Message-Id: <D821D090-C230-4D34-BEC8-723715E3B8BE@holtmann.org>
Date:   Sat, 19 Oct 2019 23:17:07 +0200
From:   Marcel Holtmann <marcel@...tmann.org>
To:     Jeffrey Hugo <jeffrey.l.hugo@...il.com>
Cc:     Matthias Kaehlcke <mka@...omium.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        c-hbandi@...eaurora.org, bgodavar@...eaurora.org,
        linux-bluetooth@...r.kernel.org,
        MSM <linux-arm-msm@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Bluetooth: hci_qca: Add delay for wcn3990 stability

Hi Jeffrey,

>>>>>>>>> On the msm8998 mtp, the response to the baudrate change command is never
>>>>>>>>> received.  On the Lenovo Miix 630, the response to the baudrate change
>>>>>>>>> command is corrupted - "Frame reassembly failed (-84)".
>>>>>>>>> 
>>>>>>>>> Adding a 50ms delay before re-enabling flow to receive the baudrate change
>>>>>>>>> command response from the wcn3990 addesses both issues, and allows
>>>>>>>>> bluetooth to become functional.
>>>>>>>> 
>>>>>>>> From my earlier debugging on sdm845 I don't think this is what happens.
>>>>>>>> The problem is that the wcn3990 sends the response to the baudrate change
>>>>>>>> command using the new baudrate, while the UART on the SoC still operates
>>>>>>>> with the prior speed (for details see 2faa3f15fa2f ("Bluetooth: hci_qca:
>>>>>>>> wcn3990: Drop baudrate change vendor event"))
>>>>>>>> 
>>>>>>>> IIRC the 50ms delay causes the HCI core to discard the received data,
>>>>>>>> which is why the "Frame reassembly failed" message disappears, not
>>>>>>>> because the response was received. In theory commit 78e8fa2972e5
>>>>>>>> ("Bluetooth: hci_qca: Deassert RTS while baudrate change command")
>>>>>>>> should have fixed those messages, do you know if CTS/RTS are connected
>>>>>>>> on the Bluetooth UART of the Lenovo Miix 630?
>>>>>>> 
>>>>>>> I was testing with 5.4-rc1 which contains the indicated RTS fix.
>>>>>>> 
>>>>>>> Yes, CTS/RTS are connected on the Lenovo Miix 630.
>>>>>>> 
>>>>>>> I added debug statements which indicated that data was received,
>>>>>>> however it was corrupt, and the packet type did not match what was
>>>>>>> expected, hence the frame reassembly errors.
>>>>>> 
>>>>>> Do you know if any data is received during the delay? In theory that
>>>>>> shouldn't be the case since RTS is deasserted, just double-checking.
>>>>> 
>>>>> I don't think so, but I've run so many tests, I'm not 100% positive.
>>>>> Let me go double check and get back to you.
>>> 
>>> Apparently I'd be wrong.  I instrumented the uart driver so that it
>>> would indicate when it got data from the bam.  Apparently its getting
>>> the data during the 50ms sleep, approximately right after the host
>>> baud rate is set.
>> 
>> Good finding!
>> 
>>>>> 
>>>>>> 
>>>>>> What happens if you add a longer delay (e.g. 1s) before/after setting
>>>>>> the host baudrate?
>>>>> 
>>>>> Hmm, not exactly sure.  I will test.
>>> 
>>> Adding a 1 second delay before setting the host baud rate did not
>>> change the observed results - still received the data during the 50ms
>>> sleep after the host baud rate set operation.
>>> Adding a 1 second delay after setting the host baud rate did not
>>> change when the data was received.
>> 
>> Thanks for testing!
>> 
>>>>> 
>>>>>> 
>>>>>>> In response to this patch, Balakrishna pointed me to a bug report
>>>>>>> which indicated that some of the UART GPIO lines need to have a bias
>>>>>>> applied to prevent errant data from floating lines -
>>>>>>> 
>>>>>>> https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1391888
>>>>>> 
>>>>>> Yeah, that was another source of frame reassembly errors that we were
>>>>>> seeing on SDM845.
>>>>>> 
>>>>>> Balakrishna, please post these kind of replies on-list, so that
>>>>>> everybody can benefit from possible solutions or contribute to the
>>>>>> discussion.
>>>>>> 
>>>>>>> It turns out this fix was never applied to msm8998.  Applying the fix
>>>>>>> does cause the the frame reassembly errors to go away, however then
>>>>>>> the host SoC never receives the baud rate change response (I increased
>>>>>>> the timeout from 2faa3f15fa2f ("Bluetooth: hci_qca: wcn3990: Drop
>>>>>>> baudrate change vendor event") to 5 seconds).  As of now, this patch
>>>>>>> is still required.
>>>>>> 
>>>>>> Interesting.
>>>>>> 
>>>>>> FTR, this is the full UART pin configuration for cheza (SDM845):
>>>>>> 
>>>>>> &qup_uart6_default {
>>>>>>        /* Change pinmux to all 4 pins since CTS and RTS are connected */
