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SubjectRe: [PATCH 09/14] Blackfin SPI driver: Fix SPI driver to work with SPI flash ST25P16 on bf548
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;

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
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