linux-kernel - Re: [EXT] Re: [PATCH v1 3/4] serial: 8250

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Date:   Fri, 16 Jul 2021 17:27:36 +0200
From:   Ralf Ramsauer <ralf.ramsauer@...-regensburg.de>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
CC:     Jiri Slaby <jirislaby@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        <linux-serial@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [EXT] Re: [PATCH v1 3/4] serial: 8250_pci: Always try MSI/MSI-X



On 16/07/2021 17:01, Andy Shevchenko wrote:
> On Fri, Jul 16, 2021 at 4:07 PM Ralf Ramsauer
> <ralf.ramsauer@...-regensburg.de> wrote:
>> On 14/07/2021 15:35, Andy Shevchenko wrote:
>>> On Wed, Jul 14, 2021 at 3:56 PM Ralf Ramsauer
>>> <ralf.ramsauer@...-regensburg.de> wrote:
>>>> On 14/07/2021 08:54, Jiri Slaby wrote:
>>>>> On 13. 07. 21, 12:40, Andy Shevchenko wrote:
>>>
>>>>> Hmm, have you checked the commit which introduced the whitelist?
>>>>>
>>>>>     Nevertheless, this needs to handled with care: while many 8250 devices
>>>>>     actually claim to support MSI(-X) interrupts it should not be
>>>>> enabled be
>>>>>     default. I had at least one device in my hands with broken MSI
>>>>>     implementation.
>>>>>
>>>>>     So better introduce a whitelist with devices that are known to support
>>>>>     MSI(-X) interrupts. I tested all devices mentioned in the patch.
>>>>>
>>>>>
>>>>> You should have at least CCed the author for an input.
>>>>
>>>> Yep, back then I was testing three different 8250 pci cards. All of them
>>>> claimed to support MSI, while one really worked with MSI, the one that I
>>>> whitelisted. So I thought it would be better to use legacy IRQs as long
>>>> as no one tested a specific card to work with MSI.
>>>
>>> Can you shed a light eventually what those cards are?
> 
>> So I found a no-name el-cheapo card that has some issues with MSI:
> 
> Win Chip Head (WCH)
> 
>> 18:00.0 Serial controller: Device 1c00:3253 (rev 10) (prog-if 05 [16850])
>>
>> The card comes with two serial lines. It comes perfectly up, if I enable
>> it to use MSI in the whitelist:
>>
>> serial 0000:18:00.0: Using MSI(-X) interrupts
>> serial 0000:18:00.0: Setup PCI port: port 40c0, irq 104, type 0
>> 0000:18:00.0: ttyS6 at I/O 0x40c0 (irq = 104, base_baud = 115200) is a
>> XR16850
>> serial 0000:18:00.0: Setup PCI port: port 40c8, irq 104, type 0
>> 0000:18:00.0: ttyS7 at I/O 0x40c8 (irq = 104, base_baud = 115200) is a
>> XR16850
>>
>> After loading 8250_pci, lspci -vvs 18:0.0 tells:
>>
>>         Capabilities: [68] MSI: Enable+ Count=1/32 Maskable+ 64bit+
>>                 Address: 00000000fee000b8  Data: 0000
>>                 Masking: ffffffff  Pending: 00000000
>>
>> Looks good so far. Now let's echo to the device.
>>
>> $ echo asdf > /dev/ttyS6
>>
>> -- stuck. The echoing process stucks at close():
>>
>> write(1, "asdf\n", 5)                   = 5
>> close(1
>>
>> Stuck in the sense of: the echo is still killable, no crashes. The same
>> happens if I try to access the device with stty. So something is odd
>> here. However, the Netmos cards that I whitelisted do a great job.
> 
> Can you share somehow the `lspci -vvv -xx -nk; lspci -t` with and
> without MSI enabled? (I want to understand what is going on with it)

Sure, please find it attached.

  Ralf

> 
>> So I can't tell if I was just unlucky to grab a card that has issues
>> with MSI, and this is an exception rather than the rule…
> 

View attachment "w-msi.txt" of type "text/plain" (191628 bytes)

View attachment "wo-msi.txt" of type "text/plain" (191628 bytes)