Subject: Re: "memory" binding issues
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From: Kumar Gala <galak@codeaurora.org>
In-Reply-To: <1379300274.4098.77.camel@pasglop>
Date: Mon, 16 Sep 2013 10:22:48 -0500
Cc: devicetree@vger.kernel.org,
        Linux Kernel list <linux-kernel@vger.kernel.org>,
        m.szyprowski@samsung.com, swarren@nvidia.com, rob.herring@calxeda.com,
        Grant Likely <grant.likely@secretlab.ca>
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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On Sep 15, 2013, at 9:57 PM, Benjamin Herrenschmidt wrote:

> [resent to the right list this time around]
> 
> Hi folks !
> 
> So I don't have the bandwidth to follow closely what's going on, but I
> just today noticed the crackpot that went into 3.11 as part of commit:
> 
> 9d8eab7af79cb4ce2de5de39f82c455b1f796963
> drivers: of: add initialization code for dma reserved memory
> 
> Fist of all, do NOT add (or change) a binding as part of a patch
> implementing code, it's gross.
> 
> Secondly, I don't know how much that binding was discussed on the list
> (I assume it was and I just missed it) but it's gross.
> 
> It duplicates a binding that Jeremy Kerr had proposed a while ago for
> a /reserved-ranges (and /reserved-names) pair of properties, possibly in
> a better way but the fact is that the original binding received little
> or no feedback and we went on and implemented support for it in powerpc
> back in early 3.11 merge window.
> 
> Additionally, it has the following issues:
> 
> - It describes the "memory" node as /memory, which is WRONG
> 
> It should be "/memory@unit-address, this is important because the Linux
> kernel of_find_device_by_path() isn't smart enough to do partial
> searches (unlike the real OFW one) and thus to ignore the unit address
> for search purposes, and you *need* the unit address if you have
> multiple memory nodes (which you typically do on NUMA machines).
> 
> - To add to the above mistake, it defines "reserved memory" as a child
> node of the "/memory" node. That is wrong or at least poorly thought
> out. There can be several "memory" nodes, representing different areas
> of memory, possibly even interleaved, having the reserved ranges as
> children of a specific memory nodes thus doesn't work very well.
> Breaking them up into regions based on what memory nodes they cover is
> really nasty. Basically, the "reserved-memory" node should have been at
> the root of the device-tree.
> 
> - It provides no indication of what a given region is used for (or used
> by). In the example, "display_region" is a label (thus information that
> is lost) and unless it's referenced by another node there is no good way
> to know what this region is about which is quite annoying.
> 
> - The "memory-region" property is a phandle to a "reserved-memory"
> region, this is not intuitive. If anything, the property should have
> been called "reserved-memory-region".
> 
> - The way the "compatible" property is documented breaks a basic
> premise that the device-tree is a hardware description and not some
> convenient way to pass linux specific kernel parameters accross. It is
> *ok* to pass some linux specific stuff and to make compromise based on
> what a driver generally might expect but the whole business of using
> that to describe what to put in CMA is pushing it pretty far ...
> 
> - The implementation of early_init_dt_scan_reserved_mem() will not work
> on a setup whose /memory node has a unit address (typically /memory@0)
> 
> Now, I'd like to understand why not use the simpler binding originally
> proposed by Jeremy, which had the advantage of proposing a unique name
> per region in the traditional form "vendor,name", which then allows
> drivers to pick up the region directly if they wish to query, remove or
> update it in the tree for example (because they changed the framebuffer
> address for example and want kexec to continue working).
> 
> I don't object to having a node per region, though it seemed unnecessary
> at the time, but in any case, the current binding is crap and need to be
> fixed urgently before its use spreads.
> 
> Ben.


Where is Jermey's binding documented ?

Is there concern of "breaking" whatever got merged in powerpc?

- k

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation

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