Vincent Palatin wrote at Friday, June 03, 2011 4:19 PM:
> On Fri, Jun 3, 2011 at 18:01, Stephen Warren <swarren@xxxxxxxxxx> wrote:
> >Tested-by: Stephen Warren <swarren@xxxxxxxxxx>
> >
> > (using code based on 3.0-rc1, on Harmony, ran "speaker-test -c 2", and
> > then adjusted the volume a lot using alsamixer, thus causing quite a few
> > I2C transactions)
>
> Thanks for the testing and the review !
>
> >> > @@ -213,38 +213,41 @@ static int tegra_i2c_empty_rx_fifo(struct
> tegra_i2c_dev *i2c_dev)
> >> > u32 val;
> >> > int rx_fifo_avail;
> >> > u8 *buf = i2c_dev->msg_buf;
> >> > - size_t buf_remaining = i2c_dev->msg_buf_remaining;
> >
> > The old code read msg_buf_remaining once up front and did everything
> > based on that.
> >
> >> > int words_to_transfer;
> >> > + int bytes_to_transfer;
> >> >
> >> > val = i2c_readl(i2c_dev, I2C_FIFO_STATUS);
> >> > rx_fifo_avail = (val & I2C_FIFO_STATUS_RX_MASK) >>
> >> > I2C_FIFO_STATUS_RX_SHIFT;
> >> >
> >> > /* Rounds down to not include partial word at the end of buf
> */
> >> > - words_to_transfer = buf_remaining / BYTES_PER_FIFO_WORD;
> >> > + words_to_transfer = atomic_read(&i2c_dev->msg_buf_remaining)
> /
> >> > + BYTES_PER_FIFO_WORD;
> >
> > Whereas the new code reads msg_buf_remaining once here...
> >
> >> > if (words_to_transfer > rx_fifo_avail)
> >> > words_to_transfer = rx_fifo_avail;
> >> >
> >> > + atomic_sub(words_to_transfer * BYTES_PER_FIFO_WORD,
> >> > + &i2c_dev->msg_buf_remaining);
> >> > i2c_readsl(i2c_dev, buf, I2C_RX_FIFO, words_to_transfer);
> >> >
> >> > buf += words_to_transfer * BYTES_PER_FIFO_WORD;
> >> > - buf_remaining -= words_to_transfer * BYTES_PER_FIFO_WORD;
> >> > rx_fifo_avail -= words_to_transfer;
> >> >
> >> > /*
> >> > * If there is a partial word at the end of buf, handle it manually to
> >> > * prevent overwriting past the end of buf
> >> > */
> >> > - if (rx_fifo_avail > 0 && buf_remaining > 0) {
> >> > - BUG_ON(buf_remaining > 3);
> >> > + bytes_to_transfer = atomic_read(&i2c_dev->msg_buf_remaining);
> >
> > And again here...
> >
> >> > + if (rx_fifo_avail > 0 && bytes_to_transfer > 0) {
> >> > + BUG_ON(bytes_to_transfer > 3);
> >
> > That means that if msg_buf_remaining increases between those two reads,
> > this BUG_ON could trigger.
> >
> > I assume this isn't possible, because the I2C core only sends one
> > transaction to the I2C driver and doesn't send any more requests down
> > until the previous is complete. If so, then the new code seems fine, but
> > I did want to double-check this.
>
> The transfers are serialized in the i2c_transfer function of the core
> (which calls the tegra_i2c_xfer callback) and msg_buf_remaining can
> only increase when it is set at the beginning of tegra_i2c_xfer_msg.
> So yes we have at most one transaction and I don't think we can
> trigger this BUG_ON.
Great, that's what I figured. So, the change looks good to me, so
Acked-by: Stephen Warren <swarren@xxxxxxxxxx>
too!
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nvpublic
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