linux-kernel - Re: [PATCH 0/2] serial: 8250: Let drivers request full 16550A feature probing

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Message-ID: <0B189972-4FD8-4245-BF2F-ADEAB18AAAE0@joshtriplett.org>
Date:   Sat, 17 Sep 2022 16:00:59 +0100
From:   Josh Triplett <josh@...htriplett.org>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
CC:     Anders Blomdell <anders.blomdell@...trol.lth.se>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 0/2] serial: 8250: Let drivers request full 16550A feature probing

On September 17, 2022 11:07:18 AM GMT+01:00, "Maciej W. Rozycki" <macro@...am.me.uk> wrote:
>Hi,
>
> A recent change has added a SERIAL_8250_16550A_VARIANTS option, which 
>lets one request the 8250 driver not to probe for 16550A device features 
>so as to reduce the driver's device startup time in virtual machines.  
>This has turned out problematic to a more recent update for the OxSemi 
>Tornado series PCIe devices, whose new baud rate generator handling code 
>actually requires switching hardware into the enhanced mode for correct 
>operation, which actually requires 16550A device features to have been 
>probed for.
>
> This small patch series fixes the issue by letting individual device 
>subdrivers to request full 16550A device feature probing by means of a 
>flag regardless of the SERIAL_8250_16550A_VARIANTS setting chosen.
>
> The changes have been verified with an OXPCIe952 device, in the native 
>UART mode and a 64-bit RISC-V system as well as in the legacy UART mode 
>and a 32-bit x86 system.

Seems reasonable to me, as long as the flag is only set by drivers that know they've found their hardware.