Subject: Re: [PATCH 2/6] PowerPC 440EPx: Sequoia DTS

From: Sergei Shtylyov <[email-masked]>
Date: Fri, 03 Aug 2007 19:47:43 +0400

Hello.

David Gibson wrote:

>>>Index: working-2.6/Documentation/powerpc/booting-without-of.txt
>>>===================================================================
>>>--- working-2.6.orig/Documentation/powerpc/booting-without-of.txt	2007-07-30 17:07:14.000000000 +1000
>>>+++ working-2.6/Documentation/powerpc/booting-without-of.txt	2007-07-30 17:07:14.000000000 +1000
>>>@@ -1757,45 +1757,23 @@ platforms are moved over to use the flat
>>> 		};
>>> 	};
>>> 
>>>-    j) Flash chip nodes
>>>+   j) CFI or JEDEC memory-mapped NOR flash
>>> 
>>>     Flash chips (Memory Technology Devices) are often used for solid state
>>>     file systems on embedded devices.

>>>-    Required properties:
>>>+     - compatible : should contain the specific model of flash chip(s) used
>>>+       followed by either "cfi-flash" or "jedec-flash"

>>    This "compatible" prop (and the node in whole) doesn't say a
>>thing about how the flash is mapped into the CPU address space.  I
>>strongly disagree that this node provides enough info to select a
>>driver. :-/

> To be honest, I'm not sure that describing the mapping is really the
> job of the compatible property.  That the flash is mapped into the
> address space is kind of implicit in the way the reg interacts with
> the parents' ranges property.

    Ah, I keep forgetting about implied 1:1 parent/child address 
correspondence... :-<
    But does it really imply how the (low) address bits of the *child* bus 
("ebc" in this case) are connected to the chip?  I don't think so...

> Can you describe some of the options for *not* direct mapped flash
> chips - I can't reasonably come up with a way of describing the
> distinction when I've never seen NOR flash other than direct mapped.

    You're lucky to live in the single-endian world.  Once you're in bi-endian 
one, all kinds of strange mappings become possible.  I've seen the MIPS 
mapping driver which required swapping flash bytes in s/w in BE mode, i.e. 
couldn't be served by physmap, yet that's not all...  here's the code of its 
map read*() methods:

static __u8 xxx_map_read8(struct map_info *map, unsigned long offs)
{
         u16 val;

         val = readw(map->map_priv_1 + (offs & ~1));
         if (offs & 1)
                 return ((val >> 8) & 0xff);
         else
                 return (val & 0xff);
}

static __u16 bcm947xx_map_read16(struct map_info *map, unsigned long offs)
{
         return readw(map->map_priv_1 + offs);
}

static __u32 bcm947xx_map_read32(struct map_info *map, unsigned long offs)
{
         return readl(map->map_priv_1 + offs);
}

... while the simple map (used by physmap) has just __raw_read*() for those?

>>>+     - reg : Address range of the flash chip
>>>+     - bank-width : Width (in bytes) of the flash bank.  Equal to the device width
>>>+       times the number of interleaved chips.
>>>+     - device-width : (optional) Width of a single flash chip.  If omitted,
>>>+       assumed to be equal to 'bank-width'.

>>    Why then not just introduce the "interleave" prop and obsolete the
>>"bank-width"?

> Because they're equivalent information, and bank-width is what the
> code ends up actually caring about.  I don't see any reason to prefer
> giving the interleave.

    Well, "device-width" is not the thing that we care about either. ;-)

>>>Index: working-2.6/drivers/mtd/maps/physmap_of.c
>>>===================================================================
>>>--- working-2.6.orig/drivers/mtd/maps/physmap_of.c	2007-07-30 17:07:11.000000000 +1000
>>>+++ working-2.6/drivers/mtd/maps/physmap_of.c	2007-07-30 17:07:14.000000000 +1000
[...]
>>>+	for (pp = dp->child, i = 0 ; pp; pp = pp->sibling, i++) {
>>>+		const u32 *reg;
>>>+		const char *name;
>>>+		const void *ro;
>>
>>    We hardly need the above 2 variables.

> They're all used...

    I meant that there's no necessity in them.

[...]

>>    Oh, I see that the new partition representation have really simplified
>>parsing -- this function is hardly shorter than the old one... :-P

> They new format isn't supposed to be simpler to parse: it's supposed
> to be more flexible if we ever need to add more per-partition
> information than just the offset / size / read-only.

    Well, if we ever need that indeed... :-)

[...]

>>>@@ -221,6 +297,14 @@ err_out:
>>> 
>>> static struct of_device_id of_physmap_match[] = {
>>> 	{
>>>+		.compatible	= "cfi-flash",
>>>+		.data		= (void *)"cfi_probe",
>>>+	},
>>>+	{
>>>+		.compatible	= "jedec-flash",
>>>+		.data		= (void *)"jedec_probe",
>>>+	},
>>>+	{

>>    This would also trigger on non-linearly mapped CFI or JEDEC
>>flashes which is not a good idea -- however, as we're using the MTD
>>probing code anyway, it will just fail, so it's not luckily is not a
>>fatal design flaw.

> Well, if there's nothing else to distinguish them.  Which there isn't
> yet, but will need to be: see above about incomplete - helpful
> suggestions about how to describe the mapping would be more useful
> than merely pointing out the lack.

    I was thinking about adding "direct-mapped" prop... but maybe adding 
"ranges" to the parent node (that's "ebc") would indeed ensure that the flash 
is mapped 1:1 to the EBC's parent bus also.

>>>Index: working-2.6/arch/powerpc/boot/dts/ebony.dts
>>>===================================================================
>>>--- working-2.6.orig/arch/powerpc/boot/dts/ebony.dts	2007-07-30 17:07:14.000000000 +1000
>>>+++ working-2.6/arch/powerpc/boot/dts/ebony.dts	2007-07-30 17:07:14.000000000 +1000

>>[...]

>>>@@ -158,14 +161,20 @@
>>> 				};

>>> 				large-flash@2,0 {

>>    Hmm... what does @2,0 mean? :-O

> EBC chip select 2, offset 0.

    Well, so this node is under some kind of local bus node -- that's good.
Didn't know that the spec allows commas after @...

>>>-					device_type = "rom";
>>>-					compatible = "direct-mapped";
>>>-					probe-type = "JEDEC";
>>>+					compatible = "jedec-flash";
>>> 					bank-width = <1>;
>>>-					partitions = <0 380000
>>>-						      380000 80000>;
>>>-					partition-names = "fs", "firmware";
>>>+//					partitions = <0 380000
>>>+//						      380000 80000>;
>>>+//					partition-names = "fs", "firmware";
>>> 					reg = <2 0 400000>;
>>>+					#address-cells = <1>;
>>>+					#size-cells = <1>;

>>    Heh...

> Yeah, that bit's a bit ugly, I'll grant you.

    Don't we need "ranges" here, at least from the formal PoV -- as the parent 
and child address spaces differ? I know the physmap_of parser doesn't care but 
still...

>>>+					};
>>> 				};

>>    So, I don't see what we're really winning with your changes...

> "direct-mapped" is simply not a sufficiently specific compatible
> property, no two ways about it.

    Yes, for example "direct-mapped-cfi" and "direct-mapped-jedec" would have 
been better...

> This spec still needs more specific
> description of some properties, at least for JEDEC flashes.

    Yes, of course...

WBR, Sergei