>>>>>>        pinmux {
>>>>>>                pins = "gpio45", "gpio46",
>>>>>>                       "gpio47", "gpio48";
>>>>>>        };
>>>>>> 
>>>>>>        pinconf-cts {
>>>>>>                /*
>>>>>>                 * Configure a pull-down on 45 (CTS) to match the pull of
>>>>>>                 * the Bluetooth module.
>>>>>>                 */
>>>>>>                pins = "gpio45";
>>>>>>                bias-pull-down;
>>>>>>        };
>>>>>> 
>>>>>>        pinconf-rts-tx {
>>>>>>                /* We'll drive 46 (RTS) and 47 (TX), so no pull */
>>>>>>                pins = "gpio46", "gpio47";
>>>>>>                drive-strength = <2>;
>>>>>>                bias-disable;
>>>>>>        };
>>>>>> 
>>>>>>        pinconf-rx {
>>>>>>                /*
>>>>>>                 * Configure a pull-up on 48 (RX). This is needed to avoid
>>>>>>                 * garbage data when the TX pin of the Bluetooth module is
>>>>>>                 * in tri-state (module powered off or not driving the
>>>>>>                 * signal yet).
>>>>>>                 */
>>>>>>                pins = "gpio48";
>>>>>>                bias-pull-up;
>>>>>>        };
>>>>>> };
>>>>>> 
>>>>>> Does this correspond to what you tried on the Lenovo Miix 630?
>>>>> 
>>>>> Which GPIO maps to which pin is different -
>>>>> 45 - TX
>>>>> 46 - RX
>>>>> 47 - CTS
>>>>> 48 - RFR (RTS)
>>>>> 
>>>>> However, accounting for that, yes that corresponds to what I used.
>>>> 
>>>> Thanks for re-confirming.
>>>> 
>>>>>>> I have no idea why the delay is required, and was hoping that posting
>>>>>>> this patch would result in someone else providing some missing pieces
>>>>>>> to determine the real root cause.  I suspect that asserting RTS at the
>>>>>>> wrong time may cause an issue for the wcn3990, but I have no data nor
>>>>>>> documentation to support this guess.  I welcome any further insights
>>>>>>> you may have.
>>>>>> 
>>>>>> Unfortunately I don't have a clear suggestion at this point, debugging
>>>>>> the original problem which lead to 2faa3f15fa2f ("Bluetooth: hci_qca:
>>>>>> wcn3990: Drop baudrate change vendor event") involved quite some time
>>>>>> and hooking up a scope/logic analyzer ...
>>>>>> 
>>>>>> I also suspect RTS is involved, and potentially the configuration of
>>>>>> the pulls. It might be interesting to analyze the data that leads to
>>>>>> the frame assembly error and determine if it is just noise (wrong
>>>>>> pulls/drive strength?) or received with a non-matching baud-rate.
>>>>> 
>>>>> I don't have a scope/logic analyzer, but since I hooked up the
>>>>> blsp_bam I'm able to see the raw data from the uart before it gets to
>>>>> the HCI stack or anything.  As a side note, having the bam or not
>>>>> seemed to have no effect on the issue.
>>>> 
>>>> It's not exactly the same though. I suppose with the blsp_bam you only
>>>> see the actual data when the UART runs at the same speed as it's
>>>> counterpart. With a logic analyzer you can change the speed after
>>>> data capture, which might convert apparent garbage into reasonable
>>>> data.
>>>> 
>>>>> Most of the time the data was one byte (zero), some times it was a
>>>>> string of zero bytes.  Rarely it would be random data.
>>>> 
>>>> In terms of data ss there difference between a string of zero bytes
>>>> and a single zero byte?
>>> 
>>> Per my notes, the bam would indicate that it processed one byte, which
>>> was a 0, or 6 bytes, all of which were zero.
>> 
>> ok
>> 
>>>> From my notes the response (vendor event) to a baudrate change
>>>> command on the WCN3990 is:
>>>> 
>>>> 04 0e 04 01 00 00 00
>>>> 
>>>> The tail *might* be the zero(s) you are seeing, and the first part gets
>>>> lost for some reason?
>>> 
>>> So, if that were the case, then the number of processed bytes would
>>> probably 1, 2, or 3 which doesn't seem to line up fully with 1 or 6.
>> 
>> ack
>> 
>>>> A simplified version of the code in question:
>>>> 
>>>>  set_RTS(false)
>>>> 
>>>>  hci_set_baudrate(br)
>>>>  host_set_baudrate(br)
>>>> 
>>>>  msleep(50); // why is this needed???
>>>>  set_RTS(true)
>>>> 
>>>>  // supposedly wcn3990 now sends vendor event using the new baudrate
>>>> 
>>>>  wait_for_vendor_event()
>>>>    // ok with msleep, otherwise frame reassembly error
>>> 
>>> Yep, I'm with you here.
>>> 
>>>> 
>>>> Maybe the MSM8998 UART (driver) currently needs the delay to fully switch to
>>>> the new baudrate? Perhaps the pinconfig still needs tweaking of some kind?
