Date:   Fri, 2 Apr 2021 17:41:01 +0200
From:   Loic Poulain <loic.poulain@...aro.org>
To:     Greg KH <gregkh@...uxfoundation.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, 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.

>
>
> > +};
> > +
> > +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.

>
> > +
> > +     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.

>
> 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).

>
> > +
> > +     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.

>
>
> > +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.

Thanks,
Loic

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