RE: [RFC v2 4/4] i3c: add i3cdev module to expose i3c dev in /dev

From: Vitor Soares <Vitor.Soares@synopsys.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Jose Abreu <Jose.Abreu@synopsys.com>,
	Joao Pinto <Joao.Pinto@synopsys.com>,
	Boris Brezillon <bbrezillon@kernel.org>,
	gregkh <gregkh@linuxfoundation.org>,
	Wolfram Sang <wsa@the-dreams.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Mark Brown <broonie@kernel.org>,
	"linux-i3c@lists.infradead.org" <linux-i3c@lists.infradead.org>
Subject: RE: [RFC v2 4/4] i3c: add i3cdev module to expose i3c dev in /dev
Date: Wed, 29 Jan 2020 17:00:11 +0000	[thread overview]
Message-ID: <CH2PR12MB4216ED068AD93C43B2C421A8AE050@CH2PR12MB4216.namprd12.prod.outlook.com> (raw)
In-Reply-To: <CAK8P3a0uFjhuO7e-i3r_RiA_ni=S8MfYoZUwZzmbXRcd=+kMKw@mail.gmail.com>

Hi Arnd,

From: Arnd Bergmann <arnd@arndb.de>
Date: Wed, Jan 29, 2020 at 14:30:56

> On Wed, Jan 29, 2020 at 1:17 PM Vitor Soares <Vitor.Soares@synopsys.com> wrote:
> >
> > +
> > +struct i3cdev_data {
> > +       struct list_head list;
> > +       struct i3c_device *i3c;
> > +       struct cdev cdev;
> > +       struct device *dev;
> > +       int id;
> > +};
> > +
> > +static DEFINE_IDA(i3cdev_ida);
> > +static dev_t i3cdev_number;
> > +#define I3C_MINORS 16 /* 16 I3C devices supported for now */
> > +
> > +static LIST_HEAD(i3cdev_list);
> > +static DEFINE_SPINLOCK(i3cdev_list_lock);
> 
> Please try to avoid arbitrarily limiting the number of devices you support.

Should I use all minors range instead?

> 
> Searching through the list feels a little clumsy. If the i3c user interface is
> supposed to become a standard feature of the subsystem, it would seem
> appropriate to put a pointer into the device to simplify the lookup, 

Do you mean i3c->dev ?

> or
> just embed the cdev inside of i3c_device.

I would prefer to have a pointer in i3c_device for i3cdev_data, but I see 
others using it in drvdata.

> 
> > +static int
> > +i3cdev_do_priv_xfer(struct i3c_device *dev, struct i3c_ioc_priv_xfer *xfers,
> > +                   unsigned int nxfers)
> > +{
> > +       struct i3c_priv_xfer *k_xfers;
> > +       u8 **data_ptrs;
> > +       int i, ret = 0;
> > +
> > +       k_xfers = kcalloc(nxfers, sizeof(*k_xfers), GFP_KERNEL);
> > +       if (!k_xfers)
> > +               return -ENOMEM;
> > +
> > +       data_ptrs = kcalloc(nxfers, sizeof(*data_ptrs), GFP_KERNEL);
> > +       if (!data_ptrs) {
> > +               ret = -ENOMEM;
> > +               goto err_free_k_xfer;
> > +       }
> 
> Maybe use a  combined allocation to simplify the error handling?

Could you please provide an example?

> 
> > +       for (i = 0; i < nxfers; i++) {
> > +               data_ptrs[i] = memdup_user((const u8 __user *)
> > +                                          (uintptr_t)xfers[i].data,
> > +                                          xfers[i].len);
> 
> > +               if (xfers[i].rnw) {
> > +                       if (copy_to_user((void __user *)(uintptr_t)xfers[i].data,
> > +                                        data_ptrs[i], xfers[i].len))
> 
> Use u64_to_user_ptr() here.

You are right, it wasn't available went I did it.

