Date:   Wed, 16 Jun 2021 00:06:08 +0200
From:   Stephan Gerhold <stephan@...hold.net>
To:     Loic Poulain <loic.poulain@...aro.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Aleksander Morgado <aleksander@...ksander.es>,
        Sergey Ryazanov <ryazanov.s.a@...il.com>,
        Johannes Berg <johannes.berg@...el.com>,
        M Chetan Kumar <m.chetan.kumar@...el.com>,
        Ohad Ben-Cohen <ohad@...ery.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Network Development <netdev@...r.kernel.org>,
        linux-remoteproc@...r.kernel.org,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        phone-devel@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>,
        ~postmarketos/upstreaming@...ts.sr.ht
Subject: Re: [PATCH net-next 3/3] net: wwan: Allow WWAN drivers to provide
 blocking tx and poll function

On Tue, Jun 15, 2021 at 11:24:41PM +0200, Loic Poulain wrote:
> On Tue, 15 Jun 2021 at 15:34, Stephan Gerhold <stephan@...hold.net> wrote:
> >
> > At the moment, the WWAN core provides wwan_port_txon/off() to implement
> > blocking writes. The tx() port operation should not block, instead
> > wwan_port_txon/off() should be called when the TX queue is full or has
> > free space again.
> >
> > However, in some cases it is not straightforward to make use of that
> > functionality. For example, the RPMSG API used by rpmsg_wwan_ctrl.c
> > does not provide any way to be notified when the TX queue has space
> > again. Instead, it only provides the following operations:
> >
> >   - rpmsg_send(): blocking write (wait until there is space)
> >   - rpmsg_trysend(): non-blocking write (return error if no space)
> >   - rpmsg_poll(): set poll flags depending on TX queue state
> >
> > Generally that's totally sufficient for implementing a char device,
> > but it does not fit well to the currently provided WWAN port ops.
> >
> > Most of the time, using the non-blocking rpmsg_trysend() in the
> > WWAN tx() port operation works just fine. However, with high-frequent
> > writes to the char device it is possible to trigger a situation
> > where this causes issues. For example, consider the following
> > (somewhat unrealistic) example:
> >
> >  # dd if=/dev/zero bs=1000 of=/dev/wwan0p2QMI
> >  dd: error writing '/dev/wwan0p2QMI': Resource temporarily unavailable
> >  1+0 records out
> >
> > This fails immediately after writing the first record. It's likely
> > only a matter of time until this triggers issues for some real application
> > (e.g. ModemManager sending a lot of large QMI packets).
> >
> > The rpmsg_char device does not have this problem, because it uses
> > rpmsg_trysend() and rpmsg_poll() to support non-blocking operations.
> > Make it possible to use the same in the RPMSG WWAN driver by extending
> > the tx() operation with a "nonblock" parameter and adding an optional
> > poll() callback. This integrates nicely with the RPMSG API and does
> > not break other WWAN drivers.
> >
> > With these changes, the dd example above blocks instead of exiting
> > with an error.
> >
> > Cc: Loic Poulain <loic.poulain@...aro.org>
> > Signed-off-by: Stephan Gerhold <stephan@...hold.net>
> > ---
> > Note that rpmsg_poll() is an optional callback currently only implemented
> > by the qcom_smd RPMSG provider. However, it should be easy to implement
> > this for other RPMSG providers when needed.
> >
> > Another potential solution suggested by Loic Poulain in [1] is to always
> > use the blocking rpmsg_send() from a workqueue/kthread and disable TX
> > until it is done. I think this could also work (perhaps a bit more
> > difficult to implement) but the main disadvantage is that I don't see
> > a way to return any kind of error to the client with this approach.
> > I assume we return immediately from the write() to the char device
> > after scheduling the rpmsg_send(), so we already reported success
> > when rpmsg_send() returns.
> >
> > At the end all that matters to me is that it works properly, so I'm
> > open for any other suggestions. :)
> >
> > [1]: https://lore.kernel.org/linux-arm-msm/CAMZdPi_-Qa=JnThHs_h-144dAfSAjF5s+QdBawdXZ3kk8Mx8ng@mail.gmail.com/
> > ---
> >  drivers/net/wwan/iosm/iosm_ipc_port.c |  3 ++-
> >  drivers/net/wwan/mhi_wwan_ctrl.c      |  3 ++-
> >  drivers/net/wwan/rpmsg_wwan_ctrl.c    | 17 +++++++++++++++--
> >  drivers/net/wwan/wwan_core.c          |  9 ++++++---
> >  drivers/net/wwan/wwan_hwsim.c         |  3 ++-
> >  include/linux/wwan.h                  | 13 +++++++++----
> >  6 files changed, 36 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/net/wwan/iosm/iosm_ipc_port.c b/drivers/net/wwan/iosm/iosm_ipc_port.c
> > index beb944847398..2f874e41ceff 100644
> > --- a/drivers/net/wwan/iosm/iosm_ipc_port.c
> > +++ b/drivers/net/wwan/iosm/iosm_ipc_port.c
> > @@ -31,7 +31,8 @@ static void ipc_port_ctrl_stop(struct wwan_port *port)
> >  }
> >
> >  /* transfer control data to modem */
