[Arm-netbook] Keyboard news

[Luke Kenneth Casson Leighton]

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crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68


On Sun, Aug 7, 2016 at 11:17 PM, Albert ARIBAUD <albert.aribaud at free.fr> wrote:
> Le Fri, 5 Aug 2016 01:33:52 +0100
> Luke Kenneth Casson Leighton <lkcl at lkcl.net> a écrit:
>
>> > I'll push the changes tomorrow.
>>
>>  awesome.
>
> Did it only now, because of a slight annoyance: sometimes the build
> would not run at all on the target, in a reproducible way: a given
> state of the source code would either always work, or never work. And
> the change that would make a working source code stop working could be
> as simple as declaring a static uint8_t variable even without using it,
> which made me look for alignment issues.
>
> So I set up non-working case, broke with GDB into it, found the core
> in a hardfault indeed, and did a bit of climbing up and down the stack.
>
> Turns out the uint8_t array usbd_control_buffer[] declared in
> usbifaces.c is sometimes cast to an uint16_t array, and so undergoes
> word accesses. However, an array of uint8_t does not have alignment
> constraints, so depending on the size and alignment of other globals or
> static locals, usbd_control_buffer could begin on an odd address, which
> would make the uint16_t alias misaligned, and the first read or write
> to it would hardfault.

 bleugh!

> I fixed this by aligning usbd_control_buffer on a 4-byte boundary (so
> that even a cast to uint32_t would not cause misaligments).

 awesome

> I then rebuilt and measured the transposing of the keyboard (using
> Timer2 as a free-running 1/48th-of-a-microsecond counter, see branch
> aaribaud/kbd_timing for an example).
>
> Before transposing, a whole keyboard scan would be 16 columns times
> about 425 cycles per column, total about 141 us. After transposing, it
> is 8 columns times about 686 cycles per column, that is, about 115 us,
> an 18% reduction.

 that's pretty damn good... 0.000115 seconds...

> All the code is in the repo now.
>
> I /think/ there might be some finer improvements to make.

 ... is it worth it at this phase?

l.