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Date:   Sat, 17 Sep 2022 16:00:59 +0100
From:   Josh Triplett <josh@joshtriplett.org>
To:     "Maciej W. Rozycki" <macro@orcam.me.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>
CC:     Anders Blomdell <anders.blomdell@control.lth.se>,
        linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Subject: =?US-ASCII?Q?Re=3A_=5BPATCH_0/2=5D_serial=3A_8250=3A_Let_driv?= =?US-ASCII?Q?ers_request_full_16550A_feature_probing?=
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References: <alpine.DEB.2.21.2209162317180.19473@angie.orcam.me.uk>
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On September 17, 2022 11:07:18 AM GMT+01:00, "Maciej W=2E Rozycki" <macro@o=
rcam=2Eme=2Euk> wrote:
>Hi,
>
> A recent change has added a SERIAL_8250_16550A_VARIANTS option, which=20
>lets one request the 8250 driver not to probe for 16550A device features=
=20
>so as to reduce the driver's device startup time in virtual machines=2E =
=20
>This has turned out problematic to a more recent update for the OxSemi=20
>Tornado series PCIe devices, whose new baud rate generator handling code=
=20
>actually requires switching hardware into the enhanced mode for correct=
=20
>operation, which actually requires 16550A device features to have been=20
>probed for=2E
>
> This small patch series fixes the issue by letting individual device=20
>subdrivers to request full 16550A device feature probing by means of a=20
>flag regardless of the SERIAL_8250_16550A_VARIANTS setting chosen=2E
>
> The changes have been verified with an OXPCIe952 device, in the native=
=20
>UART mode and a 64-bit RISC-V system as well as in the legacy UART mode=
=20
>and a 32-bit x86 system=2E

Seems reasonable to me, as long as the flag is only set by drivers that kn=
ow they've found their hardware=2E