Message-ID: <d1a61e61-d101-e8ce-0fa1-ff4fb60ee654@gmail.com>
Date:   Sun, 30 Aug 2020 15:55:27 +0200
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Nicholas Piggin <npiggin@...il.com>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "broonie@...nel.org" <broonie@...nel.org>,
        Chris Packham <Chris.Packham@...iedtelesis.co.nz>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "paulus@...ba.org" <paulus@...ba.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>
Subject: Re: fsl_espi errors on v5.7.15

On 30.08.2020 14:30, Nicholas Piggin wrote:
> Excerpts from Chris Packham's message of August 28, 2020 8:07 am:
>> On 27/08/20 7:12 pm, Nicholas Piggin wrote:
>>> Excerpts from Heiner Kallweit's message of August 26, 2020 4:38 pm:
>>>> On 26.08.2020 08:07, Chris Packham wrote:
>>>>> On 26/08/20 1:48 pm, Chris Packham wrote:
>>>>>> On 26/08/20 10:22 am, Chris Packham wrote:
>>>>>>> On 25/08/20 7:22 pm, Heiner Kallweit wrote:
>>>>>>>
>>>>>>> <snip>
>>>>>>>> I've been staring at spi-fsl-espi.c for while now and I think I've
>>>>>>>>> identified a couple of deficiencies that may or may not be related
>>>>>>>>> to my
>>>>>>>>> issue.
>>>>>>>>>
>>>>>>>>> First I think the 'Transfer done but SPIE_DON isn't set' message
>>>>>>>>> can be
>>>>>>>>> generated spuriously. In fsl_espi_irq() we read the ESPI_SPIE
>>>>>>>>> register.
>>>>>>>>> We also write back to it to clear the current events. We re-read it in
>>>>>>>>> fsl_espi_cpu_irq() and complain when SPIE_DON is not set. But we can
>>>>>>>>> naturally end up in that situation if we're doing a large read.
>>>>>>>>> Consider
>>>>>>>>> the messages for reading a block of data from a spi-nor chip
>>>>>>>>>
>>>>>>>>>     tx = READ_OP + ADDR
>>>>>>>>>     rx = data
>>>>>>>>>
>>>>>>>>> We setup the transfer and pump out the tx_buf. The first interrupt
>>>>>>>>> goes
>>>>>>>>> off and ESPI_SPIE has SPIM_DON and SPIM_RXT set. We empty the rx fifo,
>>>>>>>>> clear ESPI_SPIE and wait for the next interrupt. The next interrupt
>>>>>>>>> fires and this time we have ESPI_SPIE with just SPIM_RXT set. This
>>>>>>>>> continues until we've received all the data and we finish with
>>>>>>>>> ESPI_SPIE
>>>>>>>>> having only SPIM_RXT set. When we re-read it we complain that SPIE_DON
>>>>>>>>> isn't set.
>>>>>>>>>
>>>>>>>>> The other deficiency is that we only get an interrupt when the
>>>>>>>>> amount of
>>>>>>>>> data in the rx fifo is above FSL_ESPI_RXTHR. If there are fewer than
>>>>>>>>> FSL_ESPI_RXTHR left to be received we will never pull them out of
>>>>>>>>> the fifo.
>>>>>>>>>
>>>>>>>> SPIM_DON will trigger an interrupt once the last characters have been
>>>>>>>> transferred, and read the remaining characters from the FIFO.
>>>>>>> The T2080RM that I have says the following about the DON bit
>>>>>>>
>>>>>>> "Last character was transmitted. The last character was transmitted
>>>>>>> and a new command can be written for the next frame."
>>>>>>>
>>>>>>> That does at least seem to fit with my assertion that it's all about
>>>>>>> the TX direction. But the fact that it doesn't happen all the time
>>>>>>> throws some doubt on it.
>>>>>>>
>>>>>>>> I think the reason I'm seeing some variability is because of how fast
>>>>>>>>> (or slow) the interrupts get processed and how fast the spi-nor
>>>>>>>>> chip can
>>>>>>>>> fill the CPUs rx fifo.
>>>>>>>>>
>>>>>>>> To rule out timing issues at high bus frequencies I initially asked
>>>>>>>> for re-testing at lower frequencies. If you e.g. limit the bus to 1 MHz
>>>>>>>> or even less, then timing shouldn't be an issue.
>>>>>>> Yes I've currently got spi-max-frequency = <1000000>; in my dts. I
>>>>>>> would also expect a slower frequency would fit my "DON is for TX"
>>>>>>> narrative.
>>>>>>>> Last relevant functional changes have been done almost 4 years ago.
>>>>>>>> And yours is the first such report I see. So question is what could
>>>>>>>> be so
>>>>>>>> special with your setup that it seems you're the only one being
>>>>>>>> affected.
>>>>>>>> The scenarios you describe are standard, therefore much more people
>>>>>>>> should be affected in case of a driver bug.
>>>>>>> Agreed. But even on my hardware (which may have a latent issue
>>>>>>> despite being in the field for going on 5 years) the issue only
>>>>>>> triggers under some fairly specific circumstances.
>>>>>>>> You said that kernel config impacts how frequently the issue happens.
>>>>>>>> Therefore question is what's the diff in kernel config, and how could
>>>>>>>> the differences be related to SPI.
>>>>>>> It did seem to be somewhat random. Things like CONFIG_PREEMPT have an
>>>>>>> impact but every time I found something that seemed to be having an
>>>>>>> impact I've been able to disprove it. I actually think its about how
>>>>>>> busy the system is which may or may not affect when we get round to
>>>>>>> processing the interrupts.
>>>>>>>
>>>>>>> I have managed to get the 'Transfer done but SPIE_DON isn't set!' to
>>>>>>> occur on the T2080RDB.
>>>>>>>
>>>>>>> I've had to add the following to expose the environment as a mtd
>>>>>>> partition
>>>>>>>
>>>>>>> diff --git a/arch/powerpc/boot/dts/fsl/t208xrdb.dtsi
>>>>>>> b/arch/powerpc/boot/dts/fsl/t208xrdb.dtsi
>>>>>>> index ff87e67c70da..fbf95fc1fd68 100644
>>>>>>> --- a/arch/powerpc/boot/dts/fsl/t208xrdb.dtsi
>>>>>>> +++ b/arch/powerpc/boot/dts/fsl/t208xrdb.dtsi
>>>>>>> @@ -116,6 +116,15 @@ flash@0 {
>>>>>>>                                  compatible = "micron,n25q512ax3",
>>>>>>> "jedec,spi-nor";
>>>>>>>                                  reg = <0>;
>>>>>>>                                  spi-max-frequency = <10000000>; /*
>>>>>>> input clock */
>>>>>>> +
>>>>>>> +                               partition@...oot {
>>>>>>> +                                        reg = <0x00000000 0x00100000>;
>>>>>>> +                                        label = "u-boot";
>>>>>>> +                                };
>>>>>>> +                                partition@...oot-env {
>>>>>>> +                                        reg = <0x00100000 0x00010000>;
>>>>>>> +                                        label = "u-boot-env";
>>>>>>> +                                };
>>>>>>>                          };
>>>>>>>                  };
>>>>>>>
>>>>>>> And I'm using the following script to poke at the environment
>>>>>>> (warning if anyone does try this and the bug hits it can render your
>>>>>>> u-boot environment invalid).
>>>>>>>
>>>>>>> cat flash/fw_env_test.sh
>>>>>>> #!/bin/sh
>>>>>>>
>>>>>>> generate_fw_env_config()
>>>>>>> {
>>>>>>>    cat /proc/mtd | sed 's/[:"]//g' | while read dev size erasesize
>>>>>>> name ; do
>>>>>>>       echo "$dev $size $erasesize $name"
>>>>>>>       [ "$name" = "u-boot-env" ] && echo "/dev/$dev 0x0000 0x2000
>>>>>>> $erasesize" >/flash/fw_env.config
>>>>>>>    done
>>>>>>> }
>>>>>>>
>>>>>>> cycles=10
>>>>>>> [ $# -ge 1 ] && cycles=$1
>>>>>>>
>>>>>>> generate_fw_env_config
>>>>>>>
>>>>>>> fw_printenv -c /flash/fw_env.config
>>>>>>>
>>>>>>> dmesg -c >/dev/null
>>>>>>> x=0
>>>>>>> while [ $x -lt $cycles ]; do
>>>>>>>      fw_printenv -c /flash/fw_env.config >/dev/null || break
>>>>>>>      fw_setenv -c /flash/fw_env.config foo $RANDOM || break;
>>>>>>>      dmesg -c | grep -q fsl_espi && break;
>>>>>>>      let x=x+1
>>>>>>> done
>>>>>>>
>>>>>>> echo "Ran $x cycles"
>>>>>> I've also now seen the RX FIFO not empty error on the T2080RDB
>>>>>>
>>>>>> fsl_espi ffe110000.spi: Transfer done but SPIE_DON isn't set!
>>>>>> fsl_espi ffe110000.spi: Transfer done but SPIE_DON isn't set!
>>>>>> fsl_espi ffe110000.spi: Transfer done but SPIE_DON isn't set!
>>>>>> fsl_espi ffe110000.spi: Transfer done but SPIE_DON isn't set!
>>>>>> fsl_espi ffe110000.spi: Transfer done but rx/tx fifo's aren't empty!
>>>>>> fsl_espi ffe110000.spi: SPIE_RXCNT = 1, SPIE_TXCNT = 32
>>>>>>
>>>>>> With my current workaround of emptying the RX FIFO. It seems
>>>>>> survivable. Interestingly it only ever seems to be 1 extra byte in the
>>>>>> RX FIFO and it seems to be after either a READ_SR or a READ_FSR.
>>>>>>
>>>>>> fsl_espi ffe110000.spi: tx 70
>>>>>> fsl_espi ffe110000.spi: rx 03
>>>>>> fsl_espi ffe110000.spi: Extra RX 00
>>>>>> fsl_espi ffe110000.spi: Transfer done but SPIE_DON isn't set!
>>>>>> fsl_espi ffe110000.spi: Transfer done but rx/tx fifo's aren't empty!
>>>>>> fsl_espi ffe110000.spi: SPIE_RXCNT = 1, SPIE_TXCNT = 32
>>>>>> fsl_espi ffe110000.spi: tx 05
>>>>>> fsl_espi ffe110000.spi: rx 00
>>>>>> fsl_espi ffe110000.spi: Extra RX 03
>>>>>> fsl_espi ffe110000.spi: Transfer done but SPIE_DON isn't set!
>>>>>> fsl_espi ffe110000.spi: Transfer done but rx/tx fifo's aren't empty!
>>>>>> fsl_espi ffe110000.spi: SPIE_RXCNT = 1, SPIE_TXCNT = 32
>>>>>> fsl_espi ffe110000.spi: tx 05
>>>>>> fsl_espi ffe110000.spi: rx 00
>>>>>> fsl_espi ffe110000.spi: Extra RX 03
>>>>>>
>>>>>>  From all the Micron SPI-NOR datasheets I've got access to it is
>>>>>> possible to continually read the SR/FSR. But I've no idea why it
>>>>>> happens some times and not others.
>>>>> So I think I've got a reproduction and I think I've bisected the problem
>>>>> to commit 3282a3da25bd ("powerpc/64: Implement soft interrupt replay in
>>>>> C"). My day is just finishing now so I haven't applied too much scrutiny
>>>>> to this result. Given the various rabbit holes I've been down on this
>>>>> issue already I'd take this information with a good degree of skepticism.
>>>>>
>>>> OK, so an easy test should be to re-test with a 5.4 kernel.
>>>> It doesn't have yet the change you're referring to, and the fsl-espi driver
>>>> is basically the same as in 5.7 (just two small changes in 5.7).
>>> There's 6cc0c16d82f88 and maybe also other interrupt related patches
>>> around this time that could affect book E, so it's good if that exact
>>> patch is confirmed.
>>
>> My confirmation is basically that I can induce the issue in a 5.4 kernel 
>> by cherry-picking 3282a3da25bd. I'm also able to "fix" the issue in 
>> 5.9-rc2 by reverting that one commit.
>>
>> I both cases it's not exactly a clean cherry-pick/revert so I also 
>> confirmed the bisection result by building at 3282a3da25bd (which sees 
>> the issue) and the commit just before (which does not).
> 
> Thanks for testing, that confirms it well.
> 
> [snip patch]
> 
>> I still saw the issue with this change applied. PPC_IRQ_SOFT_MASK_DEBUG 
>> didn't report anything (either with or without the change above).
> 
> Okay, it was a bit of a shot in the dark. I still can't see what
> else has changed.
> 
> What would cause this, a lost interrupt? A spurious interrupt? Or
> higher interrupt latency?
> 
> I don't think the patch should cause significantly worse latency,
> (it's supposed to be a bit better if anything because it doesn't set
> up the full interrupt frame). But it's possible.
> 
> Thanks,
> Nick
> 

As additional background:
When I refactored and extended the fsl-espi driver my test device was
a P1014-based WiFi router (running at 800MHz CPU clock). I still use it
and operate the SPI NOR (s25fl128s) at 50MHz in dual read mode
(with a 4.9 kernel). I never had a problem. 
If the issue now occurs on a system with most likely faster CPU and at
a much lower SPI bus frequency, then something must have a significant
impact on interrupt performance. However I can't say exactly which kind
of interrupt issue should cause the SPI issue we see.

Heiner

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