> 
> > +
> > +static struct i3c_ioc_priv_xfer *
> > +i3cdev_get_ioc_priv_xfer(unsigned int cmd, struct i3c_ioc_priv_xfer *u_xfers,
> > +                        unsigned int *nxfers)
> > +{
> > +       u32 tmp = _IOC_SIZE(cmd);
> > +
> > +       if ((tmp % sizeof(struct i3c_ioc_priv_xfer)) != 0)
> > +               return ERR_PTR(-EINVAL);
> > +
> > +       *nxfers = tmp / sizeof(struct i3c_ioc_priv_xfer);
> > +       if (*nxfers == 0)
> > +               return NULL;
> > +
> > +       return memdup_user(u_xfers, tmp);
> > +}
> > +
> > +static int
> > +i3cdev_ioc_priv_xfer(struct i3c_device *i3c, unsigned int cmd,
> > +                    struct i3c_ioc_priv_xfer *u_xfers)
> > +{
> > +       struct i3c_ioc_priv_xfer *k_xfers;
> > +       unsigned int nxfers;
> > +       int ret;
> > +
> > +       k_xfers = i3cdev_get_ioc_priv_xfer(cmd, u_xfers, &nxfers);
> > +       if (IS_ERR_OR_NULL(k_xfers))
> > +               return PTR_ERR(k_xfers);
> > +
> > +       ret = i3cdev_do_priv_xfer(i3c, k_xfers, nxfers);
> 
> The IS_ERR_OR_NULL() usage looks suspicious. It's generally
> better to avoid interfaces that require this. What does it mean to
> return NULL from i3cdev_get_ioc_priv_xfer() and turn that into
> success? Could you handle this condition in the caller instead,
> or turn it into an error?

In both cases something is not correct. I will turn both conditions to 
return ERR_PTR(-EINVAL) and them just check if (IS_ERR(k_xfer)).

> 
> > +       /* Keep track of busses which have devices to add or remove later */
> > +       res = bus_register_notifier(&i3c_bus_type, &i3c_notifier);
> > +       if (res)
> > +               goto out_unreg_class;
> > +
> > +       /* Bind to already existing device without driver right away */
> > +       i3c_for_each_dev(NULL, i3cdev_attach);
> 
> The combination of the notifier and searching through the devices
> seems to be racy. What happens when a device appears just before
> or during the i3c_for_each_dev() traversal?

The i3c core is locked during this phase.

> 
> What happens when a driver attaches to a device that is currently
> transferring data on the user interface?
> 

It may lost references for inode and file. I need to guarantee there no 
tranfer going on during the detach.
Do you have any suggestion?

> Is there any guarantee that the notifiers for attach and detach
> are serialized?
> 

Sorry I didn't get this part. 

> > +/**
> > + * struct i3c_ioc_priv_xfer - I3C SDR ioctl private transfer
> > + * @data: Holds pointer to userspace buffer with transmit data.
> > + * @len: Length of data buffer buffers, in bytes.
> > + * @rnw: encodes the transfer direction. true for a read, false for a write
> > + */
> > +struct i3c_ioc_priv_xfer {
> > +       __u64 data;
> > +       __u16 len;
> > +       __u8 rnw;
> > +       __u8 pad[5];
> > +};
> > +
> > +
> > +#define I3C_PRIV_XFER_SIZE(N)  \
> > +       ((((sizeof(struct i3c_ioc_priv_xfer)) * (N)) < (1 << _IOC_SIZEBITS)) \
> > +       ? ((sizeof(struct i3c_ioc_priv_xfer)) * (N)) : 0)
> > +
> > +#define I3C_IOC_PRIV_XFER(N)   \
> > +       _IOC(_IOC_READ|_IOC_WRITE, I3C_DEV_IOC_MAGIC, 30, I3C_PRIV_XFER_SIZE(N))
> 
> This looks like a reasonable ioctl definition, avoiding the usual problems
> with compat mode etc.

Do you think I should add more reserved fields for future?

> 
>       Arnd
> 
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Thanks for your comments 😊

Best regards,
Vitor Soares