linux-kernel - Re: [PATCH v6 02/16] chardev: introduce cdev_get_by

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Date:   Thu, 25 Jul 2019 12:08:27 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Stephen Bates <sbates@...thlin.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path()



On 2019-07-25 11:58 a.m., Matthew Wilcox wrote:
> On Thu, Jul 25, 2019 at 11:53:20AM -0600, Logan Gunthorpe wrote:
>>
>>
>> On 2019-07-25 11:40 a.m., Greg Kroah-Hartman wrote:
>>> On Thu, Jul 25, 2019 at 11:23:21AM -0600, Logan Gunthorpe wrote:
>>>> cdev_get_by_path() attempts to retrieve a struct cdev from
>>>> a path name. It is analagous to blkdev_get_by_path().
>>>>
>>>> This will be necessary to create a nvme_ctrl_get_by_path()to
>>>> support NVMe-OF passthru.
>>>
>>> Ick, why?  Why would a cdev have a "pathname"?
>>
>> So we can go from "/dev/nvme0" (which points to a char device) to its
>> struct cdev and eventually it's struct nvme_ctrl. Doing it this way also
>> allows supporting symlinks that might be created by udev rules.
> 
> But you're not really trying to go from a string to a chardev.  You're
> trying to go from a nvmet_subsys to a chardev.  Isn't there a better
> way to link the two somewhere else?
> 
> (I must confess that once I would have known the answer to this, but
> the NVMe subsystem has grown ridiculously complex and I can no longer
> fit it in my head)

Well the nvmet_subsys isn't related to the nvme_ctrl (and thus char dev)
at all. An nvmet_subsys is created via configfs and the user has to
specify an NVMe controller for it to use (by writting a string to a
config attribute). The best handle the user has is a path to the
controller's cdev (/dev/nvmeX) so the fabrics code has to be able to
lookup the corresponding struct nvme_ctrl from the path.

This is directly analogous to the way NVMe-of works today: it uses
blkdev_get_by_path() to translate a user provided path to a struct
block_device. The only difference here is that, for passthru, we need a
nvme_ctrl, not a block device.

Logan