>>> 
>>> So, I kinda wonder if its something else.  The uart driver on msm8998
>>> is not the same as on sdm845.  The msm8998 one behaves strangely in my
>>> opinion.  Any configuration (set the baud, change the flow control,
>>> etc) results in a complete reinit of the entire uart.  I wonder if
>>> there is a glitch, and the driver ends up inadvertently enabling flow
>>> during the host baud rate switch.  If I look at the timing between
>>> 50ms delay and no 50ms delay, it looks like the set_RTS(true)
>>> operation occurs at the approximate same time the data would be
>>> received had the 50ms delay been in place.  I wonder if since the uart
>>> driver completely reconfigures everything as a result of the
>>> set_RTS(true) operation, that is causing the data to be dropped -
>>> either in hardware or software.  More investigation would be required
>>> to find data to validate or disprove my guesses about these two
>>> possible issues.
>> 
>> A RTS glitch and data being dropped during the port reconfiguration is a
>> possiblity. However, with the delay the data that otherwise would cause
>> the frame reassembly error is received during the delay and discarded
>> (I don't recall what exactly leads to discarding), but we still receive
>> the vendor event we are waiting for (to drop it). This strongly suggests
>> that the data received during the delay is not the vendor event, but
>> something else. My first guess would be garbage as artifact of the
>> baudrate switch and/or pin config. My second guess would be an additional
>> HCI event, but I don't recall having seen that when I investigated the
>> problem with changing the baudrate on cheza.
> 
> After more digging, I'm convinced there is a RTS glitch in the uart driver.
> 
> To be clear, after porting the cheza pin configuration, I don't see
> any frame reassembly errors.  With the updated pinconfig, the
> remaining questions would be why is the 50 ms delay necessary to
> receive the event, and why does the event come when flow is disabled?
> 
> I investigated the possibility of a RTS issue.  After studying the
> uart hardware documentation, I concluded that the driver was doing the
> wrong thing as part of "reset".  Instead of de-asserting RTS, it was
> asserting it, thus enabling flow.  Disabling flow would asset RTS as
> part of the reset from the termios operation, then disable the
> hardware flow management as part of a mctrl operation, which would end
> up deasserting RTS (although this appear to be a concidence and not
> guarenteed by the hardware documentation).  Then the host baud change
> would invoke the termios operation, which would reset the hardware and
> re-assert RTS, but there would be no mctrl operation to de-assert RTS
> again, which is what would be expected.  Thus the wcn3990 would be
> free to send the response, which would line up with the data showing
> an event received immediately after the host baud change, even though
> flow is expected to be disabled at that point.
> 
> I still have no logic analyzer, nor a platform that allows me to get
> at the actual signals, however I hacked up the pinctrl driver to allow
> me to read the raw gpio state at any point from code.  I was able to
> use this to confirm that RTS was not behaving as expected, and what
> exact operations in the uart driver was causing the errant behavior.
> 
> By changing the uart driver to de-assert RTS instead of asserting RTS
> as part of "reset", I see that the RTS gpio line behaves as expected,
> and the expected event always comes after flow is re-enabled.  This
> behavior does not change despite having a 1 second delay after host
> baud change, a 50ms delay, or no delay.
> 
> Therefore, I believe we have root caused why the 50ms delay was having
> an effect, and determined a proper fix.  I will be formulating a
> proper patch to the uart driver, and sending it upstream
> 
> Marcel, I appreciate that you picked up this change (adding a 50 ms
> delay).  However, since I believe we have root caused the issue and
> formulated a proper fix, this change is now unnecessary, and I believe
> it should be dropped.  How would you like to handle that?  Would you
> like me to post a revert?

if you have a solution, then I would just use a new patch with fixes tag.

Dropping the patch is not an option since we have other commits referencing other commit ids now.

Regards

Marcel

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