Re: [Question] m25p80 driver versus spi clock rate

Steven A. Falco
2009-06-23T18:41:50Z

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

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

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

Thanks,
Steve