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

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Message-ID: <alpine.DEB.2.21.2209162317180.19473@angie.orcam.me.uk>
Date:   Sat, 17 Sep 2022 11:07:18 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
cc:     Josh Triplett <josh@...htriplett.org>,
        Anders Blomdell <anders.blomdell@...trol.lth.se>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: [PATCH 0/2] serial: 8250: Let drivers request full 16550A feature
 probing

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.

 Credit to Anders for reporting this issue and then working through the 
resolution.

  Maciej