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Date: Tue, 16 Apr 2024 19:40:40 +0000
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
From: Michael Pratt <mcpratt@pm.me>
Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Jiri Slaby <jirislaby@kernel.org>, Wander Lairson Costa <wander@redhat.com>, =?utf-8?Q?Ilpo_J=C3=A4rvinen?= <ilpo.jarvinen@linux.intel.com>, Vamshi Gajjela <vamshigajjela@google.com>
Subject: Re: [PATCH v2 2/3] serial: 8250: Store whether fifo device is enabled
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Hi Andy,

On Tuesday, April 16th, 2024 at 15:18, Andy Shevchenko <andriy.shevchenko@l=
inux.intel.com> wrote:

>=20
>=20
> On Tue, Apr 16, 2024 at 07:09:52PM +0000, Michael Pratt wrote:
>=20
> > On Tuesday, April 16th, 2024 at 14:55, Andy Shevchenko andriy.shevchenk=
o@linux.intel.com wrote:
>=20
> > > > @@ -3392,6 +3392,8 @@ void serial8250_console_write(struct uart_825=
0_port *up, const char *s,
> > >=20
> > > > + up->fifo_enable =3D use_fifo;
> > >=20
> > > This seems incorrect / not the only one place to assign this. What if=
 the
> > > console not enabled at compile time? What if it's not enabled at boot=
 time?
> >=20
> > This is 8250 specific, and currently, it's the only place there
> > where it's decided whether or not to use the fifo device
> > by checking a bunch of flags and values.
>=20
>=20
> Exactly, as initial commit is related to the kernel console only.
> While your code, IIUC (correct me, if I'm wrong) is for any use of the po=
rt.
>=20
> > If you're suggesting that these checks are moved out of this function s=
omewhere else,
> > I would probably agree with that, but let's save that idea for the futu=
re...
>=20
>=20
> Not really (again, IIUC above), as console can be not enabled, and hence
> serial8250_console_write() never been called and you will have false impr=
ession
> that there is no FIFO in use.

Ah ok, I understand now...

So there are cases where the function with the checks will never be called,
yet the device itself will be configured the same way and the struct member=
 I am adding
will still be instantiated with value of 0 and never be set elsewhere... an=
d because
it is declared in a major struct "uart_8250_port", it appears to apply to a=
 larger scope
compared to the way it is actually being used...
(or at the very least, the name "fifo_enable" would be misleading).

Thanks for pointing that out, I'll take a deeper dive into the file...


>=20
> --
> With Best Regards,
> Andy Shevchenko

--
MCP