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

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Date:   Fri, 14 Jun 2019 02:50:27 -0700
From:   Bitan Biswas <bbiswas@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Thierry Reding <treding@...dia.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        <linux-i2c@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Peter Rosin <peda@...ntia.se>,
        Wolfram Sang <wsa@...-dreams.de>
CC:     Shardar Mohammed <smohammed@...dia.com>,
        Sowjanya Komatineni <skomatineni@...dia.com>,
        Mantravadi Karthik <mkarthik@...dia.com>
Subject: Re: [PATCH V5 6/7] i2c: tegra: fix PIO rx/tx residual transfer check



On 6/13/19 5:28 AM, Dmitry Osipenko wrote:
> 13.06.2019 14:30, Bitan Biswas пишет:
>>
>>
>> On 6/12/19 7:30 AM, Dmitry Osipenko wrote:
>>> 11.06.2019 13:51, Bitan Biswas пишет:
>>>> Fix expression for residual bytes(less than word) transfer
>>>> in I2C PIO mode RX/TX.
>>>>
>>>> Signed-off-by: Bitan Biswas <bbiswas@...dia.com>
>>>> ---
>>>
>>> [snip]
>>>
>>>>            /*
>>>> -         * 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;
>>>
>>> Looks like the comment could be removed altogether because it doesn't
>>> make sense since interrupt handler is under xfer_lock which is kept
>>> locked during of tegra_i2c_xfer_msg().
>> I would push a separate patch to remove this comment because of
>> xfer_lock in ISR now.
>>
>>>
>>> Moreover the comment says that "PACKET_XFER_COMPLETE is not maskable",
>>> but then what I2C_INT_PACKET_XFER_COMPLETE masking does?
>>>
>> I2C_INT_PACKET_XFER_COMPLETE masking support available in Tegra chips
>> newer than Tegra30 allows one to not see interrupt after Packet transfer
>> complete. With the xfer_lock in ISR the scenario discussed in comment
>> can be ignored.
> 
> Also note that xfer_lock could be removed and replaced with a just
> irq_enable/disable() calls in tegra_i2c_xfer_msg() because we only care
> about IRQ not firing during of the preparation process.
This should need sufficient testing hence let us do it in a different 
series.

> 
> It also looks like tegra_i2c_[un]nmask_irq isn't really needed and all
> IRQ's could be simply unmasked during the driver's probe, in that case
> it may worth to add a kind of "in-progress" flag to catch erroneous
> interrupts.
> 
TX interrupt needs special handling if this change is done. Hence I 
think it should be taken up after sufficient testing in a separate patch.

-regards,
  Bitan