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

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Date:   Tue, 20 Oct 2020 09:37:15 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Thierry Reding <thierry.reding@...il.com>
CC:     <jonathanh@...dia.com>, <digetx@...il.com>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH v1] i2c: tegra: Fix i2c_writesl() to use writel() instead
 of writesl()


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@...dia.com>
>> ---
>>   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