linux-kernel - Re: Device tree node to major/minor?

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Message-ID: <CAPnjgZ3jF7bdwqDCNzJXvSrwoP0PZVjcFO+nGOiLACLDX07Z7A@mail.gmail.com>
Date:	Tue, 20 Nov 2012 15:48:24 -0800
From:	Simon Glass <sjg@...omium.org>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	lk <linux-kernel@...r.kernel.org>,
	Devicetree Discuss <devicetree-discuss@...ts.ozlabs.org>,
	Che-liang Chiou <clchiou@...omium.org>
Subject: Re: Device tree node to major/minor?

Hi Grant,

On Tue, Nov 20, 2012 at 2:32 PM, Grant Likely <grant.likely@...retlab.ca> wrote:
> On Tue, Nov 20, 2012 at 10:23 PM, Simon Glass <sjg@...omium.org> wrote:
>> Hi,
>>
>> I hope this is a stupid question with an easy answer, but I cannot find it.
>>
>> I have a device tree node for an mmc block device and I want to use
>> that block device from another driver. I have a phandle which lets me
>> get the node of the mmc device, but I am not sure how to convert that
>> into a block_device. In order to do so, I think I need a major/minor
>> number. Of course the phandle might in fact point to a SCSI driver and
>> I want that to work correctly also.
>>
>> I imagine I might be able to search through the wonders of sysfs in
>> user space, but is there a better way?
>
> Do you /want/ to do it from userspace? What is your use case? Mounting
> the rootfs?

The use case is storing some raw data on a block device from within a
driver in the kernel. It is used to keep track of the verified boot
state.

>
> Regardless, userspace can monitor the uevents when devices are added
> (that's what udev does) and watch for the full path of the node you
> want in the uevent attribute. Then you can look for the child device
> with the block major/minor numbers in it.

So is there a way to do this entirely in the kernel ex post? It might
need to happen during kernel boot, before user space.

>
> g.

Thanks,
Simon
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