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From: Tim HRM on 15 Jul 2010 20:50
On Thu, Jul 15, 2010 at 4:55 AM, Russell King - ARM Linux
<linux(a)arm.linux.org.uk> wrote:
> On Wed, Jul 14, 2010 at 06:29:58PM -0700, Zach Pfeffer wrote:
>> The VCM ensures that all mappings that map a given physical buffer:
>> IOMMU mappings, CPU mappings and one-to-one device mappings all map
>> that buffer using the same (or compatible) attributes. At this point
>> the only attribute that users can pass is CACHED. In the absence of
>> CACHED all accesses go straight through to the physical memory.
>
> So what you're saying is that if I have a buffer in kernel space
> which I already have its virtual address, I can pass this to VCM and
> tell it !CACHED, and it'll setup another mapping which is not cached
> for me?
>
> You are aware that multiple V:P mappings for the same physical page
> with different attributes are being outlawed with ARMv6 and ARMv7
> due to speculative prefetching. �The cache can be searched even for
> a mapping specified as 'normal, uncached' and you can get cache hits
> because the data has been speculatively loaded through a separate
> cached mapping of the same physical page.
>
> FYI, during the next merge window, I will be pushing a patch which makes
> ioremap() of system RAM fail, which should be the last core code creator
> of mappings with different memory types. �This behaviour has been outlawed
> (as unpredictable) in the architecture specification and does cause
> problems on some CPUs.
>
> We've also the issue of multiple mappings with differing cache attributes
> which needs addressing too...
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>

Interesting, since I seem to remember the MSM devices mostly conduct
IO through regions of normal RAM, largely accomplished through
ioremap() calls.

Without more public domain documentation of the MSM chips and AMSS
interfaces I wouldn't know how to avoid this, but I can imagine it
creates a bit of urgency for Qualcomm developers as they attempt to
upstream support for this most interesting SoC.

--
Timothy Meade
tmzt #htc-linux
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From: Tim HRM on 19 Jul 2010 05:30
On Fri, Jul 16, 2010 at 8:01 PM, Larry Bassel <lbassel(a)codeaurora.org> wrote:
> On 16 Jul 10 08:58, Russell King - ARM Linux wrote:
>> On Thu, Jul 15, 2010 at 08:48:36PM -0400, Tim HRM wrote:
>> > Interesting, since I seem to remember the MSM devices mostly conduct
>> > IO through regions of normal RAM, largely accomplished through
>> > ioremap() calls.
>> >
>> > Without more public domain documentation of the MSM chips and AMSS
>> > interfaces I wouldn't know how to avoid this, but I can imagine it
>> > creates a bit of urgency for Qualcomm developers as they attempt to
>> > upstream support for this most interesting SoC.
>>
>> As the patch has been out for RFC since early April on the linux-arm-kernel
>> mailing list (Subject: [RFC] Prohibit ioremap() on kernel managed RAM),
>> and no comments have come back from Qualcomm folk.
>
> We are investigating the impact of this change on us, and I
> will send out more detailed comments next week.
>
>>
>> The restriction on creation of multiple V:P mappings with differing
>> attributes is also fairly hard to miss in the ARM architecture
>> specification when reading the sections about caches.
>>
>
> Larry Bassel
>
> --
> Sent by an employee of the Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
>

Hi Larry and Qualcomm people.
I'm curious what your reason for introducing this new api (or adding
to dma) is. Specifically how this would be used to make the memory
mapping of the MSM chip dynamic in contrast to the fixed _PHYS defines
in the Android and Codeaurora trees.

I'm also interested in how this ability to map memory regions as files
for devices like KGSL/DRI or PMEM might work and why this is better
suited to that purpose than existing methods, where this fits into
camera preview and other issues that have been dealt with in these
trees in novel ways (from my perspective).

