[PATCH] cpm2: Round the baud-rate clock divider to the nearest integer.

[Laurent Pinchart]

On Thursday 20 November 2008 17:00:54 Kumar Gala wrote:
> On Nov 20, 2008, at 9:57 AM, Laurent Pinchart wrote:
> > On Thursday 20 November 2008 16:32:21 Kumar Gala wrote:
> >> On Nov 20, 2008, at 6:32 AM, Laurent Pinchart wrote:
> >>> Instead of rounding the divider down, improve the baud-rate
> >>> generators
> >>> accuracy by rounding to the nearest integer.
> >>>
> >>> Signed-off-by: Laurent Pinchart <laurentp at cse-semaphore.com>
> >>> ---
> >>> arch/powerpc/sysdev/cpm2.c |    3 ++-
> >>> 1 files changed, 2 insertions(+), 1 deletions(-)
> >>
> >> is this fixing a bug or just making things better?
> >
> > I guess it depends on your definition of bug :-) The problem has
> > always been present in the kernel sources, and people seem to have coped
> > with it until today. It is still a bug in my opinion, but I suppose the
> > patch can wait until 2.6.29 especially if we want to get it tested (it
> > slightly changes baud rates after all).
>
> Ok. I'll put this in the .29 queue.  Can you provide a bit more detail
> on the issue this is resolving.

Sure.

Let's assume a 25 MHz BRG clock. When setting a 115200 bds baud rate for an 
SMC port in UART mode (16x oversampling), the divisor should be

25e6/(16*115200) ~= 13.563368

The current code rounds this down. The resulting baud rate is

25e6/(16*13) ~= 120192 bds

The relative error is thus (115200 - 120192)/115200 ~= 4.33%

With the new code, the divisor is rounded to the nearest integer (14). The 
resulting baud rate is

25e6/(16*14) ~= 111607 bds

and the relative error becomes (115200 - 111607)/115200 ~= 3.12%

-- 
Laurent Pinchart
CSE Semaphore Belgium

Chaussee de Bruxelles, 732A
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