Device tree node to major/minor?

[Simon Glass]

Hi Grant,

On Wed, Nov 21, 2012 at 7:47 AM, Grant Likely <grant.likely at secretlab.ca> wrote:
> On Tue, 20 Nov 2012 15:48:24 -0800, Simon Glass <sjg at chromium.org> wrote:
>> Hi Grant,
>>
>> On Tue, Nov 20, 2012 at 2:32 PM, Grant Likely <grant.likely at secretlab.ca> wrote:
>> > On Tue, Nov 20, 2012 at 10:23 PM, Simon Glass <sjg at chromium.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.
>
> Yes, it is certainly doable within the kernel. First, you'll need to use
> a notifier to get called back whenever a new device is created. Then
> you'll need to look at the dev->of_node(->full_name) to see if it is the
> node you actually want. You might need/want to resolve it from an alias
> or something, but I presume you already have a way to find the
> device_node before seaching for a struct device.

OK thank you. Was hoping to find a simple way to find a block device
from a device tree node (yes I know the right one) but I suppose in
general this is impossible, since nodes may create more than one
device, and each has its own data structures leading to the block
device.

So it seems like a notifier is the best way. Thanks for looking at this Grant.

Regards,
Simon

>
> g.