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: <1193816110.6971.50.camel@roc-laptop>
Date:	Wed, 31 Oct 2007 15:35:10 +0800
From:	Bryan Wu <bryan.wu@...log.com>
To:	David Brownell <david-b@...bell.net>
Cc:	bryan.wu@...log.com, 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 Wed, 2007-10-31 at 00:11 -0700, David Brownell wrote:
> 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.
> 

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.

In your explanation, the max_speed_hz is just a default value. the
transfer speed_hz can beyond max_speed_hz. We found the bug in M25P16
should be related this missing checking of the transfer  speed_hz.

> 
> > 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.)
> 
OK, We check it on Blackfin board.
-Bryan Wu
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