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

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Date:	Wed, 09 Dec 2015 14:44:15 -0700
From:	Toshi Kani <toshi.kani@....com>
To:	Dan Williams <dan.j.williams@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Tony Luck <tony.luck@...el.com>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/3] resource: Add @flags to region_intersects()

On Wed, 2015-12-09 at 08:25 -0800, Dan Williams wrote:
> On Thu, Dec 3, 2015 at 12:35 PM, Toshi Kani <toshi.kani@....com> wrote:
> > 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).
> 
> IORESOURCE_PMEM is not descriptive enough for the two different types
> of pmem in the kernel.  How about we go with just
> IORESOURCE_SYSTEM_RAM for now since "is_ram()" checks are common.  Let
> the rest continue to be checked by strcmp().
> 
> For example the nvdimm-e820 driver cares about "Persistent Memory
> (legacy)", while other forms of pmem may just be "reserved" and only
> the driver knows that it is pmem.  An IORESOURCE_PMEM would not be
> reliable nor descriptive enough.

Agreed.  I will introduce a new type for System RAM, and leave the strcmp
check for other types.

Thanks,
-Toshi
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