> > -static int ipc_port_ctrl_tx(struct wwan_port *port, struct sk_buff *skb)
> > +static int ipc_port_ctrl_tx(struct wwan_port *port, struct sk_buff *skb,
> > +                           bool nonblock)
> >  {
> >         struct iosm_cdev *ipc_port = wwan_port_get_drvdata(port);
> >
> > diff --git a/drivers/net/wwan/mhi_wwan_ctrl.c b/drivers/net/wwan/mhi_wwan_ctrl.c
> > index 1bc6b69aa530..9754f014d348 100644
> > --- a/drivers/net/wwan/mhi_wwan_ctrl.c
> > +++ b/drivers/net/wwan/mhi_wwan_ctrl.c
> > @@ -139,7 +139,8 @@ static void mhi_wwan_ctrl_stop(struct wwan_port *port)
> >         mhi_unprepare_from_transfer(mhiwwan->mhi_dev);
> >  }
> >
> > -static int mhi_wwan_ctrl_tx(struct wwan_port *port, struct sk_buff *skb)
> > +static int mhi_wwan_ctrl_tx(struct wwan_port *port, struct sk_buff *skb,
> > +                           bool nonblock)
> >  {
> >         struct mhi_wwan_dev *mhiwwan = wwan_port_get_drvdata(port);
> >         int ret;
> > diff --git a/drivers/net/wwan/rpmsg_wwan_ctrl.c b/drivers/net/wwan/rpmsg_wwan_ctrl.c
> > index de226cdb69fd..63f431eada39 100644
> > --- a/drivers/net/wwan/rpmsg_wwan_ctrl.c
> > +++ b/drivers/net/wwan/rpmsg_wwan_ctrl.c
> > @@ -54,12 +54,16 @@ static void rpmsg_wwan_ctrl_stop(struct wwan_port *port)
> >         rpwwan->ept = NULL;
> >  }
> >
> > -static int rpmsg_wwan_ctrl_tx(struct wwan_port *port, struct sk_buff *skb)
> > +static int rpmsg_wwan_ctrl_tx(struct wwan_port *port, struct sk_buff *skb,
> > +                             bool nonblock)
> >  {
> >         struct rpmsg_wwan_dev *rpwwan = wwan_port_get_drvdata(port);
> >         int ret;
> >
> > -       ret = rpmsg_trysend(rpwwan->ept, skb->data, skb->len);
> > +       if (nonblock)
> > +               ret = rpmsg_trysend(rpwwan->ept, skb->data, skb->len);
> > +       else
> > +               ret = rpmsg_send(rpwwan->ept, skb->data, skb->len);
> >         if (ret)
> >                 return ret;
> >
> > @@ -67,10 +71,19 @@ static int rpmsg_wwan_ctrl_tx(struct wwan_port *port, struct sk_buff *skb)
> >         return 0;
> >  }
> >
> > +static __poll_t rpmsg_wwan_ctrl_poll(struct wwan_port *port, struct file *filp,
> > +                                    poll_table *wait)
> > +{
> > +       struct rpmsg_wwan_dev *rpwwan = wwan_port_get_drvdata(port);
> > +
> > +       return rpmsg_poll(rpwwan->ept, filp, wait);
> > +}
> > +
> >  static const struct wwan_port_ops rpmsg_wwan_pops = {
> >         .start = rpmsg_wwan_ctrl_start,
> >         .stop = rpmsg_wwan_ctrl_stop,
> >         .tx = rpmsg_wwan_ctrl_tx,
> > +       .poll = rpmsg_wwan_ctrl_poll,
> >  };
> >
> >  static struct device *rpmsg_wwan_find_parent(struct device *dev)
> > diff --git a/drivers/net/wwan/wwan_core.c b/drivers/net/wwan/wwan_core.c
> > index 7e728042fc41..c7fd0b897f87 100644
> > --- a/drivers/net/wwan/wwan_core.c
> > +++ b/drivers/net/wwan/wwan_core.c
> > @@ -500,7 +500,8 @@ static void wwan_port_op_stop(struct wwan_port *port)
> >         mutex_unlock(&port->ops_lock);
> >  }
> >
> > -static int wwan_port_op_tx(struct wwan_port *port, struct sk_buff *skb)
> > +static int wwan_port_op_tx(struct wwan_port *port, struct sk_buff *skb,
> > +                          bool nonblock)
> >  {
> >         int ret;
> >
> > @@ -510,7 +511,7 @@ static int wwan_port_op_tx(struct wwan_port *port, struct sk_buff *skb)
> >                 goto out_unlock;
> >         }
> >
> > -       ret = port->ops->tx(port, skb);
> > +       ret = port->ops->tx(port, skb, nonblock);
> >
> >  out_unlock:
> >         mutex_unlock(&port->ops_lock);
> > @@ -637,7 +638,7 @@ static ssize_t wwan_port_fops_write(struct file *filp, const char __user *buf,
> >                 return -EFAULT;
> >         }
> >
> > -       ret = wwan_port_op_tx(port, skb);
> > +       ret = wwan_port_op_tx(port, skb, !!(filp->f_flags & O_NONBLOCK));
> >         if (ret) {
> >                 kfree_skb(skb);
> >                 return ret;
> > @@ -659,6 +660,8 @@ static __poll_t wwan_port_fops_poll(struct file *filp, poll_table *wait)
> >                 mask |= EPOLLIN | EPOLLRDNORM;
> >         if (!port->ops)
> >                 mask |= EPOLLHUP | EPOLLERR;
> > +       else if (port->ops->poll)
> > +               mask |= port->ops->poll(port, filp, wait);
> 
> I'm not sure it useful here because EPOLLOUT flag is already set above, right?
> 

Oops, you're right - sorry! I thought the flags are inverted (only set
if (is_write_blocked())), then it would have worked fine. :)

I think this should be easy to fix though, I can just make the

	if (!is_write_blocked(port))
		mask |= EPOLLOUT | EPOLLWRNORM;

if statement conditional to (port->ops->poll == NULL). It only makes
sense to supply the poll() op if the built-in write-blocking cannot be
used easily (like in my case).

I can add that in v2, will wait a bit more in case someone has some
other comments or a better suggestion.

Thanks for the review!
Stephan

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