linux-kernel - Re: [RESEND x2][PATCH v2] block: partition: Add partition specific uevent callbacks for partition info

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Date:	Fri, 5 Feb 2016 10:59:00 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	lkml <linux-kernel@...r.kernel.org>, San Mehat <san@...gle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Rom Lemarchand <romlem@...gle.com>,
	Android Kernel Team <kernel-team@...roid.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Harald Hoyer <harald@...hat.com>,
	Kees Cook <keescook@...omium.org>, Kay Sievers <kay@...y.org>,
	Dima Zavin <dima@...roid.com>, Greg KH <greg@...ah.com>,
	Cai Zhiyong <caizhiyong@...wei.com>
Subject: Re: [RESEND x2][PATCH v2] block: partition: Add partition specific
 uevent callbacks for partition info

On Fri, Feb 5, 2016 at 10:18 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Fri,  5 Feb 2016 10:01:43 -0800 John Stultz <john.stultz@...aro.org> wrote:
>
>> From: San Mehat <san@...gle.com>
>>
>> This patch has been carried in the Android tree for quite some
>> time and is one of the few patches required to get a mainline
>> kernel up and running with an exsiting Android userspace. So I
>> wanted to submit it for review and consideration if it should
>> be merged.
>>
>> For partitions, add new uevent parameters 'PARTN' which
>> specifies the partitions index in the table, and 'PARTNAME',
>> which specifies PARTNAME specifices the partition name of a
>> partition device.
>>
>> Android's userspace uses this for creating device node links from the
>> partition name and number: ie:
>>       /dev/block/platform/soc/by-name/system
>> or
>>       /dev/block/platform/soc/by-num/p1
>>
>> One can see its usage here:
>> https://android.googlesource.com/platform/system/core/+/master/init/devices.cpp#355
>> and
>> https://android.googlesource.com/platform/system/core/+/master/init/devices.cpp#494
>>
>> ...
>>
>> --- a/block/partition-generic.c
>> +++ b/block/partition-generic.c
>> @@ -216,10 +216,21 @@ static void part_release(struct device *dev)
>>       kfree(p);
>>  }
>>
>> +static int part_uevent(struct device *dev, struct kobj_uevent_env *env)
>> +{
>> +     struct hd_struct *part = dev_to_part(dev);
>> +
>> +     add_uevent_var(env, "PARTN=%u", part->partno);
>> +     if (part->info && part->info->volname[0])
>> +             add_uevent_var(env, "PARTNAME=%s", part->info->volname);
>> +     return 0;
>> +}
>> +
>>  struct device_type part_type = {
>>       .name           = "partition",
>>       .groups         = part_attr_groups,
>>       .release        = part_release,
>> +     .uevent         = part_uevent,
>>  };
>>
>>  static void delete_partition_rcu_cb(struct rcu_head *head)
>
> Interestingly, this feature appears to already be documented in
> Documentation/block/cmdline-partition.txt.  I wonder how that happened.
> Maybe we used to do this but it got taken away?

Heh. Looks like the documentation was added not too long ago (by Cai -
cc'ed). I suspect they had been working w/ the Android tree and
assumed the functionality was already upstream?

> It seems bad that we don't document uevents in any organized fashion.
> But the audience is very small and knows how to find kernel source code
> so I guess it doesn't matter.
>
> Anyway, please do check that the conveniently self-adding documentation
> is accurate and complete.

It does match the behavior this patch provides from Android. It is
somewhat tangential to the functionality described in the
documentation, so I'm not sure of its measure of completeness (for
example, it doesn't talk about PARTN parameter, but again, the
documentation is covering how to specify partition info via the boot
cmdline, and isn't really covering the uevents - the uevent was just a
mentioned side-effect for the partition name portion of the cmdline
option).

thanks
-john