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

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Message-ID: <20190725182701.GA11547@kroah.com>
Date:   Thu, 25 Jul 2019 20:27:01 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     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 Thu, Jul 25, 2019 at 12:14:33PM -0600, Logan Gunthorpe wrote:
> 
> 
> On 2019-07-25 12:08 p.m., Greg Kroah-Hartman 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.
> > 
> > Why do you have a "string" within the kernel and are not using the
> > normal open() call from userspace on the character device node on the
> > filesystem in your namespace/mount/whatever?
> 
> NVMe-OF is configured using configfs. The target is specified by the
> user writing a path to a configfs attribute. This is the way it works
> today but with blkdev_get_by_path()[1]. For the passthru code, we need
> to get a nvme_ctrl instead of a block_device, but the principal is the same.

Why isn't a fd being passed in there instead of a random string?

Seems odd, but oh well, that ship sailed a long time ago for block
devices I guess.

So what do you actually _do_ with that char device once you have it?

greg k-h