linux-kernel - Re: I/O page faults from 8250_mid PCIe UART after TIOCVHANGUP

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Message-ID: <YyhyvazTBBmMSnXk@smile.fi.intel.com>
Date:   Mon, 19 Sep 2022 16:46:37 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Lennert Buytenhek <buytenh@...tstofly.org>
Cc:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: I/O page faults from 8250_mid PCIe UART after TIOCVHANGUP

On Fri, Sep 16, 2022 at 02:47:08PM +0300, Lennert Buytenhek wrote:
> On Thu, Sep 15, 2022 at 07:27:45PM +0300, Ilpo Järvinen wrote:

...

> Thanks for the fix!
> 
> > [...] I'm far from sure if it's the 
> > best fix though as I don't fully understand what causes the faults during 
> > the THRE tests because the port->irq is disabled by the THRE test block.
> 
> If the IRQ hasn't been set up yet, the UART will have zeroes in its MSI
> address/data registers.  Disabling the IRQ at the interrupt controller
> won't stop the UART from performing a DMA write to the address programmed
> in its MSI address register (zero) when it wants to signal an interrupt.
> 
> (These UARTs (in Ice Lake-D) implement PCI 2.1 style MSI without masking
> capability, so there is no way to mask the interrupt at the source PCI
> function level, except disabling the MSI capability entirely, but that
> would cause it to fall back to INTx# assertion, and the PCI specification
> prohibits disabling the MSI capability as a way to mask a function's
> interrupt service request.)

This sounds to me like a good part to be injected into commit message of
the proposed fix.

-- 
With Best Regards,
Andy Shevchenko