Date:	Tue, 23 Dec 2008 17:09:42 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Guennadi Liakhovetski <g.liakhovetski@....de>
Cc:	linux-kernel@...r.kernel.org,
	linux-fbdev-devel@...ts.sourceforge.net, adaplas@...il.com,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 2/4 v4] i.MX31: Image Processing Unit DMA and IRQ
	drivers

On Tue, Dec 23, 2008 at 03:55:36PM +0100, Guennadi Liakhovetski wrote:
> On Tue, 23 Dec 2008, Sascha Hauer wrote:
> 
> > On Tue, Dec 23, 2008 at 02:14:12PM +0100, Guennadi Liakhovetski wrote:
> > > On Tue, 23 Dec 2008, Sascha Hauer wrote:
> > > 
> > > > On Tue, Dec 23, 2008 at 12:21:54PM +0100, Guennadi Liakhovetski wrote:
> > > > > Hi Sascha
> > > > > 
> > > > > On Tue, 23 Dec 2008, Sascha Hauer wrote:
> > > > > 
> > > > > > On Mon, Dec 22, 2008 at 09:10:03PM +0100, Guennadi Liakhovetski wrote:
> > > > > > > 
> > > > > > > Ok, so, what would we like to have there? We agree that the proper way to 
> > > > > > > serve them is a irq-chip driver, right?
> > > > > > 
> > > > > > In case of the *_EOF interrupts when they can be used outside the idmac
> > > > > > driver then yes. If not then not for the reasons I explained.
> > > > > 
> > > > > Wait a minute, are you suggesting to handle interrupts that are exported 
> > > > > to client drivers and that are "internal" to ipu_idmac differently? Like 
> > > > > exported once - properly using the irq chip machinery, and internal once 
> > > > > just demux in the driver hiding them from the kernel?... Or have I 
> > > > > misunderstood you? If this is indeed what you mean, then that doesn't 
> > > > > sound like a good idea to me, sorry. Like you configure a chained handler, 
> > > > > then when it is called on an IRQ, you check if the reason is bits 0, 1, or 
> > > > > 2 you call generic_handle_irq(), for other bits you handle them 
> > > > > internally... grrrr... 
> > > > 
> > > > I'm not suggesting that. The *_EOF registers are all 32 bits on the *same*
> > > > register. And these 32 interrupts are inherently occupied by ipu_idmac.c
> > > > as you cannot request a idmac channel without requesting this interrupt.
> > > > So at the moment you are passing 32 bits of the same register through a
> > > > chained interrupt handler and *all* these interrupts go to the very same
> > > > interrupt handler.
> > > > For the corresponding 32 error interrupts it's probably also the idmac
> > > > engine that has to react to these interrupts, not the drivers using it.
> > > > Not sure about the remaining assorted interrupts.
> > > 
> > > Yes, I understand that. So, you're suggesting to put all EOF interrupts 
> > > under 1 irq-number. Just for example: now I have
> > > 
> > > 167:        351     ipu_irq  idmac
> > > 174:        752     ipu_irq  idmac
> > > 175:          0     ipu_irq  idmac
> > > 230:          1     ipu_irq  csi
> > > 
> > > where 167 - camera IRQs, 174 - framebuffer (panning), 175 - overlay (ok, 
> > > this one will go), 230 - CSI_EOF from IPU_INT_STAT_3 (ok, it is not used 
> > > ATM, can go too).
> > > 
> > > So, there are at least two valid users of EOF interrupts. 
> > 
> > No, there are no two users, there is *one* user, namely idmac, as
> > the output from /proc/interrupts clearly shows. The users are *not* the
> > client drivers but the idmac code.
> > 
> > Hm, what else can I say that you understand what I mean?
> > 
> > Why would you dispatch one physical interrupt to 32 interrupts (only
> > counting the *_EOF irqs) which all lead back to the same interrupt handler?
> > 
> > The client drivers are not interested in the interrupts, only the idmac
> > code is. Even if they were, you provided a callback function for this
> > case in your idmac interrupt handling code:
> > 
> > +	if (done && (desc->txd.flags & DMA_PREP_INTERRUPT) && callback)
> > +		callback(callback_param);
> 
> Ok, fair enough, you can see all *_EOF interrupts as being consumed by one 
> user - the dma channel driver. So then it is similar to a serial (or 
> whatever else) driver handling multiple instances of the hardware on one 
> system.
> 
> Let's consider what we would get, if we don't use the irq chip API:
> 
> 1. we request two interrupts - function and error - normally using 
>    request_irq in the ipu irq driver
> 
> 2. in the dma channel driver you have to register new clients with the ipu 
>    irq driver so it can call the dma channel driver EOF irq handler with 
>    the correct argument - the number of interrupting DMA channel
> 
> 3. on an interrupt you have to implement masking / unmasking / acking of 
>    interrupts, list locking, whatever it takes yourself
> 
> so you end up with an API similar to ipu_request_irq() from Freescale... 
> Yes, you can allocate your interrupt descriptors dynamically, you can save 
> a few more bytes in every descriptor object... But is it all worth it?
> 
> Ok, we can make this differently:
> 
> 1. I create a static constant array of virtual irq number <-> ipu irq bit 
>    maps, with currently only two elements - for the framebuffer and the 
>    camera, and use it in irq demultiplexing.
> 
> 2. I increment NR_IRQS by two and put a comment explaining where to look 
>    for implementing support for more IPU interrupts.
> 
> 3. as new users come and implement support for more IPU interrupts, they 
>    add entries to the array and increment NR_IRQS.
> 
> This way we add only as many irq descriptors as we really use, but the 
> mapping becomes absolutely internal, so looking in /proc/interupts gives 
> you little understanding of whose interrupts you see there.

