linux-kernel - Re: [PATCH 09/14] Blackfin SPI driver: Fix SPI driver to work with SPI flash ST25P16 on bf548

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Message-Id: <200710310102.36542.david-b@pacbell.net>
Date:	Wed, 31 Oct 2007 01:02:36 -0700
From:	David Brownell <david-b@...bell.net>
To:	bryan.wu@...log.com
Cc:	spi-devel-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, Sonic Zhang <sonic.zhang@...log.com>
Subject: Re: [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;
	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
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