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

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Message-ID: <CAHp75VcLicxAz5BjP+Lp2yHxEGiKcT9OUZbPeRUgZVcYLdY0FA@mail.gmail.com>
Date:   Fri, 16 Jul 2021 18:01:42 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Ralf Ramsauer <ralf.ramsauer@...-regensburg.de>
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 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)

> 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…

-- 
With Best Regards,
Andy Shevchenko