You're still not getting it. Consider an implementation of the chained
interrupt handler like this:

static struct ipu_irq_bank irq_bank_fn[IPU_IRQ_NR_FN_BANKS] = {
        {
                .nr_irqs        = 1,	/* IDMAC EOF and NF irq */
        }, {
                .nr_irqs        = 24,	/* misc interrupts the IPU is not
					 * interested in, but clients drivers
					 * may be.
					 */
        },
};

static struct ipu_irq_bank irq_bank_err[IPU_IRQ_NR_ERR_BANKS] = {
        /* 2 groups of error interrupts */
        {
                .nr_irqs        = 1,	/* IDMAC channel error irq */
        }, {
                .nr_irqs        = 17,	/* misc interrupts the IPU is not
                                         * interested in, but clients drivers
                                         * may be.
                                         */
        },
};

/* Chained IRQ handler for IPU error interrupt */
static void ipu_irq_err(unsigned int irq, struct irq_desc *desc)
{
        struct ipu *ipu = get_irq_data(irq);
        u32 status;
        int i, line;

	/* These interrupts are handled exclusively by the IPU code */
	if (ipu_read_reg(ipu, IPU_INT_STAT_4))
		generic_handle_irq(IPU_IRQ);

	status = ipu_read_reg(ipu, IPU_INT_STAT_5);
        while ((line = ffs(status))) {
                status &= ~(1UL << (line - 1));
                generic_handle_irq(irq_bank_err[i].irq_base + line - 1);
        }
}

/* Chained IRQ handler for IPU function interrupt */
static void ipu_irq_fn(unsigned int irq, struct irq_desc *desc)
{
        struct ipu *ipu = get_irq_data(irq);
        u32 status;
        int i, line;

        /* These interrupts are handled exclusively by the IPU code */
        if (ipu_read_reg(ipu, IPU_INT_STAT_1) || ipu_read_reg(ipu, IPU_INT_STAT_2))
                generic_handle_irq(IPU_ERR_IRQ);

        status = ipu_read_reg(ipu, IPU_INT_STAT_3);
        while ((line = ffs(status))) {
                status &= ~(1UL << (line - 1));
                generic_handle_irq(irq_bank_err[i].irq_base + line - 1);
        }
}


and in ipu_idmac.c:

request_irq(IPU_ERR_IRQ, &ipu_err_handler);
request_irq(IPU_IRQ, &ipu_irq);


That said I'm not sure whether we need a chained interrupt handler at all.
Looking at the remaining interrupts it seems that for example the
CSI_EOF (IPU_INT_STAT3 bit 5) interrupt is redundant to the corresponding
channels interrupt. In the camera driver you already realized that it's
the dma end event you're interested in, not the CSI_EOF event. Not sure
if that holds for the other interrupts though.

Sascha

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