Re: [Question] m25p80 driver versus spi clock rate

Mike Frysinger
2009-06-23T18:46:35Z

On Tue, Jun 23, 2009 at 14:41, Steven A. Falco wrote:
> Mike Frysinger wrote:
>> On Mon, Jun 22, 2009 at 16:50, Steven A. Falco wrote:
>>> I am trying to figure out how the mtd/devices/m25p80.c driver is supposed
>>> to set the spi clock speed.  (Perhaps I'm making a bad assuption even to
>>> think that it _should_ set the clock speed.  If so, please say so.)
>>
>> it shouldnt.  this is done in the board resources via the speed_hz
>> field of the spi_board_info struct on a per-spi device setting.
>
> Thank you for the hint.  spi_board_info has a max_speed_hz field - it does
> not have a speed_hz field.  The various platforms all seem to set
> max_speed_hz, so perhaps that is what you meant to say.
>
> In my case, max_speed_hz is being correctly set, but that doesn't seem to be
> enough.  I have traced through the calling hierarchy, and this is what I got:
>
> 1) m25p80_read builds a spi_message, and calls spi_sync to do the transfer.
>
> 2) spi_sync calls spi_async.  I added some printk, and saw speed_hz=0 and
>   max_speed_hz=50000000.  This is consistent with my platform setup (set via
>   a dts file).
>
> 3) spi_async calls through pointer "transfer" to spi_bitbang_transfer (because
>   the PPC4xx driver doesn't set its own transfer handler).
>
> 4) spi_bitbang_transfer calls spi_master_get_devdata, then enqueues the work
>
> 5) bitbang_work iterates through the queued transfers, and if speed_hz is
>   non-zero, bitbang_work calls through setup_transfer to spi_ppc4xx_setupxfer
>   which would set the divisor.  But, as noted in step 2, speed_hz=0, so the
>   spi bus speed is not set.  Rather, it remains at whatever speed some other
>   device chose.
>
> Note that bitbang_work looks at speed_hz, not max_speed_hz.  So, I come back to
> the same problem.  Somehow speed_hz must be set in order to make bitbang_work
> call setup_transfer, yet the only place that seems to happen is in spidev.

sounds like the bitbang SPI bus driver is broken. if speed_hz is 0,
then the bus driver should fall back to the max_speed_hz from the spi
resources.
-mike