It is and has been working for more than a month. It's not suitable
for submission yet though, mainly since the UIO kernel interface is
unstable until we've agreed on this patch. The kernel interface
changed with the write() re-enabling method, remember? And I feel this
may happen again since it seems very hard for us to agree on things.

>> > It's not a good idea to add nonsense code and tell the users to ignore
>> > it whenever they can...
>>
>> This nonsense code, exactly which code are you talking about?
>
> The concept of having a generic irq handler that works only with
> non-shared interrupts.

That may look like nonsense to you, but you fail to see the big picture if so.

>> Could it
>> be one of your significant kernel contributions? =)
>
> You think I have some? Thank you ;-)

No, I don't think so. But I do think you should work a bit on
understanding your position in the community and pay closer attention
to people with more experience than you. I may understand that you
feel the need to argue with me - since I only have a few small
contributions behind me - but when you're not listening to what Paul
has to say then you're just stupid. Stop ignoring people above you in
the food chain.

You can use git shortlog to get statistics.

/ magnus
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