linux-kernel - Re: [PATCH v13 4/4] bus: mhi: Add userspace client interface driver

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Date:   Tue, 1 Dec 2020 18:52:57 +0100
From:   Loic Poulain <loic.poulain@...aro.org>
To:     Jeffrey Hugo <jhugo@...eaurora.org>
Cc:     Hemant Kumar <hemantk@...eaurora.org>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Bhaumik Bhatt <bbhatt@...eaurora.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v13 4/4] bus: mhi: Add userspace client interface driver

On Tue, 1 Dec 2020 at 18:37, Jeffrey Hugo <jhugo@...eaurora.org> wrote:
>
> On 12/1/2020 10:36 AM, Loic Poulain wrote:
> > On Tue, 1 Dec 2020 at 02:16, Hemant Kumar <hemantk@...eaurora.org> wrote:
> >>
> >> Hi Loic,
> >>
> >> On 11/30/20 10:22 AM, Loic Poulain wrote:
> >>> On Sat, 28 Nov 2020 at 04:26, Hemant Kumar <hemantk@...eaurora.org> wrote:
> >>>>
> >>>> This MHI client driver allows userspace clients to transfer
> >>>> raw data between MHI device and host using standard file operations.
> >>>> Driver instantiates UCI device object which is associated to device
> >>>> file node. UCI device object instantiates UCI channel object when device
> >>>> file node is opened. UCI channel object is used to manage MHI channels
> >>>> by calling MHI core APIs for read and write operations. MHI channels
> >>>> are started as part of device open(). MHI channels remain in start
> >>>> state until last release() is called on UCI device file node. Device
> >>>> file node is created with format
> >>>
> >>> [...]
> >>>
> >>>> +struct uci_chan {
> >>>> +       struct uci_dev *udev;
> >>>> +       wait_queue_head_t ul_wq;
> >>>> +
> >>>> +       /* ul channel lock to synchronize multiple writes */
> >>>> +       struct mutex write_lock;
> >>>> +
> >>>> +       wait_queue_head_t dl_wq;
> >>>> +
> >>>> +       /* dl channel lock to synchronize multiple reads */
> >>>> +       struct mutex read_lock;
> >>>> +
> >>>> +       /*
> >>>> +        * protects pending list in bh context, channel release, read and
> >>>> +        * poll
> >>>> +        */
> >>>> +       spinlock_t dl_pending_lock;
> >>>> +
> >>>> +       struct list_head dl_pending;
> >>>> +       struct uci_buf *cur_buf;
> >>>> +       size_t dl_size;
> >>>> +       struct kref ref_count;
> >>>> +};
> >>>
> >>> [...]
> >>>
> >>>> + * struct uci_dev - MHI UCI device
> >>>> + * @minor: UCI device node minor number
> >>>> + * @mhi_dev: associated mhi device object
> >>>> + * @uchan: UCI uplink and downlink channel object
> >>>> + * @mtu: max TRE buffer length
> >>>> + * @enabled: Flag to track the state of the UCI device
> >>>> + * @lock: mutex lock to manage uchan object
> >>>> + * @ref_count: uci_dev reference count
> >>>> + */
> >>>> +struct uci_dev {
> >>>> +       unsigned int minor;
> >>>> +       struct mhi_device *mhi_dev;
> >>>> +       struct uci_chan *uchan;
> >>>
> >>> Why a pointer to uci_chan and not just plainly integrating the
> >>> structure here, AFAIU uci_chan describes the channels and is just a
> >>> subpart of uci_dev. That would reduce the number of dynamic
> >>> allocations you manage and the extra kref. do you even need a separate
> >>> structure for this?
> >>
> >> This goes back to one of my patch versions i tried to address concern
> >> from Greg. Since we need to ref count the channel as well as the uci
> >> device i decoupled the two objects and used two reference counts for two
> >> different objects.
> >
> > What Greg complained about is the two kref in the same structure and
> > that you were using kref as an open() counter. But splitting your
> > struct in two in order to keep the two kref does not make the much
> > code better (and simpler). I'm still a bit puzzled about the driver
> > complexity, it's supposed to be just a passthrough interface to MHI
> > after all.
> >
> > I would suggest several changes, that IMHO would simplify reviewing:
> > - Use only one structure representing the 'uci' context (uci_dev)
> > - Keep the read path simple (mhi_uci_read), do no use an intermediate
> > cur_buf pointer, only dequeue the buffer when it is fully consumed.
> > - As I commented before, take care of the dl_pending list access
> > concurrency, even in wait_event.
> > - You don't need to count the number of open() calls, AFAIK,
> > mhi_prepare_for_transfer() simply fails if channels are already
> > started...
>
> Unless I missed something, you seem to have ignored the root issue that
> Hemant needs to solve, which is when to call
> mhi_unprepare_for_transfer().  You can't just call that when close() is
> called because there might be multiple users, and each one is going to
> trigger a close(), so you need to know how many close() instances to
> expect, and only call mhi_unprepare_for_transfer() for the last one.

That one part of his problem, yes, but if you unconditionally call
mhi_prepare_for_transfer in open(), it should fail for subsequent
users, and so only one user will successfully open the device.

Regards,
Loic