Re: [PATCH v2 4/5] i2c: tegra: Add support for SW mutex register

[Kartik Rajput <kkartik@nvidia.com>]

Thanks for reviewing the patch Krzysztof!

On Thu, 2025-01-30 at 17:12 +0100, Krzysztof Kozlowski wrote:
> External email: Use caution opening links or attachments
> 
> 
> On 30/01/2025 15:34, Kartik Rajput wrote:
> > From: Akhil R <akhilrajeev@nvidia.com>
> > 
> > Add support for SW mutex register introduced in Tegra264 to provide
> > an option to share the interface between multiple firmwares and/or
> > VMs.
> > 
> > However, the hardware does not ensure any protection based on the
> > values. The driver/firmware should honor the peer who already holds
> > the mutex.
> > 
> > Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
> > Signed-off-by: Kartik Rajput <kkartik@nvidia.com>
> > ---
> > v1 -> v2:
> >       * Fixed typos.
> >       * Fix tegra_i2c_mutex_lock() logic.
> >       * Add a timeout in tegra_i2c_mutex_lock() instead of polling
> > for
> >         mutex indefinitely.
> > ---
> >  drivers/i2c/busses/i2c-tegra.c | 132
> > +++++++++++++++++++++++++++++----
> >  1 file changed, 117 insertions(+), 15 deletions(-)
> > 
> > diff --git a/drivers/i2c/busses/i2c-tegra.c
> > b/drivers/i2c/busses/i2c-tegra.c
> > index 7c8b76406e2e..aa92faa6f5cb 100644
> > --- a/drivers/i2c/busses/i2c-tegra.c
> > +++ b/drivers/i2c/busses/i2c-tegra.c
> > @@ -135,6 +135,14 @@
> >  #define I2C_MST_FIFO_STATUS_TX                       GENMASK(23,
> > 16)
> >  #define I2C_MST_FIFO_STATUS_RX                       GENMASK(7, 0)
> > 
> > +#define I2C_SW_MUTEX                         0x0ec
> > +#define I2C_SW_MUTEX_REQUEST                 GENMASK(3, 0)
> > +#define I2C_SW_MUTEX_GRANT                   GENMASK(7, 4)
> > +#define I2C_SW_MUTEX_ID                              9
> > +
> > +/* SW mutex acquire timeout value in milliseconds. */
> > +#define I2C_SW_MUTEX_TIMEOUT                 25
> > +
> >  /* configuration load timeout in microseconds */
> >  #define I2C_CONFIG_LOAD_TIMEOUT                      1000000
> > 
> > @@ -203,6 +211,7 @@ enum msg_end_type {
> >   * @has_interface_timing_reg: Has interface timing register to
> > program the tuned
> >   *           timing settings.
> >   * @has_hs_mode_support: Has support for high speed (HS) mode
> > transfers.
> > + * @has_mutex: Has mutex register for mutual exclusion with other
> > firmwares or VM.
> >   */
> >  struct tegra_i2c_hw_feature {
> >       bool has_continue_xfer_support;
> > @@ -229,6 +238,7 @@ struct tegra_i2c_hw_feature {
> >       u32 setup_hold_time_hs_mode;
> >       bool has_interface_timing_reg;
> >       bool has_hs_mode_support;
> > +     bool has_mutex;
> >  };
> > 
> >  /**
> > @@ -372,6 +382,103 @@ static void i2c_readsl(struct tegra_i2c_dev
> > *i2c_dev, void *data,
> >       readsl(i2c_dev->base + tegra_i2c_reg_addr(i2c_dev, reg),
> > data, len);
> >  }
> > 
> > +static int tegra_i2c_poll_register(struct tegra_i2c_dev *i2c_dev,
> > +                                u32 reg, u32 mask, u32 delay_us,
> > +                                u32 timeout_us)
> > +{
> > +     void __iomem *addr = i2c_dev->base +
> > tegra_i2c_reg_addr(i2c_dev, reg);
> > +     u32 val;
> > +
> > +     if (!i2c_dev->atomic_mode)
> > +             return readl_relaxed_poll_timeout(addr, val, !(val &
> > mask),
> > +                                               delay_us,
> > timeout_us);
> > +
> > +     return readl_relaxed_poll_timeout_atomic(addr, val, !(val &
> > mask),
> > +                                              delay_us,
> > timeout_us);
> > +}
> > +
> > +static int tegra_i2c_mutex_trylock(struct tegra_i2c_dev *i2c_dev)
> > +{
> > +     u32 val, id;
> > +
> > +     val = i2c_readl(i2c_dev, I2C_SW_MUTEX);
> > +     id = FIELD_GET(I2C_SW_MUTEX_GRANT, val);
> > +     if (id != 0 && id != I2C_SW_MUTEX_ID)
> > +             return 0;
> > +
> > +     val = FIELD_PREP(I2C_SW_MUTEX_REQUEST, I2C_SW_MUTEX_ID);
> > +     i2c_writel(i2c_dev, val, I2C_SW_MUTEX);
> 
> And how do you exactly prevent concurrent, overwriting write? This
> looks
> like pure race.
> 

The I2C_SW_MUTEX_GRANT field reflects the id of the current mutex
owner. The I2C_SW_MUTEX_GRANT field does not change with overwrites to
the I2C_SW_MUTEX_REQUEST field, unless I2C_SW_MUTEX_REQUEST field is
cleared.

> > +
> > +     val = i2c_readl(i2c_dev, I2C_SW_MUTEX);
> > +     id = FIELD_GET(I2C_SW_MUTEX_GRANT, val);
> > +
> > +     if (id != I2C_SW_MUTEX_ID)
> > +             return 0;
> > +
> > +     return 1;
> > +}
> > +
> > +static void tegra_i2c_mutex_lock(struct tegra_i2c_dev *i2c_dev)
> > +{
> > +     unsigned int num_retries = I2C_SW_MUTEX_TIMEOUT;
> > +
> > +     /* Poll until mutex is acquired or timeout. */
> > +     while (--num_retries && !tegra_i2c_mutex_trylock(i2c_dev))
> > +             usleep_range(1000, 2000);
> > +
> > +     WARN_ON(!num_retries);
> 
> 
> Blocked thread is not a reason to reboot entire system (see panic on
> warn). Drop or change to some dev_warn.
> 
> 

Ack. Will change this to dev_warn in v3.

> > +}
> > +
> > +static void tegra_i2c_mutex_unlock(struct tegra_i2c_dev *i2c_dev)
> > +{
> > +     u32 val, id;
> > +
> > +     val = i2c_readl(i2c_dev, I2C_SW_MUTEX);
> > +     id = FIELD_GET(I2C_SW_MUTEX_GRANT, val);
> > +
> > +     if (WARN_ON(id != I2C_SW_MUTEX_ID))
> 
> Same problem here.
> 

Ack.

> 
> 
> Best regards,
> Krzysztof

Thanks & Regards,
Kartik