linux-kernel - Re: [PATCH] uio: User IRQ Mode

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Date:	Thu, 3 Jul 2008 19:23:34 +0900
From:	"Magnus Damm" <magnus.damm@...il.com>
To:	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Cc:	linux-kernel@...r.kernel.org, Uwe.Kleine-Koenig@...i.com,
	gregkh@...e.de, akpm@...ux-foundation.org, hjk@...utronix.de,
	lethal@...ux-sh.org, tglx@...utronix.de
Subject: Re: [PATCH] uio: User IRQ Mode

Hi again Alan,

On Wed, Jul 2, 2008 at 8:11 PM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> On Wed, 02 Jul 2008 19:59:51 +0900
> Magnus Damm <magnus.damm@...il.com> wrote:
>
>> From: Uwe Kleine-König <Uwe.Kleine-Koenig@...i.com>
>>
>> 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.
>> Shared interrupts are not supported by this mode.
>
> This doesn't work even for some non shared interrupts.
>
> If I take a level triggered interrupt then the IRQ handler code must
> clear the IRQ before the line can be unmasked.
>
> It might work for edge triggered providing you don't get too many edges
> before you respond (in which case we will decide its a stuck IRQ and turn
> it off for good).

Below is a little description of how this is supposed to work, see
point 5, 8, 9.

--- Regular in-kernel driver approach ---

1. hardware device signals the processor
   -> interrupt signal to interrupt controller gets asserted

2. interrupt controller passes through interrupt signal
   the source is enabled so the interrupt controller does not mask
   -> interrupt signal to processor gets asserted

3. processor saves context and starts executing kernel interrupt code
   the processor will mask interrupts for this certain interrupt level

4. mask_ack_irq()
   this modifies the state of the interrupt controller
   -> interrupt signal to processor is no longer asserted

5. kernel interrupt handler acknowledges interrupt
   this modifies the state of the hardware device
   -> interrupt signal to interrupt controller is no longer asserted

6. unmask_irq() - unmask interrupt controller
   this modifies the state of the interrupt controller
   -> interrupt signal to processor is let through, but is no longer asserted

7. processor restores context
   this will unmask interrupts for the current interrupt level

--- UIO "User IRQ Mode" approach ---

1. hardware device signals the processor
   -> interrupt signal to interrupt controller gets asserted

2. interrupt controller passes through interrupt signal
   the source is enabled so the interrupt controller does not mask
   -> interrupt signal to processor gets asserted

3. processor saves context and starts executing kernel interrupt code
   the processor will mask interrupts for this certain interrupt level

4. mask_ack_irq()
   this modifies the state of the interrupt controller
   -> interrupt signal to processor is no longer asserted

5. kernel interrupt handler _disables_ interrupts, notifies user space
   this may modify the state of the interrupt controller
   -> interrupt signal from device to interrupt controller is still asserted
   -> interrupt source is still masked by interrupt controller

6. unmask_irq() - unmask interrupt controller
   this may modify the sate of the interrupt controller
   -> interrupt source is still masked by interrupt controller

7. processor restores context
   this will unmask interrupts for the current interrupt level
   -> interrupt signal from device to interrupt controller is still asserted
   -> interrupt source is still masked by interrupt controller

8. user space driver get scheduled, acknowledges interrupt
   this modifies the state of the hardware device
   -> interrupt signal from device to interrupt controller gets deasserted
   -> interrupt source is still masked by interrupt controller

9. user space driver uses write() to re-enable the interrupt
   this modifies the state of the interrupt controller
   -> interrupt source is no longer masked by interrupt controller

Hope this clarifies a bit. Thank for your feedback!

/ magnus
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