Date:   Fri, 2 Apr 2021 17:43:11 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Loic Poulain <loic.poulain@...aro.org>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Aleksander Morgado <aleksander@...ksander.es>,
        open list <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Subject: Re: [PATCH net-next v8 2/2] net: Add Qcom WWAN control driver

On Fri, Apr 02, 2021 at 05:41:01PM +0200, Loic Poulain wrote:
> On Fri, 2 Apr 2021 at 16:05, Greg KH <gregkh@...uxfoundation.org> wrote:
> >
> > On Fri, Apr 02, 2021 at 04:06:37PM +0200, Loic Poulain wrote:
> > > The MHI WWWAN control driver allows MHI QCOM-based modems to expose
> > > different modem control protocols/ports via the WWAN framework, so that
> > > userspace modem tools or daemon (e.g. ModemManager) can control WWAN
> > > config and state (APN config, SMS, provider selection...). A QCOM-based
> > > modem can expose one or several of the following protocols:
> > > - AT: Well known AT commands interactive protocol (microcom, minicom...)
> > > - MBIM: Mobile Broadband Interface Model (libmbim, mbimcli)
> > > - QMI: QCOM MSM/Modem Interface (libqmi, qmicli)
> > > - QCDM: QCOM Modem diagnostic interface (libqcdm)
> > > - FIREHOSE: XML-based protocol for Modem firmware management
> > >         (qmi-firmware-update)
> > >
> > > Note that this patch is mostly a rework of the earlier MHI UCI
> > > tentative that was a generic interface for accessing MHI bus from
> > > userspace. As suggested, this new version is WWAN specific and is
> > > dedicated to only expose channels used for controlling a modem, and
> > > for which related opensource userpace support exist.
> > >
> > > Signed-off-by: Loic Poulain <loic.poulain@...aro.org>
> > > ---
> > >  v2: update copyright (2021)
> > >  v3: Move driver to dedicated drivers/net/wwan directory
> > >  v4: Rework to use wwan framework instead of self cdev management
> > >  v5: Fix errors/typos in Kconfig
> > >  v6: - Move to new wwan interface, No need dedicated call to wwan_dev_create
> > >      - Cleanup code (remove legacy from mhi_uci, unused defines/vars...)
> > >      - Remove useless write_lock mutex
> > >      - Add mhi_wwan_wait_writable and mhi_wwan_wait_dlqueue_lock_irq helpers
> > >      - Rework locking
> > >      - Add MHI_WWAN_TX_FULL flag
> > >      - Add support for NONBLOCK read/write
> > >  v7: Fix change log (mixed up 1/2 and 2/2)
> > >  v8: - Implement wwan_port_ops (instead of fops)
> > >      - Remove all mhi wwan data obsolete members (kref, lock, waitqueues)
> > >      - Add tracking of RX buffer budget
> > >      - Use WWAN TX flow control function to stop TX when MHI queue is full
> > >
> > >  drivers/net/wwan/Kconfig         |  14 +++
> > >  drivers/net/wwan/Makefile        |   2 +
> > >  drivers/net/wwan/mhi_wwan_ctrl.c | 253 +++++++++++++++++++++++++++++++++++++++
> > >  3 files changed, 269 insertions(+)
> > >  create mode 100644 drivers/net/wwan/mhi_wwan_ctrl.c
> > >
> > > diff --git a/drivers/net/wwan/Kconfig b/drivers/net/wwan/Kconfig
> > > index 545fe54..ce0bbfb 100644
> > > --- a/drivers/net/wwan/Kconfig
> > > +++ b/drivers/net/wwan/Kconfig
> > > @@ -19,4 +19,18 @@ config WWAN_CORE
> > >         To compile this driver as a module, choose M here: the module will be
> > >         called wwan.
> > >
> > > +config MHI_WWAN_CTRL
> > > +     tristate "MHI WWAN control driver for QCOM-based PCIe modems"
> > > +     select WWAN_CORE
> > > +     depends on MHI_BUS
> > > +     help
> > > +       MHI WWAN CTRL allows QCOM-based PCIe modems to expose different modem
> > > +       control protocols/ports to userspace, including AT, MBIM, QMI, DIAG
> > > +       and FIREHOSE. These protocols can be accessed directly from userspace
> > > +       (e.g. AT commands) or via libraries/tools (e.g. libmbim, libqmi,
> > > +       libqcdm...).
> > > +
> > > +       To compile this driver as a module, choose M here: the module will be
> > > +       called mhi_wwan_ctrl
> > > +
> > >  endif # WWAN
> > > diff --git a/drivers/net/wwan/Makefile b/drivers/net/wwan/Makefile
> > > index 934590b..556cd90 100644
> > > --- a/drivers/net/wwan/Makefile
> > > +++ b/drivers/net/wwan/Makefile
> > > @@ -5,3 +5,5 @@
> > >
> > >  obj-$(CONFIG_WWAN_CORE) += wwan.o
> > >  wwan-objs += wwan_core.o
> > > +
> > > +obj-$(CONFIG_MHI_WWAN_CTRL) += mhi_wwan_ctrl.o
> > > diff --git a/drivers/net/wwan/mhi_wwan_ctrl.c b/drivers/net/wwan/mhi_wwan_ctrl.c
> > > new file mode 100644
> > > index 0000000..f2fab23
> > > --- /dev/null
> > > +++ b/drivers/net/wwan/mhi_wwan_ctrl.c
> > > @@ -0,0 +1,253 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> > > +/* Copyright (c) 2021, Linaro Ltd <loic.poulain@...aro.org> */
> > > +#include <linux/kernel.h>
> > > +#include <linux/mhi.h>
> > > +#include <linux/mod_devicetable.h>
> > > +#include <linux/module.h>
> > > +#include <linux/wwan.h>
> > > +
> > > +/* MHI wwan flags */
> > > +#define MHI_WWAN_DL_CAP              BIT(0)
> > > +#define MHI_WWAN_UL_CAP              BIT(1)
> > > +#define MHI_WWAN_STARTED     BIT(2)
> > > +
> > > +#define MHI_WWAN_MAX_MTU     0x8000
> > > +
> > > +struct mhi_wwan_dev {
> > > +     /* Lower level is a mhi dev, upper level is a wwan port */
> > > +     struct mhi_device *mhi_dev;
> > > +     struct wwan_port *wwan_port;
> > > +
> > > +     /* State and capabilities */
> > > +     unsigned long flags;
> > > +     size_t mtu;
> > > +
> > > +     /* Protect against concurrent TX and TX-completion (bh) */
> > > +     spinlock_t tx_lock;
> > > +
> > > +     struct work_struct rx_refill;
> > > +     atomic_t rx_budget;
> >
> > Why is this atomic if you have a real lock already?
> 
> Access to rx_budget value is not under any locking protection and can
> be modified (dec/inc) from different and possibly concurrent places.

