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

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Message-ID: <CAHp75VdoaE7hCOzsRvuf=7A4mmv0NWBmwqK_mM8vO-K3YZKTUQ@mail.gmail.com>
Date:   Wed, 14 Jul 2021 16:35:33 +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 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?

> 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?

-- 
With Best Regards,
Andy Shevchenko