Messages in this thread | | | From | David Brownell <> | Subject | Re: [PATCH 09/14] Blackfin SPI driver: Fix SPI driver to work with SPI flash ST25P16 on bf548 | Date | Wed, 31 Oct 2007 01:02:36 -0700 |
| |
On Wednesday 31 October 2007, Bryan Wu wrote: > IMO, the spi_transfer.speed_hz <= spi_board_info.max_speed_hz and if > spi_trasnfer.speed_hz is 0, we should use spi_board_info.max_speed_hz. > From the meaning of max_speed_hz, the spi_transfer.speed_hz should not > beyond max_speed_hz.
According to the interface spec that's not how it works; so I'm not sure what you base your opinion on. It's not defined with such constraints.
That might be a reasonable policy to adopt in many cases, and nothing prevents any given protocol driver from choosing to follow it. But likewise, if the driver chooses _not_ to follow it, it's wrong for a controller driver to add its own private rules.
> In your explanation, the max_speed_hz is just a default value. the > transfer speed_hz can beyond max_speed_hz.
Kerneldoc for board info says;
* @max_speed_hz: Initializes spi_device.max_speed_hz; based on limits * from the chip datasheet and board-specific signal quality issues.
So yes -- initial value, it can be changed. If a driver says
spi->max_speed_hz = X; spi_setup(spi);
that's how it requests a different clock rate ceiling. It can lower the rate; it can raise it.
The reason it's called max_speed_hz is that most systems compute the clock rate by dividing a base clock, and few can achieve that exact value. The name indicates that it's to be treated as an upper limit, not a lower limit or exact value. The controller driver should get as close to that rate as it can, without going over.
> We found the bug in M25P16 > should be related this missing checking of the transfer speed_hz.
The m25p80 driver doesn't change that value...
So if there was any issue there, it must have been related to something else in your controller driver.
- Dave - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |