linux-kernel - Re: [PATCH] serial: sc16is7xx: address RX timeout interrupt errata

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Message-ID: <549319d4-5da9-4eb0-abeb-6e63b3e26e3f@zonque.org>
Date:   Wed, 15 Nov 2023 15:57:38 +0100
From:   Daniel Mack <daniel@...que.org>
To:     Hugo Villeneuve <hugo@...ovil.com>
Cc:     gregkh@...uxfoundation.org, jirislaby@...nel.org,
        lech.perczak@...lingroup.com, u.kleine-koenig@...gutronix.de,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Maxim Popov <maxim.snafu@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH] serial: sc16is7xx: address RX timeout interrupt errata

On 11/15/23 15:47, Hugo Villeneuve wrote:
> On Tue, 14 Nov 2023 16:55:33 +0100
> Daniel Mack <daniel@...que.org> wrote:
> 
>> Hi Hugo,
>>
>> On 11/14/23 16:20, Hugo Villeneuve wrote:
>>> On Tue, 14 Nov 2023 08:49:04 +0100
>>> Daniel Mack <daniel@...que.org> wrote:
>>>> This devices has a silicon bug that makes it report a timeout interrupt
>>>> but no data in FIFO.
>>>>
>>>> The datasheet states the following in the errata section 18.1.4:
>>>>
>>>>   "If the host reads the receive FIFO at the at the same time as a
>>>>   time-out interrupt condition happens, the host might read 0xCC
>>>>   (time-out) in the Interrupt Indication Register (IIR), but bit 0
>>>>   of the Line Status Register (LSR) is not set (means there is not
>>>>   data in the receive FIFO)."
>>>>
>>>> When this happens, the loop in sc16is7xx_irq() will run forever,
>>>> which effectively blocks the i2c bus and breaks the functionality
>>>> of the UART.
>>>>
>>>> From the information above, it is assumed that when the bug is
>>>> triggered, the FIFO does in fact have payload in its buffer, but the
>>>> fill level reporting is off-by-one. Hence this patch fixes the issue
>>>> by reading one byte from the FIFO when that condition is detected.
>>>
>>> From what I understand from the errata, when the problem occurs, it
>>> affects bit 0 of the LSR register. I see no mention that it
>>> also affects the RX FIFO level register (SC16IS7XX_RXLVL_REG)?
>>
>> True, the errata doesn't explicitly mention that, but tests have shown
>> that the RXLVL register is equally affected.
> 
> Hi Daniel,
> ok, now it makes more sense if RXLVL is affected.
> 
> Have you contacted NXP about this? If not, I suggest you do open a
> support case and let them know about your findings, because it is very
> strange that it is not mentioned in the errata. And doing so may led to
> an updated and better documentation on their side about this errata.

The errata is also wrong in other regards - the IIR register cannot
yield 0xcc according to their own documentation. It also makes no
suggestion on how to recover from that situation, which is common
practice usually.

We'll let them know through our FAE channels, but the latest datasheet
for this chip was released over a decade ago, and I don't expect any
update to the errata wording.

> And incorporate this new info into your commit log for an eventual
> patch V2.

It makes no sense IMO to have all users of this chip suffer from an
issue that was clearly identified to be present and which has an evident
fix. Why would we do that?


Thanks,
Daniel