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Re: [PATCH v7 2/4] MIPS: Octeon: Setup irq_domains for interrupts.

To: Grant Likely <grant.likely@secretlab.ca>
Subject: Re: [PATCH v7 2/4] MIPS: Octeon: Setup irq_domains for interrupts.
From: David Daney <david.daney@cavium.com>
Date: Wed, 28 Mar 2012 18:46:27 -0700
Cc: Rob Herring <robherring2@gmail.com>, David Daney <ddaney.cavm@gmail.com>, "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>, "ralf@linux-mips.org" <ralf@linux-mips.org>, "devicetree-discuss@lists.ozlabs.org" <devicetree-discuss@lists.ozlabs.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On 03/28/2012 03:08 PM, Grant Likely wrote:
On Wed, 28 Mar 2012 09:16:05 -0700, David Daney<david.daney@cavium.com>  wrote:
On 03/28/2012 07:21 AM, Rob Herring wrote:
On 03/27/2012 05:31 PM, David Daney wrote:
On 03/27/2012 03:05 PM, Rob Herring wrote:
On 03/27/2012 01:24 PM, David Daney wrote:
On 03/26/2012 06:56 PM, Rob Herring wrote:
On 03/26/2012 02:31 PM, David Daney wrote:
From: David Daney<david.daney@cavium.com>
[...]
+static bool octeon_irq_ciu_is_edge(unsigned int line, unsigned int
bit)
+{
+    bool edge = false;
+
+    if (line == 0)
+        switch (bit) {
+        case 48 ... 49: /* GMX DRP */
+        case 50: /* IPD_DRP */
+        case 52 ... 55: /* Timers */
+        case 58: /* MPI */
+            edge = true;
+            break;
+        default:
+            break;
+        }
+    else /* line == 1 */
+        switch (bit) {
+        case 47: /* PTP */
+            edge = true;
+            break;
+        default:
+            break;
+        }
+    return edge;

Moving in the right direction, but I still don't get why this is not in
the CIU binding as a 3rd cell?

There are a several reasons, in no particular order they are:

o There is no 3rd cell.  The bindings were discussed with Grant here:
     http://www.linux-mips.org/archives/linux-mips/2011-05/msg00355.html


Then add one.

I can't.  The dtb is already programmed into the bootloader ROMs,
changing the kernel code will not change that.  It is fait accompli.


o The edge/level thing cannot be changed, and the irq lines don't leave
the SOC, so hard coding it is possible.

Right, but DT describes h/w connections and this is an aspect of the
connection. This may be fixed for the SOC, but it's not fixed for the
CIU (i.e. could change in future chips), right?

In theory yes.  However:

1) The chip designers will not change it.

2) There will likely be no more designs with either CIU or CIU2, so we
know what all the different possibilities are today.

When CIU3 is deployed, we will use the lessons we have learned to do
things the Right Way.


There's 2 reasons why you would not put this into DTS:

- All irq lines' trigger type are the same, fixed and known.
- You can read a register to tell you the trigger type.

Even if it's not going to change ever, it's still worth putting into the
DTS as it is well suited for holding that data and it is just data.

Agreed that it could be in the DTS, and retrospect it probably should
have been put in the DTS, but it wasn't.  So I think what we have now is
a workable solution, and has the added attraction of working with
already deployed boards.

Aren't you building in the dtb to the kernel?

No.  This seems like a bit of a disconnect in this discussion.  From the
cover-letter:

o A device tree template is statically linked into the kernel image.
    Based on SOC type and board type, legacy configuration probing code
    is used to prune and patch the device tree template.

o New SOCs and boards will directly use a device tree passed by the
    bootloader.

I think the real surprise here is that Octeron is depending on a
masked-in copy of the device tree.  It has alwasy been a strong
recommendation to store the .dtb along side the kernel in reflashable
storage for exactly the reason that updates may be required.  Review
is vitally important, but nobody can foresee all the implications
until the bindings are actually used.

I'm not going to harp over this, I understand that you're hamstrung
here, but please do pass on to the firmware engineers that storing the
.dtb in masked rom is risky.


Well in most cases it is not masked ROM, but rather some sort of persistent but modifiable storage. But still getting people (both internal and external) to update it is not as easy as living with the status quo. The idea is that someone should be able to get a kernel.orq kernel and use it, without having to re-flash their board's bootloader.

David Daney


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