Message-ID: <43E1D5BF.5000807@drzeus.cx>
Date: Thu, 02 Feb 2006 10:49:51 +0100
From: Pierre Ossman <drzeus-list@drzeus.cx>
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To: Pierre Ossman <drzeus-list@drzeus.cx>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: Purpose of MMC_DATA_MULTI?
References: <43E057DA.7000909@drzeus.cx> <20060201092934.GB27735@flint.arm.linux.org.uk> <43E08148.3060003@drzeus.cx> <20060202093656.GA5034@flint.arm.linux.org.uk>
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Russell King wrote:
> On Wed, Feb 01, 2006 at 10:37:12AM +0100, Pierre Ossman wrote:
>   
>> Russell King wrote:
>>     
>>> On Wed, Feb 01, 2006 at 07:40:26AM +0100, Pierre Ossman wrote:
>>>       
>>>> I noticed that a new transfer flag was recently added to the MMC layer
>>>> without any immediate users, the MMC_DATA_MULTI flag. I'm guessing the
>>>> purpose of the flag is to indicate the difference between
>>>> MMC_READ_SINGLE_BLOCK and MMC_READ_MULTIPLE_BLOCKS with just one block.
>>>> If so, then that should probably be mentioned in a comment somewhere.
>>>>     
>>>>         
>>> There are hosts out there (Atmel AT91-based) which need to know if the
>>> transfer is going to be multiple block.  Rather than have them test
>>> the op-code (which is what they're already doing), we provide a flag
>>> instead.
>>>       
>> As far as the hardware is concerned there are two "multi-block" transfers:
>>
>>  * Multiple, back-to-back blocks.
>>  * One or more blocks that need to be terminated by some form of stop
>> command.
>>
>> The first can be identified by checking the number of blocks in the
>> request, the latter is harder to identify since it's a protocol semantic
>> (it could be just one block, but still need a stop). Does MMC_DATA_MULTI
>> indicate the latter, former or both?
>>     
>
> In short, it's defined to be whatever AT91_MCI_TRTYP_MULTIPLE means in
> the AT91RM9200 MMC host driver, which appears to be set for any of the
> multiple block commands.  They currently are doing:
>
>   

That's a bit fuzzy and bound to give problems. Why not settle for the 
first case and change their code to check the block count in the 
request? Less flags should be a good thing. And if that proves to be 
insufficient we should do a more thorough investigation once we have an 
actual failure case.

> and using that as a lookup table by command for the value to put into
> the command register.  I want to eliminate that, and not passing the
> MULTI flag prevents elimination of this table.
>
>   

Seems to be a common theme in the more recent drivers. Don't they teach 
people proper layering in the schools anymore? :)

Rgds
Pierre


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