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Subject: Re: [PATCH v13 4/4] bus: mhi: Add userspace client interface driver
To:     Loic Poulain <loic.poulain@linaro.org>,
        Hemant Kumar <hemantk@codeaurora.org>
Cc:     Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Bhaumik Bhatt <bbhatt@codeaurora.org>,
        Network Development <netdev@vger.kernel.org>
References: <1606533966-22821-1-git-send-email-hemantk@codeaurora.org>
 <1606533966-22821-5-git-send-email-hemantk@codeaurora.org>
 <CAMZdPi8z+-qFqgZ7AFJcNAUMbDQtNN5Hz-geMBcp4azrUGm9iA@mail.gmail.com>
 <c47dcd57-7576-e03e-f70b-0c4d25f724b5@codeaurora.org>
 <CAMZdPi8mUV5cFs-76K3kg=hN8ht2SKjJwzbJH-+VH4Y8QabcHQ@mail.gmail.com>
From:   Jeffrey Hugo <jhugo@codeaurora.org>
Message-ID: <1247e32e-ed67-de6b-81ec-3bde9ad93250@codeaurora.org>
Date:   Tue, 1 Dec 2020 10:37:00 -0700
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On 12/1/2020 10:36 AM, Loic Poulain wrote:
> On Tue, 1 Dec 2020 at 02:16, Hemant Kumar <hemantk@codeaurora.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@codeaurora.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.

-- 
Jeffrey Hugo
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.