linux-kernel - Re: [PATCH v8 4/4] bus: mhi: Add userspace client interface driver

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Date:   Thu, 22 Oct 2020 16:25:08 -0700
From:   Hemant Kumar <hemantk@...eaurora.org>
To:     Loic Poulain <loic.poulain@...aro.org>
Cc:     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>,
        Jeffrey Hugo <jhugo@...eaurora.org>,
        Bhaumik Bhatt <bbhatt@...eaurora.org>
Subject: Re: [PATCH v8 4/4] bus: mhi: Add userspace client interface driver

Hi Loic,

On 10/22/20 3:20 AM, Loic Poulain wrote:
> Hi Hemant,
> 
> A few comments inline.
> 
> On Thu, 22 Oct 2020 at 10:22, 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 - MHI channel for a UCI device
>> + * @udev: associated UCI device object
>> + * @ul_wq: wait queue for writer
>> + * @write_lock: mutex write lock for ul channel
>> + * @dl_wq: wait queue for reader
>> + * @read_lock: mutex read lock for dl channel
>> + * @dl_lock: spin lock
>> + * @pending: list of dl buffers userspace is waiting to read
>> + * @cur_buf: current buffer userspace is reading
>> + * @dl_size: size of the current dl buffer userspace is reading
>> + * @ref_count: uci_chan reference count
>> + */
>> +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 and cur_buf members in bh context, channel release,
>> +        * read and poll
>> +        */
>> +       spinlock_t dl_lock;
> 
> Maybe I'm wrong, but I think it would be clearer and simpler for
> dl_lock to be only a lock for the pending RX list (e.g. pending_lock),
> for protecting against concurrent access in chardev read ops
> (consumer) and MHI download callback (producer). cur_buf is the
> currently read buffer, and so could be simply protected by the
> read_lock mutex (never accessed from bh/irq callback context).
You have a good point. I can protect pending list related operations 
using spin lock and call it pending_lock which would be used in dl_xfer 
call back, channel release, read and poll. Use read lock for cur_buf in 
read().
> 
>> +
>> +       struct list_head pending;
>> +       struct uci_buf *cur_buf;
>> +       size_t dl_size;
>> +       struct kref ref_count;
>> +};
>> +
>> +/**
[..]
>> +static void mhi_dl_xfer_cb(struct mhi_device *mhi_dev,
>> +                          struct mhi_result *mhi_result)
>> +{
>> +       struct uci_dev *udev = dev_get_drvdata(&mhi_dev->dev);
>> +       struct uci_chan *uchan = udev->uchan;
>> +       struct device *dev = &mhi_dev->dev;
>> +       struct uci_buf *ubuf;
>> +       size_t dl_buf_size = udev->mtu - sizeof(*ubuf);
>> +
>> +       dev_dbg(dev, "status: %d receive_len: %zu\n",
>> +               mhi_result->transaction_status, mhi_result->bytes_xferd);
>> +
>> +       if (mhi_result->transaction_status == -ENOTCONN) {
>> +               kfree(mhi_result->buf_addr);
>> +               return;
>> +       }
> 
> It would be more robust to test only transaction_status values that
> allow you to consider the buffer as valid, AFAIU 0 and -EOVERFLOW.
> That would prevent breaking that code if new transaction_status errors
> are returned in the futur (e.g. -EIO...).
I agree, will use this instead
if (mhi_result->transaction_status &&
		mhi_result->transaction_status != -EOVERFLOW)
> 
> 
>> +
>> +       ubuf = mhi_result->buf_addr + dl_buf_size;
>> +       ubuf->data = mhi_result->buf_addr;
>> +       ubuf->len = mhi_result->bytes_xferd;
>> +       spin_lock_bh(&uchan->dl_lock);
>> +       list_add_tail(&ubuf->node, &uchan->pending);
>> +       spin_unlock_bh(&uchan->dl_lock);
>> +
>> +       wake_up(&uchan->dl_wq);
>> +}
>> +

Thanks,
Hemant
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