Re: [PATCH V8 1/1] i2c: i2c-qcom-geni: Add shutdown callback for i2c

[rojay]

Hi Stephen,

On 2021-02-24 12:36, Stephen Boyd wrote:
> Quoting rojay@xxxxxxxxxxxxxx (2021-02-18 06:15:17)
> > Hi Stephen,
> >
> > On 2021-01-13 12:24, Stephen Boyd wrote:
> > > Quoting Roja Rani Yarubandi (2021-01-08 07:05:45)
> > >> diff --git a/drivers/i2c/busses/i2c-qcom-geni.c
> > >> b/drivers/i2c/busses/i2c-qcom-geni.c
> > >> index 214b4c913a13..c3f584795911 100644
> > >> --- a/drivers/i2c/busses/i2c-qcom-geni.c
> > >> +++ b/drivers/i2c/busses/i2c-qcom-geni.c
> > >> @@ -375,6 +375,32 @@ static void geni_i2c_tx_msg_cleanup(struct
> > >> geni_i2c_dev *gi2c,
> > >>         }
> > >>  }
> > >>
> > >> +static void geni_i2c_stop_xfer(struct geni_i2c_dev *gi2c)
> > >> +{
> > >> +       int ret;
> > >> +       u32 geni_status;
> > >> +       struct i2c_msg *cur;
> > >> +
> > >> +       /* Resume device, as runtime suspend can happen anytime during
> > >> transfer */
> > >> +       ret = pm_runtime_get_sync(gi2c->se.dev);
> > >> +       if (ret < 0) {
> > >> +               dev_err(gi2c->se.dev, "Failed to resume device: %d\n",
> > >> ret);
> > >> +               return;
> > >> +       }
> > >> +
> > >> +       geni_status = readl_relaxed(gi2c->se.base + SE_GENI_STATUS);
> > >> +       if (geni_status & M_GENI_CMD_ACTIVE) {
> > >> +               cur = gi2c->cur;
> > >
> > > Why don't we need to hold the spinlock gi2c::lock here?
> > >
> >
> > I am not seeing any race here. May I know which race are you 
> > suspecting
> > here?
>
> Sorry there are long delays between posting and replies to my review
> comments. It takes me some time to remember what we're talking about
> because this patch has dragged on for many months.

Sorry for the delayed responses.

> So my understanding is that gi2c::lock protects the 'cur' pointer. I
> imagine this scenario might go bad
>
>   CPU0                      CPU1
>   ----                      ----
>   geni_i2c_stop_xfer()
>    ...                      geni_i2c_rx_one_msg()
> 			     gi2c->cur = cur1;
>    cur = gi2c->cur;
>    ...                       geni_i2c_tx_one_msg()
> 			     gi2c->cur = cur2;
>    geni_i2c_abort_xfer()
>     <uses cur2>
>    if (cur->flags & I2C_M_RD)
>     <uses cur1 for the condition and call; oops that's bad>
>
> It's almost like we should combine the geni_i2c_abort_xfer() logic with
> the rx/tx message cleanup functions so that it's all done under one
> lock. Unfortunately it's complicated by the fact that there are various
> completion waiting timeouts involved. Fun!

Thanks for the explanation. Fixed this possible race by protecting 
gi2c->cur
and calling geni_i2c_abort_xfer() with adding another parameter to 
differentiate
from which sequence is the geni_i2c_abort_xfer() called and handle the
spin_lock/spin_unlock accordingly inside geni_i2c_abort_xfer()

> But even after all that, I don't see how the geni_i2c_stop_xfer() puts 
> a
> stop to future calls to geni_i2c_rx_one_msg() or geni_i2c_tx_one_msg().

Now handled this by adding a bool variable "stop_xfer" in geni_i2c_dev 
struct,
used to put stop to upcoming geni_i2c_rx_one_msg() and 
geni_i2c_tx_one_msg() calls
once we receive the shutdown call.

> The hardware isn't disabled from what I can tell. The irq isn't
> disabled, the clks aren't turned off, etc. What is to stop an i2c 
> device
> from trying to use the bus after this shutdown function is called? If
> anything, this function looks like a "flush", where we flush out any
> pending transfer. Where's the "plug" operation that prevents any future
> operations from following this call?

We are turning off clocks and disabling irq in 
geni_i2c_runtime_suspend().

IIUC about shutdown sequence, during "remove" we will unplug the device 
with opposite
calls to "probe's" plug operations example i2c_del_adapter(). For 
"shutdown", as system
is going to shutdown, there is no need of unplug operations to be done.

> BTW, I see this is merged upstream. That's great, but it seems broken.
> Please fix it or revert it out.
>
> > >> +               geni_i2c_abort_xfer(gi2c);
> > >> +               if (cur->flags & I2C_M_RD)
> > >> +                       geni_i2c_rx_msg_cleanup(gi2c, cur);
> > >> +               else
> > >> +                       geni_i2c_tx_msg_cleanup(gi2c, cur);
> > >> +       }
> > >> +
> > >> +       pm_runtime_put_sync_suspend(gi2c->se.dev);
> > >> +}
> > >> +
> > >>  static int geni_i2c_rx_one_msg(struct geni_i2c_dev *gi2c, struct
> > >> i2c_msg *msg,
> > >>                                 u32 m_param)
> > >>  {

Thanks,
Roja