linux-kernel - Re: [PATCH] block: support embedded device command line partition

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Date:	Fri, 16 Aug 2013 10:02:39 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Caizhiyong <caizhiyong@...wei.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Karel Zak <kzak@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Wanglin (Albert)" <albert.wanglin@...wei.com>,
	Quyaxin <quyaxin@...wei.com>
Subject: Re: [PATCH] block: support embedded device command line partition

On 08/15/2013 08:54 PM, Caizhiyong wrote:
>>> +blkdevparts=<blkdev-def>[;<blkdev-def>]
>>> +  <blkdev-def> := <blkdev-id>:<partdef>[,<partdef>]
>>> +    <partdef> := <size>[@<offset>](part-name)
>>> +
>>> +<blkdev-id>
>>> +    block device disk name, embedded device used fixed block device,
>>> +    it's disk name also fixed. such as: mmcblk0, mmcblk1, mmcblk0boot0.
>>
>> The device-name isn't always fixed.
>>
>> For example, what if there are multiple SDHCI controllers, one hosting a
>> fixed eMMC device and the other an SD card? It's quite typical for the
>> eMMC's device name (which is likely what should be affected by this
>> feature) to be mmcblk0 when no SD card is present, yet be mmcblk1 when
>> an SD card is present. Is there a more precise/stable way to define
>> which device the command-line option applies to?
>   
> Yes. Fixed is for single controller.
> For multiple controllers, currently, there is not a simple way.
> I tend to do something in the eMMC driver, such as initialize order,
> but I have not tried.

There have been proposals before to try and create a fixed naming for
the controllers (or rather the block devices they generate...) but
they've been rejected. I don't think we should rely on being able to do
that.

>>> +
>>> +<offset>
>>> +    partition start address, in bytes.
>>> +
>>> +(part-name)
>>> +    partition name, kernel send uevent with "PARTNAME". application can create
>>> +    a link to block device partition with the name "PARTNAME".
>>> +    user space application can access partition by partition name.
>>
>> Do partitions usually have a PARTNAME attribute when probed through
>> normal mechanisms like MBR? If so, I guess this is fine.
>>
>> Perhaps we can just use , as the delimiter for all fields, rather than
>> special-casing part-name to use (), so:
>>
>> size,offset,partname,size,offset,partname,...
>>
>> The partname field could easily be empty if not needed.
> 
> If no need partname, your bootargs are mmcblk0:1G,1G,1G,...

Well, you always need the offset too. I don't think there's any harm in
forcing all fields to be specified in all cases; it makes the whole
system much more regular and less error-prone.

Alternatively, use a different separator between fields for a given
partition, and between partitions, e.g.:

size,offset,partname;size,offset,partname

That way, you know that if you see a ; you're looking at a new
partition, and hence the partname field need not always be specified.
Although, if you want to specify a partname but not an offset you'd
still need empty fields, so just requiring all fields to always be
present still seems safest to me.
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