linux-kernel - Re: [PATCH 1/2] USB: serial: ch341: fix lost character on LCR updates

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Message-ID: <Yw8fQz2amdKKYNvS@hovoldconsulting.com>
Date:   Wed, 31 Aug 2022 10:43:47 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     Jonathan Woithe <jwoithe@...t42.net>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] USB: serial: ch341: fix lost character on LCR updates

On Wed, Aug 31, 2022 at 10:36:18AM +0200, Jiri Slaby wrote:
> On 31. 08. 22, 10:15, Johan Hovold wrote:
> > Disable LCR updates for pre-0x30 devices which use a different (unknown)
> > protocol for line control and where the current register write causes
> > the next received character to be lost.
> > 
> > Note that updating LCR using the INIT command has no effect on these
> > devices either.
> > 
> > Reported-by: Jonathan Woithe <jwoithe@...t42.net>
> > Link: https://lore.kernel.org/r/Ys1iPTfiZRWj2gXs@marvin.atrad.com.au
> > Fixes: 4e46c410e050 ("USB: serial: ch341: reinitialize chip on reconfiguration")
> > Fixes: 55fa15b5987d ("USB: serial: ch341: fix baud rate and line-control handling")
> > Cc: stable@...r.kernel.org      # 4.10
> > Signed-off-by: Johan Hovold <johan@...nel.org>
> > ---
> >   drivers/usb/serial/ch341.c | 10 +++++++++-
> >   1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
> > index 2798fca71261..2bcce172355b 100644
> > --- a/drivers/usb/serial/ch341.c
> > +++ b/drivers/usb/serial/ch341.c
> > @@ -97,7 +97,10 @@ struct ch341_private {
> >   	u8 mcr;
> >   	u8 msr;
> >   	u8 lcr;
> > +
> >   	unsigned long quirks;
> > +	u8 version;
> 
> Could you move version above quirks? That would not create another 
> 7-byte padding in here. Actually it would not make ch341_private larger 
> on 64bit at all, if I am looking correctly.

No, I added it after quirks on purpose as it isn't protected by the
spinlock and doesn't change during runtime like the shadow registers.

And I really don't care about saving 8 bytes on 64-bit. :)

Johan