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

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

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On Wed, Oct 14, 2020 at 8:50 AM Dhananjay Phadke
<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 cases 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.
>
> 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.

Best regards,
Rayagonda

>
>

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