Re: [PATCH v1 5/6] i2c: iproc: handle master read request

[Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>]

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On Fri, Oct 23, 2020 at 10:56 PM Ray Jui <ray.jui@broadcom.com> wrote:
>
>
>
> On 10/13/2020 10:12 PM, Rayagonda Kokatanur wrote:
> >
> >
> > On Wed, Oct 14, 2020 at 8:50 AM Dhananjay Phadke
> > <dphadke@linux.microsoft.com <mailto:dphadke@linux.microsoft.com>> wrote:
> >
> >     On Sun, 11 Oct 2020 23:52:53 +0530, Rayagonda Kokatanur wrote:
> >     > --- a/drivers/i2c/busses/i2c-bcm-iproc.c
> >     > +++ b/drivers/i2c/busses/i2c-bcm-iproc.c
> >     >
> >     > -             } else if (status & BIT(IS_S_RD_EVENT_SHIFT)) {
> >     > -                     /* Start of SMBUS for Master Read */
> >     > +                                     I2C_SLAVE_WRITE_REQUESTED,
> >     &rx_data);
> >     > +                     iproc_i2c->rx_start_rcvd = true;
> >     > +                     iproc_i2c->slave_read_complete = false;
> >     > +             } else if (rx_status == I2C_SLAVE_RX_DATA &&
> >     > +                        iproc_i2c->rx_start_rcvd) {
> >     > +                     /* Middle of SMBUS Master write */
> >     >                       i2c_slave_event(iproc_i2c->slave,
> >     > -                                     I2C_SLAVE_READ_REQUESTED,
> >     &value);
> >     > -                     iproc_i2c_wr_reg(iproc_i2c, S_TX_OFFSET, value);
> >     > +                                     I2C_SLAVE_WRITE_RECEIVED,
> >     &rx_data);
> >     > +             } else if (rx_status == I2C_SLAVE_RX_END &&
> >     > +                        iproc_i2c->rx_start_rcvd) {
> >     > +                     /* End of SMBUS Master write */
> >     > +                     if (iproc_i2c->slave_rx_only)
> >     > +                             i2c_slave_event(iproc_i2c->slave,
> >     > +
> >      I2C_SLAVE_WRITE_RECEIVED,
> >     > +                                             &rx_data);
> >     > +
> >     > +                     i2c_slave_event(iproc_i2c->slave,
> >     I2C_SLAVE_STOP,
> >     > +                                     &rx_data);
> >     > +             } else if (rx_status == I2C_SLAVE_RX_FIFO_EMPTY) {
> >     > +                     iproc_i2c->rx_start_rcvd = false;
> >     > +                     iproc_i2c->slave_read_complete = true;
> >     > +                     break;
> >     > +             }
> >     >
> >     > -                     val = BIT(S_CMD_START_BUSY_SHIFT);
> >     > -                     iproc_i2c_wr_reg(iproc_i2c, S_CMD_OFFSET, val);
> >     > +             rx_bytes++;
> >
> >     rx_bytes should be incremented only along with
> >     I2C_SLAVE_WRITE_RECEIVED event?
> >
> >
> > It should be incremented in both I2C_SLAVE_WRITE_REQUESTED and
> > I2C_SLAVE_WRITE_RECEIVED cases because in both case it is reading valid
> > bytes from rx fifo.
> >
> >
> >     >
> >     > +static bool bcm_iproc_i2c_slave_isr(struct bcm_iproc_i2c_dev
> >     *iproc_i2c,
> >     > +                                 u32 status)
> >     > +{
> >     > +     u32 val;
> >     > +     u8 value;
> >     > +
> >     > +     /*
> >     > +      * Slave events in case of master-write, master-write-read and,
> >     > +      * master-read
> >     > +      *
> >     > +      * Master-write     : only IS_S_RX_EVENT_SHIFT event
> >     > +      * Master-write-read: both IS_S_RX_EVENT_SHIFT and
> >     IS_S_RD_EVENT_SHIFT
> >     > +      *                    events
> >     > +      * Master-read      : both IS_S_RX_EVENT_SHIFT and
> >     IS_S_RD_EVENT_SHIFT
> >     > +      *                    events or only IS_S_RD_EVENT_SHIFT
> >     > +      */
> >     > +     if (status & BIT(IS_S_RX_EVENT_SHIFT) ||
> >     > +         status & BIT(IS_S_RD_EVENT_SHIFT)) {
> >     > +             /* disable slave interrupts */
> >     > +             val = iproc_i2c_rd_reg(iproc_i2c, IE_OFFSET);
> >     > +             val &= ~iproc_i2c->slave_int_mask;
> >     > +             iproc_i2c_wr_reg(iproc_i2c, IE_OFFSET, val);
> >     > +
> >     > +             if (status & BIT(IS_S_RD_EVENT_SHIFT))
> >     > +                     /* Master-write-read request */
> >     > +                     iproc_i2c->slave_rx_only = false;
> >     > +             else
> >     > +                     /* Master-write request only */
> >     > +                     iproc_i2c->slave_rx_only = true;
> >     > +
> >     > +             /* schedule tasklet to read data later */
> >     > +             tasklet_schedule(&iproc_i2c->slave_rx_tasklet);
> >     > +
> >     > +             /* clear only IS_S_RX_EVENT_SHIFT interrupt */
> >     > +             iproc_i2c_wr_reg(iproc_i2c, IS_OFFSET,
> >     > +                              BIT(IS_S_RX_EVENT_SHIFT));
> >     >
> >
> >     Both tasklet and isr are writing to status (IS_OFFSET) reg.
> >
> >
> > Yes this is required.
> >
> > For ex, If IS_S_RD_EVENT_SHIFT interrupt, this should be cleared once
> > the driver completes reading all data from rx fifo.
> > After this the driver can start sending data to master.
> >
>
> If both tasklet and isr are accessing the IS_OFFSET register, don't you
> need lock protection against race condition? That is, ISR can interrupt
> tasklet.

All interrupts are disbaled when the tasklet is running.
Interrupts are re-enabled at the end of the tasklet.
So no race condition between tasklet and isr.

Best regards,
Rayagonda

>
> >
> >
> >     The tasklet seems to be batching up rx fifo reads because of
> >     time-sensitive
> >     Master-write-read transaction? Linux I2C framework is byte interface
> >     anyway.
> >     Can the need to batch reads be avoided by setting slave rx threshold for
> >     interrupt (S_FIFO_RX_THLD) to 1-byte?
> >
> >
> > To process more data with a single interrupt we are batching up rx fifo
> > reads.
> > This will reduce the number of interrupts.
> >
> > Also to avoid tasklet running more time (20us) we have a threshold of 10
> > bytes for batching read.
> > This is a better/optimised approach than reading single byte data per
> > interrupt.
> >
> >
> >     Also, wouldn't tasklets be susceptible to other interrupts? If fifo
> >     reads
> >     have to be batched up, can it be changed to threaded irq?
> >
> >
> > tasklets have higher priority than threaded irq, since i2c is time
> > sensitive so using a tasklet is preferred over threaded irq.
> >

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