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SubjectRe: [PATCH v5 4/4] misc: sram: add support for configurable allocation order
On Thu, Nov 15, 2012 at 02:11:35PM +0100, Philipp Zabel wrote:
> Am Mittwoch, den 14.11.2012, 19:15 +0000 schrieb Grant Likely:
> > On Thu, 18 Oct 2012 16:27:33 +0200, Philipp Zabel <p.zabel@pengutronix.de> wrote:
> > > From: Matt Porter <mporter@ti.com>
> > >
> > > Adds support for setting the genalloc pool's minimum allocation
> > > order via DT or platform data. The allocation order is optional
> > > for both the DT property and platform data case. If it is not
> > > present then the order defaults to PAGE_SHIFT to preserve the
> > > current behavior.
> > >
> > > Signed-off-by: Matt Porter <mporter@ti.com>
> > > Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
> > > ---
> > > Documentation/devicetree/bindings/misc/sram.txt | 12 ++++++++++-
> > > drivers/misc/sram.c | 14 ++++++++++++-
> > > include/linux/platform_data/sram.h | 25 +++++++++++++++++++++++
> > > 3 files changed, 49 insertions(+), 2 deletions(-)
> > > create mode 100644 include/linux/platform_data/sram.h
> > >
> > > diff --git a/Documentation/devicetree/bindings/misc/sram.txt b/Documentation/devicetree/bindings/misc/sram.txt
> > > index b64136c..b1705ec 100644
> > > --- a/Documentation/devicetree/bindings/misc/sram.txt
> > > +++ b/Documentation/devicetree/bindings/misc/sram.txt
> > > @@ -8,10 +8,20 @@ Required properties:
> > >
> > > - reg : SRAM iomem address range
> > >
> > > -Example:
> > > +Optional properties:
> > > +
> > > +- alloc-order : Minimum allocation order for the SRAM pool
> >
> > Looks okay, but I think the property name is confusing. I for one had
> > no idea what 'order' would be and why it was important. I had to read
> > the code to figure it out.
> >
> > It does raise the question though of what is this binding actually
> > for? Does it reflect a limitation of the SRAM? or of the hardware using
> > the SRAM? Or is it an optimization? How do you expect to use it?
>
> If I am not mistaken, it is about the expected use case. A driver
> allocating many small buffers would quickly fill small SRAMs if the
> allocations were of PAGE_SIZE granularity.
>
> I wonder if a common allocation size (say, 512 bytes instead of
> PAGE_SIZE) can be found that every prospective user could be reasonably
> happy with?

Unfortunately, no, 512 bytes doesn't work either. Although it'll meet
the needs of converting davinci to this driver, I have a driver under
development (the 6502 remoteproc) which needs finer grained allocation
of SRAM yet. I suspect people will object if the default is 32 bytes as
that bloats the genalloc bitmap for all users...however that's a size
that would work for me. Another possibility is to not do the
gen_pool_create() at probe time but rather provide an interface for client
drivers where the pool is created with the appropriate client-specific
parameters. This may be a bit messy though when the point of the pool is
to share it amongst several client drivers.

> > Assuming it is appropriate to put into the device tree, I'd suggest a
> > different name. Instead of 'order', how about 'sram-alloc-align' (in
> > address bits) or 'sram-alloc-min-size' (in bytes).
>
> A size in bytes would be the most obvious to me, although that allows to
> enter values that are not a power of two.

I think the implication is that this isn't even a h/w characteristic of
SRAM and, as such, does not belong in a DT binding (for that reason I
don't mind seeing that it's been dropped in v6). It's unfortunate since
it's otherwise a very clean solution. I sure wish I had a "Software
Tree" I could pass in too. ;)

-Matt


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