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

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Date:	Wed, 9 Dec 2015 08:25:58 -0800
From:	Dan Williams <dan.j.williams@...il.com>
To:	Toshi Kani <toshi.kani@....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 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.
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