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Subject: Re: [EXT] Re: [PATCH v1 3/4] serial: 8250_pci: Always try MSI/MSI-X
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
CC:     Jiri Slaby <jirislaby@kernel.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        <linux-serial@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
References: <20210713104026.58560-1-andriy.shevchenko@linux.intel.com>
 <20210713104026.58560-3-andriy.shevchenko@linux.intel.com>
 <9af24b96-8119-7ccf-f0d0-d725af80aa0b@kernel.org>
 <784629f9-677e-ee53-aceb-89397ce0951a@oth-regensburg.de>
 <CAHp75VdoaE7hCOzsRvuf=7A4mmv0NWBmwqK_mM8vO-K3YZKTUQ@mail.gmail.com>
From:   Ralf Ramsauer <ralf.ramsauer@oth-regensburg.de>
Message-ID: <0a15be25-d4e1-f2f5-22f7-6144b7100bf1@oth-regensburg.de>
Date:   Wed, 14 Jul 2021 18:49:44 +0200
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On 14/07/2021 15:35, Andy Shevchenko wrote:
> On Wed, Jul 14, 2021 at 3:56 PM Ralf Ramsauer
> <ralf.ramsauer@oth-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?

That's been a while. Let me check that if I can still find them, and
I'll test them once again against MSI being enabled. But this can take
some days.

  Ralf

> 
>> Don't do that… And don't convert it to a blacklist. A blacklist will
>> break users until they report that something doesn't work.
> 
> White list is not okay either. MSI in general is a right thing to do.
> preventing users from MSI is asking for the performance degradation
> and IRQ resource conflicts (in case the IRQ line is shared).
> 
> Besides that, shouldn't it be rather the specific field in private (to
> 8250_pci) structure than constantly growing list?