[Question] m25p80 driver versus spi clock rate

Steven A. Falco sfalco at harris.com
Tue Jun 23 14:41:30 EDT 2009

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.
> -mike

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.

Stefan - I believe you wrote the SPI_PPC4XX driver.  Can you comment on how
this is supposed to work?  My device tree has:

				m25p16 at 1 {
					compatible = "st,m25p80";
					reg = <1>;
					spi-max-frequency = <50000000>;

				fpdlite at 2 {
					compatible = "linux,spidev";
					reg = <2>;
					spi-max-frequency = <200000>;

and the issue is that fpdlite at 2 drops the frequency to 200000, but m25p16 at 1
does not raise the frequency to 50000000.


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