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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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