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

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Message-ID: <5951e0f5-cc90-f3de-0083-088fecfd43bb@grimberg.me>
Date:   Thu, 25 Jul 2019 12:02:30 -0700
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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>,
        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()


>>>> 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?
> 
> I wouldn't know the answer to this but I assume because once we decided
> to use configfs, there was no way for the user to pass the kernel an fd.

That's definitely not changing. But this is not different than how we
use the block device or file configuration, this just happen to need the
nvme controller chardev now to issue I/O.