Then use the lock you have instead of creating a "fake lock" with the
atomic value.  If that's really even working (you can't check it and do
something based on it as it could change right afterward).

> > > +};
> > > +
> > > +static bool mhi_wwan_ctrl_refill_needed(struct mhi_wwan_dev *mhiwwan)
> > > +{
> > > +     if (!test_bit(MHI_WWAN_STARTED, &mhiwwan->flags))
> > > +             return false;
> > > +
> > > +     if (!test_bit(MHI_WWAN_DL_CAP, &mhiwwan->flags))
> > > +             return false;
> >
> > What prevents these bits from being changed right after reading them?
> 
> Nothing, I've think (maybe wrongly) it's not a problem in the current code.

Why not?  Why isn't the lock being used here?

Where is the lock used?

> > > +
> > > +     if (!atomic_read(&mhiwwan->rx_budget))
> > > +             return false;
> >
> > Why is this atomic?  What happens if it changes right after returning?
> 
> 
> If rx_budget was null and becomes non-null, it has been incremented by
> __mhi_skb_destructor() which will anyway call
> mhi_wwan_ctrl_refill_needed() again, so that's not a problem. On the
> other hand, if rx_budget was non-null and becomes null, the
> refill_work that will be unnecessarily scheduled will check the value
> again and will just return without doing anything.

You should document the heck out of this as it's not obvious :(

> >
> > This feels really odd.
> >
> > > +
> > > +     return true;
> > > +}
> > > +
> > > +void __mhi_skb_destructor(struct sk_buff *skb)
> > > +{
> > > +     struct mhi_wwan_dev *mhiwwan = skb_shinfo(skb)->destructor_arg;
> > > +
> > > +     /* RX buffer has been consumed, increase the allowed budget */
> > > +     atomic_inc(&mhiwwan->rx_budget);
> >
> > So this is a reference count?  What is this thing?
> 
> This represents the remaining number of buffers that can be allocated
> for RX. It is decremented When a buffer is allocated/queued and
> incremented when a buffer is consumed (e.g. on WWAN port reading).

Why have any budget at all?

That being said, budgets are fine, but properly lock things please.

> > > +
> > > +     if (mhi_wwan_ctrl_refill_needed(mhiwwan))
> > > +             schedule_work(&mhiwwan->rx_refill);
> >
> > What if refill is needed right after this check?  Did you just miss the
> > call?
> 
> In running condition, refill is allowed when rx_budget is non-zero,
> and __mhi_skb_destructor() is the only path that increments the budget
> (and so allow refill) and schedules the refill,  so for this scenario
> to happen it would mean that a parallel  __mhi_skb_destructor() is
> executed (and incremented rx_budget), so this second parallel call
> will schedule the refill.
> 
> I realize it's probably odd, but I don't see any scenario in which we
> can end badly (e.g. missing refill scheduling, queueing too many
> buffers), but I admit it would be certainly simpler and less
> error-prone with regular locking.

Document this all please.

> > > +static const struct mhi_device_id mhi_wwan_ctrl_match_table[] = {
> > > +     { .chan = "DUN", .driver_data = WWAN_PORT_AT },
> > > +     { .chan = "MBIM", .driver_data = WWAN_PORT_MBIM },
> > > +     { .chan = "QMI", .driver_data = WWAN_PORT_QMI },
> > > +     { .chan = "DIAG", .driver_data = WWAN_PORT_QCDM },
> > > +     { .chan = "FIREHOSE", .driver_data = WWAN_PORT_FIREHOSE },
> >
> > Wait, I thought these were all going to be separate somehow.  Now they
> > are all muxed back together?
> 
> A WWAN 'port driver' abstracts the method for accessing WWAN control
> protocols, so that userspace can e.g. talk MBIM to the port without
> knowledge of the underlying bus. Here this is just about abstracting
> the MHI/PCI transport, a  MHI modem can support one or several of
> these protocols. So this MHI driver binds all MHI control devices, and
> each one is registered as a WWAN port. Other 'port drivers' can be
> created for different busses or vendors.

ok, feels odd, I'll review it again for your next submission as this
seems like just a "raw pipe" to the device and is not actually
standardizing anything, but I could be totally wrong.

thanks,

greg k-h

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