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

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Message-ID: <a3262a7f-b78e-05ba-cda3-a7587946bd91@deltatee.com>
Date:   Thu, 25 Jul 2019 12:36:24 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     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>,
        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 12:27 p.m., Greg Kroah-Hartman wrote:
>>> 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.

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

Yup.

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

We lookup the struct nvme_ctrl and use it to submit passed-through NVMe
commands directly to the controller.

Logan