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

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Date:	Wed, 31 Oct 2007 00:11:33 -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 Tuesday 30 October 2007, Bryan Wu wrote:
> Maybe there are some confusion of mixing up the spi_trasnfer.speed_hz
> with the spi_device.max_speed_hz.
> 
> spi_device.max_speed_hz comes from spi_board_info.max_speed_hz, it is
> for the default max speed value.

It's initialized from board_info, yes.  Drivers can override it
using spi_setup().

One would expect they only override _downwards_ but that's not
guaranteed anywhere.  A driver might have a way to establish that
this particular board can run faster, for example.

> spi_transfer.speed_hz comes from upper applications for each spi
> transfer setting.

Certainly; all spi_transfer records come from applications!
If that value is zero, that transfer segment uses the limit
from the spi_device ... otherwise, it can differ.  Again,
the limits can vary based on devise characteristics; maybe
it can't feed data as fast for some commands.

(ISTR the M25P16 in $SUBJECT has two read commands, one of
which is only usable at clock rates below 33 MHz or so, but
most other commands can work above that speed just fine.)

- Dave
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