Re: [PATCH] uio: User IRQ Mode

On Thu, Jul 03, 2008 at 09:10:19AM +0200, Uwe Kleine-König wrote:
Hans J. Koch wrote:
On Wed, Jul 02, 2008 at 07:59:51PM +0900, Magnus Damm wrote:
From: Uwe Kleine-König <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.


Shared interrupts are not supported by this mode.

Signed-off-by: Uwe Kleine-König <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.

- 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?

- 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.

- 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.

- I cannot understand it.

I'll try to list them all below. Please tell us if the list isn't
complete or if my comments doesn't convince you. You might have to
repeat yourself, but for me it's hard to sort your arguments because
Magnus' suggestion changed over time.

OK, the version Magnus sent last is different, so some of my arguments
are superfluous now.

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.


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.


In the first thread[1] your unique and open concerns (to the best of my
knowledge and belief) with my comments are:

- "This only works for embedded devices [...]"

OK, this doesn't work with shared IRQs which rules out x86.
I don't claim to know all the 23[2] other architectures but
IMHO if something is good for 3 archs and doesn't hurt the
other 21, you should do it.

- "This would save somebody the trouble to add the above 5 lines
to the 30 lines of board/platform support code he has to write
anyway. That's the only gain, and that is not enough."

IMHO it's worth it. Because if you add the five lines to a
central place you save 5 lines per platform using the driver.

OK, that is one argument in favor. I always admitted that, but said that
this is not enough to compensate for the disadvantages mentioned above.

Moreover this might prevent some bugs. (And obviously this
function has the potential to have a buggy implementation as
the comment by Alan Cox shows.)

For me, this shows two things:

- I never ever had to use disable_irq_nosync() in any UIO driver yet,
otherwise I would have noticed.

- Magnus turned in a patch that he never tested.


- "And if you _know_ that on your platform the irq is not
shared, this might really be a one-liner that simply calls
irq_disable. That's OK in board specific code, but not in a
generic driver."

Please note that the patch only introduces a helper that the
platform code *can* use. You still have the freedom not to
use it without any overhead.

It's not a good idea to add nonsense code and tell the users to ignore
it whenever they can...


- "I won't accept anything that changes the current UIO
behaviour."

Not valid anymore. There is no change in behaviour.

Well, at least the whole stuff would have to be explained in the docs.
You add an element to struct uio_info, together with a new #define. And
a whole class of drivers using that stuff would have a write() function
without needing it.


In the second thread[3] I cannot find any open concerns that are not
already listed above.

Changes since Uwe's last version:
- flags should be unsigned long
- simplify uio_userirq_handler()

That's nearly nothing. All you do is sending the same stuff three weeks
later in the hope somebody will merge it this time. NAK.
I think nobody really is surprised that you're not happy with the new
post. But note that Magnus just did what you told him. ("I'm [not] the
big boss who makes the final decision. I can be critized and overridden.
If Greg loves your patch and merges it, fine. Try it.")

In the hope not to have kicked off a flame,

Oh, no, stay cool ;-)

Thanks,
Hans

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