Re: [PATCH] SPI: BCM2835: clock divider can be a multiple of 2

[Martin Sperl <kernel-TqfNSX0MhmxHKSADF0wUEw@public.gmane.org>]

> On 20.03.2015, at 06:06, Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org> wrote:
> 
> On 03/19/2015 03:01 AM, kernel-TqfNSX0MhmxHKSADF0wUEw@public.gmane.org wrote:
>> From: Martin Sperl <kernel-TqfNSX0MhmxHKSADF0wUEw@public.gmane.org>
>> 
>> The official documentation is wrong in this respect.
>> Has been tested empirically for dividers 2-1024
> 
> Do you have a link to a confirmation of this from the RPi Foundation or
> Broadcom, even a forum post or something? How does the RPI Foundation's
> downstream kernel restrict the divider?

Unfortunately there are no official errata by the foundation.

Here the argument by a foundation official (Gordon Hollingworth)
why they do not (want to) provide updates:
http://www.raspberrypi.org/forums/viewtopic.php?p=447724#p447724

That is in a slightly different context, but still it also applies to
errata in general.

The foundation spi-bcm2708 driver also relies on the power-of 2 rule
following to the letter the documentation they provide.

For some more infos of experience please look here:
https://github.com/notro/spi-bcm2708/wiki
http://www.raspberrypi.org/forums/viewtopic.php?f=44&t=43442
http://elinux.org/BCM2835_datasheet_errata#p156

Notro and I have been running an out of tree driver for a long time
and that never showed any issues with this multiple of 2.

Some of these kernels/drivers have been shared with lots of people
for the use with TFT displays or CAN controllers.

I have measured this empirically for every divider between 2 to 1024
by doing some transfers, and measuring them with a SALEAE logic analyzer.

See here: for the analysis:
https://github.com/msperl/spi-bcm2835/issues/8

You can also fetch the raw data and csv exports here:
https://github.com/msperl/spi-bcm2835/blob/patch_the_upstream/images/

Note that the descriptions of how to do polled, interrupt driven and 
DMA driven SPI transfers (page 158) is also not completely accurate.
It seems to show some "procedures", but these are not optimized and
some are inconsistent: e.g: this limitation of 16/12 byte transfers
while the register documentation says 16 words not bytes - which means 
64 bytes can enter the FIFO buffer (which is also the limit when
the HW signals that it is full via BCM2835_SPI_CS_TXD)

This will be one of the next patches for improvement of the driver.

Martin

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