Re: [PATCHv2 2/4] mm: cma: Contiguous Memory Allocator added

[Hans Verkuil]

Hi Michal,

Thanks for working on this, we definitely need something along these lines.

On Monday 26 July 2010 16:40:30 Michal Nazarewicz wrote:
> The Contiguous Memory Allocator framework is a set of APIs for
> allocating physically contiguous chunks of memory.
> 
> Various chips require contiguous blocks of memory to operate.  Those
> chips include devices such as cameras, hardware video decoders and
> encoders, etc.
> 
> The code is highly modular and customisable to suit the needs of
> various users.  Set of regions reserved for CMA can be configured on
> run-time and it is easy to add custom allocator algorithms if one
> has such need.
> 
> For more details see Documentation/contiguous-memory.txt.
> 
> Signed-off-by: Michal Nazarewicz <m.nazarewicz@xxxxxxxxxxx>
> Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
> Reviewed-by: Pawel Osciak <p.osciak@xxxxxxxxxxx>
> ---
>  Documentation/00-INDEX                             |    2 +
>  .../ABI/testing/sysfs-kernel-mm-contiguous         |    9 +
>  Documentation/contiguous-memory.txt                |  646 +++++++++++
>  Documentation/kernel-parameters.txt                |    4 +
>  include/linux/cma.h                                |  445 ++++++++
>  mm/Kconfig                                         |   34 +
>  mm/Makefile                                        |    3 +
>  mm/cma-best-fit.c                                  |  407 +++++++
>  mm/cma.c                                           | 1170 ++++++++++++++++++++
>  9 files changed, 2720 insertions(+), 0 deletions(-)
>  create mode 100644 Documentation/ABI/testing/sysfs-kernel-mm-contiguous
>  create mode 100644 Documentation/contiguous-memory.txt
>  create mode 100644 include/linux/cma.h
>  create mode 100644 mm/cma-best-fit.c
>  create mode 100644 mm/cma.c
> 
> diff --git a/Documentation/00-INDEX b/Documentation/00-INDEX
> index 5405f7a..bb50209 100644
> --- a/Documentation/00-INDEX
> +++ b/Documentation/00-INDEX
> @@ -94,6 +94,8 @@ connector/
>  	- docs on the netlink based userspace<->kernel space communication mod.
>  console/
>  	- documentation on Linux console drivers.
> +contiguous-memory.txt
> +	- documentation on physically-contiguous memory allocation framework.
>  cpu-freq/
>  	- info on CPU frequency and voltage scaling.
>  cpu-hotplug.txt
> diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-contiguous b/Documentation/ABI/testing/sysfs-kernel-mm-contiguous
> new file mode 100644
> index 0000000..05e2f6a
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-kernel-mm-contiguous
> @@ -0,0 +1,9 @@
> +What:		/sys/kernel/mm/contiguous/
> +Date:		July 2008
> +Contact:	Michal Nazarewicz <m.nazarewicz@xxxxxxxxxxx>
> +Description:
> +		/sys/kernel/mm/contiguous/ contains two files: asterisk and
> +		map.  They are used to configure the Contiguous Memory
> +		Allocator framework.
> +
> +		For details see Documentation/contiguous-memory.txt.
> diff --git a/Documentation/contiguous-memory.txt b/Documentation/contiguous-memory.txt
> new file mode 100644
> index 0000000..6eb1295
> --- /dev/null
> +++ b/Documentation/contiguous-memory.txt
> @@ -0,0 +1,646 @@
> +                                                             -*- org -*-
> +
> +* Contiguous Memory Allocator
> +
> +   The Contiguous Memory Allocator (CMA) is a framework, which allows
> +   setting up a machine-specific configuration for physically-contiguous
> +   memory management. Memory for devices is then allocated according
> +   to that configuration.
> +
> +   The main role of the framework is not to allocate memory, but to
> +   parse and manage memory configurations, as well as to act as an
> +   in-between between device drivers and pluggable allocators. It is
> +   thus not tied to any memory allocation method or strategy.
> +
> +** Why is it needed?
> +
> +    Various devices on embedded systems have no scatter-getter and/or
> +    IO map support and as such require contiguous blocks of memory to
> +    operate.  They include devices such as cameras, hardware video
> +    decoders and encoders, etc.
> +
> +    Such devices often require big memory buffers (a full HD frame is,
> +    for instance, more then 2 mega pixels large, i.e. more than 6 MB
> +    of memory), which makes mechanisms such as kmalloc() ineffective.
> +
> +    Some embedded devices impose additional requirements on the
> +    buffers, e.g. they can operate only on buffers allocated in
> +    particular location/memory bank (if system has more than one
> +    memory bank) or buffers aligned to a particular memory boundary.
> +
> +    Development of embedded devices have seen a big rise recently
> +    (especially in the V4L area) and many such drivers include their
> +    own memory allocation code. Most of them use bootmem-based methods.
> +    CMA framework is an attempt to unify contiguous memory allocation
> +    mechanisms and provide a simple API for device drivers, while
> +    staying as customisable and modular as possible.
> +
> +** Design
> +
> +    The main design goal for the CMA was to provide a customisable and
> +    modular framework, which could be configured to suit the needs of
> +    individual systems.  Configuration specifies a list of memory
> +    regions, which then are assigned to devices.  Memory regions can
> +    be shared among many device drivers or assigned exclusively to
> +    one.  This has been achieved in the following ways:

