linux-kernel - Re: [PATCH v3 1/3] resource: Add @flags to region

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Date:	Thu, 03 Dec 2015 13:35:25 -0700
From:	Toshi Kani <toshi.kani@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Tony Luck <tony.luck@...el.com>,
	Vishal L Verma <vishal.l.verma@...el.com>,
	Linux MM <linux-mm@...ck.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] resource: Add @flags to region_intersects()

On Thu, 2015-12-03 at 11:01 -0800, Linus Torvalds wrote:
> On Thu, Dec 3, 2015 at 10:40 AM, Borislav Petkov <bp@...en8.de> wrote:
> > On Thu, Dec 03, 2015 at 11:54:19AM -0700, Toshi Kani wrote:
> > > Adding a new type for regular memory will require inspecting the 
> > > codes using IORESOURCE_MEM currently, and modify them to use the new 
> > > type if their target ranges are regular memory.  There are many 
> > > references to this type across multiple architectures and drivers, 
> > > which make this inspection and testing challenging.
> > 
> > What's wrong with adding a new type_flags to struct resource and not
> > touching IORESOURCE_* at all?
> 
> Bah. Both of these ideas are bogus.
> 
> Just add a new flag. The bits are already modifiers that you can
> *combine* to show what kind of resource it is, and we already have
> things like IORESOURCE_PREFETCH etc, that are in *addition* to the
> normal IORESOURCE_MEM bit.
> 
> Just add another modifier: IORESOURCE_RAM.
> 
> So it would still show up as IORESOURCE_MEM, but it would have
> additional information specifying that it's actually RAM.
> 
> If somebody does something like
> 
>      if (res->flags == IORESOURCE_MEM)
> 
> then they are already completely broken and won't work *anyway*. It's
> a bitmask, bit a set of values.

Yes, if we can assign new modifiers, that will be quite simple. :-)  I
assume we can allocate new bits from the remaining free bits as follows.

+#define IORESOURCE_SYSTEM_RAM  0x01000000      /* System RAM */
+#define IORESOURCE_PMEM        0x02000000      /* Persistent memory */
 #define IORESOURCE_EXCLUSIVE   0x08000000      /* Userland may not map
this resource */

Note, SYSTEM_RAM represents the OS memory, i.e. "System RAM", not any RAM
ranges.

With the new modifiers, region_intersect() can check these ranges.  One
caveat is that the modifiers are not very extensible for new types as they
are bit maps.  region_intersect() will no longer be capable of checking any
regions with any given name.  I think this is OK since this function was
introduced recently, and is only used for checking "System RAM" and
"Persistent Memory" (with this patch series).

Thanks,
-Toshi
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