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

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Message-ID: <d880a7b1-28e2-1bfc-2ba7-a7139455b890@nvidia.com>
Date:   Tue, 20 Oct 2020 09:39:10 -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 9:37 AM, Sowjanya Komatineni wrote:
>
> 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.

To be clear, TX slots availability in FIFO status before filling shows 8 
words empty.

With writesl() when silent hang is seen, couldn't access any registers 
and with debug messages I see silent hang happens immediate during 
accessing any of i2c controller register after writesl() call

>
> 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
>