OK, I like the idea of regions, i.e. defining memory areas with specific
properties or uses.

But why should it be possible to define regions through kernel parameters?
Regions are typically fixed for a particular platform and can be setup in the
platform specific code. Actually, one region could be setup by default:
DMA-able memory. That would be very handy in fact for many PCI-based TV
capture drivers.

I think that the only thing that you want to set in the kernel params is the
size of each region.

The same with assigning regions to drivers: why would you want to do that?
The driver should know which regions it can use (with possible fallbacks).
And it can know that provided regions are setup by the platform code and not
created dynamically. This will simplify things enormously.

> +    1. The core of the CMA does not handle allocation of memory and
> +       management of free space.  Dedicated allocators are used for
> +       that purpose.
> +
> +       This way, if the provided solution does not match demands
> +       imposed on a given system, one can develop a new algorithm and
> +       easily plug it into the CMA framework.
> +
> +       The presented solution includes an implementation of a best-fit
> +       algorithm.

Again, do we really need user-settable per-region allocators? Just provide
one with the option to later choose others through the kernel Kconfig files.

We can always add more complex scenarios later, but for an initial version
I'd keep it simple.

> +
> +    2. CMA allows a run-time configuration of the memory regions it
> +       will use to allocate chunks of memory from.  The set of memory
> +       regions is given on command line so it can be easily changed
> +       without the need for recompiling the kernel.
> +
> +       Each region has it's own size, alignment demand, a start
> +       address (physical address where it should be placed) and an
> +       allocator algorithm assigned to the region.
> +
> +       This means that there can be different algorithms running at
> +       the same time, if different devices on the platform have
> +       distinct memory usage characteristics and different algorithm
> +       match those the best way.

Seems overengineering to me. Just ensure that the code can be extended later
to such hypothetical scenarios. They are hypothetical, right?

> +    3. When requesting memory, devices have to introduce themselves.
> +       This way CMA knows who the memory is allocated for.  This
> +       allows for the system architect to specify which memory regions
> +       each device should use.
> +
> +       3a. Devices can also specify a "kind" of memory they want.
> +           This makes it possible to configure the system in such
> +           a way, that a single device may get memory from different
> +           memory regions, depending on the "kind" of memory it
> +           requested.  For example, a video codec driver might want to
> +           allocate some shared buffers from the first memory bank and
> +           the other from the second to get the highest possible
> +           memory throughput.

Not sure I understand this. Isn't this just two regions, one for each memory bank,
and the driver requests some buffers from one region and some from the other?
Not sure how a 'kind of memory' features in this.

> +    4. For greater flexibility and extensibility, the framework allows
> +       device drivers to register private regions of reserved memory
> +       which then may be used only by them.
> +
> +       As an effect, if a driver would not use the rest of the CMA
> +       interface, it can still use CMA allocators and other
> +       mechanisms.

Why would you? Is there an actual driver that will need this?

> +
> +       4a. Early in boot process, device drivers can also request the
> +           CMA framework to a reserve a region of memory for them
> +           which then will be used as a private region.
> +
> +           This way, drivers do not need to directly call bootmem,
> +           memblock or similar early allocator but merely register an
> +           early region and the framework will handle the rest
> +           including choosing the right early allocator.

The whole concept of private regions seems unnecessary to me.

<big snip>

It looks to me as if you tried to think of all possible hypothetical situations
and write a framework for that. Of course, you may know more than I do, and some
of these situations actually happen.

The basic design ideas are solid, I think. But you should get rid of all the
fancy features and go back to basics. We can always add those features later
should that become necessary. But removing features is much, much harder.

Regards,

	Hans

-- 
Hans Verkuil - video4linux developer - sponsored by TANDBERG, part of Cisco
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