Re: [PATCH v1] i2c: tegra: Fix i2c_writesl() to use writel() instead of writesl()

[Sowjanya Komatineni]

On 10/20/20 12:48 AM, Thierry Reding wrote:
> On Mon, Oct 19, 2020 at 09:03:54PM -0700, Sowjanya Komatineni wrote:
> > VI I2C don't have DMA support and uses PIO mode all the time.
> >
> > Current driver uses writesl() to fill TX FIFO based on available
> > empty slots and with this seeing strange silent hang during any I2C
> > register access after filling TX FIFO with 8 words.
> >
> > Using writel() followed by i2c_readl() in a loop to write all words
> > to TX FIFO instead of using writesl() helps for large transfers in
> > PIO mode.
> >
> > So, this patch updates i2c_writesl() API to use writel() in a loop
> > instead of writesl().
> >
> > Signed-off-by: Sowjanya Komatineni <skomatineni@xxxxxxxxxx>
> > ---
> >   drivers/i2c/busses/i2c-tegra.c | 9 ++++++---
> >   1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
> > index 6f08c0c..274bf3a 100644
> > --- a/drivers/i2c/busses/i2c-tegra.c
> > +++ b/drivers/i2c/busses/i2c-tegra.c
> > @@ -333,10 +333,13 @@ static u32 i2c_readl(struct tegra_i2c_dev *i2c_dev, unsigned int reg)
> >   	return readl_relaxed(i2c_dev->base + tegra_i2c_reg_addr(i2c_dev, reg));
> >   }
> >   
> > -static void i2c_writesl(struct tegra_i2c_dev *i2c_dev, void *data,
> > +static void i2c_writesl(struct tegra_i2c_dev *i2c_dev, u32 *data,
> >   			unsigned int reg, unsigned int len)
> >   {
> > -	writesl(i2c_dev->base + tegra_i2c_reg_addr(i2c_dev, reg), data, len);
> > +	while (len--) {
> > +		writel(*data++, i2c_dev->base + tegra_i2c_reg_addr(i2c_dev, reg));
> > +		i2c_readl(i2c_dev, I2C_INT_STATUS);
> > +	}
> >   }
> >   
> >   static void i2c_readsl(struct tegra_i2c_dev *i2c_dev, void *data,
> > @@ -811,7 +814,7 @@ static int tegra_i2c_fill_tx_fifo(struct tegra_i2c_dev *i2c_dev)
> >   		i2c_dev->msg_buf_remaining = buf_remaining;
> >   		i2c_dev->msg_buf = buf + words_to_transfer * BYTES_PER_FIFO_WORD;
> >   
> > -		i2c_writesl(i2c_dev, buf, I2C_TX_FIFO, words_to_transfer);
> > +		i2c_writesl(i2c_dev, (u32 *)buf, I2C_TX_FIFO, words_to_transfer);
> I've thought a bit more about this and I wonder if we're simply reading
> out the wrong value for tx_fifo_avail and therefore end up overflowing
> the TX FIFO. Have you checked what the value is for tx_fifo_avail when
> this silent hang occurs? Given that this is specific to the VI I2C I'm
> wondering if this is perhaps a hardware bug where we read the wrong TX
> FIFO available count.
>
> Thierry

Yes FIFO status shows all 8 slots available.

Also, HW wise VI I2C is similar to other I2C and FIFO depth is also 8 
words. Confirmed from HW designers as well.

Using writesl() causes silent hang after filling some words in FIFO and 
most of time after filling 8 words and sometime after filling it with 
around 6 words.

I am not sure if this issue is specific to VI I2C alone as other I2C 
mostly use DMA for more than 8 words and I am not sure if we ever used 
other I2C in PIO mode for ~8 words transfer for slave devices we have on 
the platform.


Thanks

Sowjanya