Re: [PATCH V5 6/7] i2c: tegra: fix PIO rx/tx residual transfer check

[Bitan Biswas]

On 6/12/19 6:55 AM, Dmitry Osipenko wrote:
> 11.06.2019 13:51, Bitan Biswas ÐÐÑÐÑ:
> > Fix expression for residual bytes(less than word) transfer
> > in I2C PIO mode RX/TX.
> >
> > Signed-off-by: Bitan Biswas <bbiswas@xxxxxxxxxx>
> > ---
> >   drivers/i2c/busses/i2c-tegra.c | 11 ++++++-----
> >   1 file changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
> > index 4dfb4c1..0596c12 100644
> > --- a/drivers/i2c/busses/i2c-tegra.c
> > +++ b/drivers/i2c/busses/i2c-tegra.c
> > @@ -514,7 +514,8 @@ static int tegra_i2c_empty_rx_fifo(struct tegra_i2c_dev *i2c_dev)
> >   	 * 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) {
> > +	if (rx_fifo_avail > 0 &&
> > +	    (buf_remaining > 0 && buf_remaining < BYTES_PER_FIFO_WORD)) {
>
> The buf_remaining >= BYTES_PER_FIFO_WORD is not possible to happen
> because there are three possible cases:
>
> 1) buf_remaining > rx_fifo_avail * 4:
>
> 	In this case rx_fifo_avail = 0
>
> 2) buf_remaining < rx_fifo_avail * 4;
>
> 	In this case buf_remaining is always < 4 because
> 	words_to_transfer is a buf_remaining rounded down to 4
> 	and then divided by 4. Hence:
>
> 	buf_remaining -= (buf_remaining / 4) * 4 always results
> 	into buf_remaining < 4.
>
> 3) buf_remaining == rx_fifo_avail * 4:
>
> 	In this case rx_fifo_avail = 0 and buf_remaining = 0.
>
> Case 2 should never happen and means that something gone wrong.
Yes I now agree with you. The first condition "rx_fifo_avail > 0" 
failure will take care and prevent need for additional checks.

> >   		BUG_ON(buf_remaining > 3);
> >   		val = i2c_readl(i2c_dev, I2C_RX_FIFO);
> >   		val = cpu_to_le32(val);
> > @@ -557,11 +558,10 @@ static int tegra_i2c_fill_tx_fifo(struct tegra_i2c_dev *i2c_dev)
> >   			words_to_transfer = tx_fifo_avail;
> >   
> >   		/*
> > -		 * Update state before writing to FIFO.  If this casues us
> > +		 * Update state before writing to FIFO.  If this causes us
> >   		 * to finish writing all bytes (AKA buf_remaining goes to 0) we
> >   		 * have a potential for an interrupt (PACKET_XFER_COMPLETE is
> > -		 * not maskable).  We need to make sure that the isr sees
> > -		 * buf_remaining as 0 and doesn't call us back re-entrantly.
> > +		 * not maskable).
> >   		 */
> >   		buf_remaining -= words_to_transfer * BYTES_PER_FIFO_WORD;
> >   		tx_fifo_avail -= words_to_transfer;
> > @@ -580,7 +580,8 @@ static int tegra_i2c_fill_tx_fifo(struct tegra_i2c_dev *i2c_dev)
> >   	 * prevent reading past the end of buf, which could cross a page
> >   	 * boundary and fault.
> >   	 */
> > -	if (tx_fifo_avail > 0 && buf_remaining > 0) {
> > +	if (tx_fifo_avail > 0 &&
> > +	    (buf_remaining > 0 && buf_remaining < BYTES_PER_FIFO_WORD)) {
> >   		BUG_ON(buf_remaining > 3);
> >   		memcpy(&val, buf, buf_remaining);
> >   		val = le32_to_cpu(val);
>
> Same as for RX.
Yes shall discard this patch from the next update.

-Thanks,
 Bitan