Thanks,
Timothy Meade
tmzt #htc-linux
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From: Timothy Meade on 20 Jul 2010 21:50
On Tue, Jul 20, 2010 at 8:44 PM, Zach Pfeffer <zpfeffer(a)codeaurora.org> wrote:
> On Mon, Jul 19, 2010 at 05:21:35AM -0400, Tim HRM wrote:
>> On Fri, Jul 16, 2010 at 8:01 PM, Larry Bassel <lbassel(a)codeaurora.org> wrote:
>> > On 16 Jul 10 08:58, Russell King - ARM Linux wrote:
>> >> On Thu, Jul 15, 2010 at 08:48:36PM -0400, Tim HRM wrote:
>> >> > Interesting, since I seem to remember the MSM devices mostly conduct
>> >> > IO through regions of normal RAM, largely accomplished through
>> >> > ioremap() calls.
>> >> >
>> >> > Without more public domain documentation of the MSM chips and AMSS
>> >> > interfaces I wouldn't know how to avoid this, but I can imagine it
>> >> > creates a bit of urgency for Qualcomm developers as they attempt to
>> >> > upstream support for this most interesting SoC.
>> >>
>> >> As the patch has been out for RFC since early April on the linux-arm-kernel
>> >> mailing list (Subject: [RFC] Prohibit ioremap() on kernel managed RAM),
>> >> and no comments have come back from Qualcomm folk.
>> >
>> > We are investigating the impact of this change on us, and I
>> > will send out more detailed comments next week.
>> >
>> >>
>> >> The restriction on creation of multiple V:P mappings with differing
>> >> attributes is also fairly hard to miss in the ARM architecture
>> >> specification when reading the sections about caches.
>> >>
>> >
>> > Larry Bassel
>> >
>> > --
>> > Sent by an employee of the Qualcomm Innovation Center, Inc.
>> > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
>> >
>>
>> Hi Larry and Qualcomm people.
>> I'm curious what your reason for introducing this new api (or adding
>> to dma) is. �Specifically how this would be used to make the memory
>> mapping of the MSM chip dynamic in contrast to the fixed _PHYS defines
>> in the Android and Codeaurora trees.
>
> The MSM has many integrated engines that allow offloading a variety of
> workloads. These engines have always addressed memory using physical
> addresses, because of this we had to reserve large (10's MB) buffers
> at boot. These buffers are never freed regardless of whether an engine
> is actually using them. As you can imagine, needing to reserve memory
> for all time on a device that doesn't have a lot of memory in the
> first place is not ideal because that memory could be used for other
> things, running apps, etc.
>
> To solve this problem we put IOMMUs in front of a lot of the
> engines. IOMMUs allow us to map physically discontiguous memory into a
> virtually contiguous address range. This means that we could ask the
> OS for 10 MB of pages and map all of these into our IOMMU space and
> the engine would still see a contiguous range.
>


I see. Much like I suspected, this is used to replace the static
regime of the earliest Android kernel. You mention placing IOMMUs in
front of the A11 engines, you are involved in this architecture as an
engineer or similar? Is there a reason a cooperative approach using
RPC or another mechanism is not used for memory reservation, this is
something that can be accomplished fully on APPS side?

> In reality, limitations in the hardware meant that we needed to map
> memory using larger mappings to minimize the number of TLB
> misses. This, plus the number of IOMMUs and the extreme use cases we
> needed to design for led us to a generic design.
>
> This generic design solved our problem and the general mapping
> problem. We thought other people, who had this same big-buffer
> interoperation problem would also appreciate a common API that was
> built with their needs in mind so we pushed our idea up.
>
>>
>> I'm also interested in how this ability to map memory regions as files
>> for devices like KGSL/DRI or PMEM might work and why this is better
>> suited to that purpose than existing methods, where this fits into
>> camera preview and other issues that have been dealt with in these
>> trees in novel ways (from my perspective).
>
> The file based approach was driven by Android's buffer passing scheme
> and the need to write userspace drivers for multimedia, etc...
>
>
So the Android file backed approach is obiviated by GEM and other mechanisms?

Thanks you for you help,
Timothy Meade
-tmzt #htc-linux (facebook.com/HTCLinux)
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