linux-kernel - Re: [PATCH 1/2] serial: 8250: Mask out floating 16/32-bit bus bits

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Date:   Fri, 11 Jun 2021 19:07:09 +0200
From:   Philippe Mathieu-Daudé <f4bug@...at.org>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] serial: 8250: Mask out floating 16/32-bit bus bits

On Thu, Jun 10, 2021 at 8:39 PM Maciej W. Rozycki <macro@...am.me.uk> wrote:
>
> Make sure only actual 8 bits of the IIR register are used in determining
> the port type in `autoconfig'.
>
> The `serial_in' port accessor returns the `unsigned int' type, meaning
> that with UPIO_AU, UPIO_MEM16, UPIO_MEM32, and UPIO_MEM32BE access types
> more than 8 bits of data are returned, of which the high order bits will
> often come from bus lines that are left floating in the data phase.  For
> example with the MIPS Malta board's CBUS UART, where the registers are
> aligned on 8-byte boundaries and which uses 32-bit accesses, data as
> follows is returned:
>
> YAMON> dump -32 0xbf000900 0x40
>
> BF000900: 1F000942 1F000942 1F000900 1F000900  ...B...B........
> BF000910: 1F000901 1F000901 1F000900 1F000900  ................
> BF000920: 1F000900 1F000900 1F000960 1F000960  ...........`...`
> BF000930: 1F000900 1F000900 1F0009FF 1F0009FF  ................
>
> YAMON>
>
> Evidently high-order 24 bits return values previously driven in the
> address phase (the 3 highest order address bits used with the command
> above are masked out in the simple virtual address mapping used here and
> come out at zeros on the external bus), a common scenario with bus lines
> left floating, due to bus capacitance.
>
> Consequently when the value of IIR, mapped at 0x1f000910, is retrieved
> in `autoconfig', it comes out at 0x1f0009c1 and when it is right-shifted
> by 6 and then assigned to 8-bit `scratch' variable, the value calculated
> is 0x27, not one of 0, 1, 2, 3 expected in port type determination.
>
> Fix the issue then, by assigning the value returned from `serial_in' to
> `scratch' first, which masks out 24 high-order bits retrieved, and only
> then right-shift the resulting 8-bit data quantity, producing the value
> of 3 in this case, as expected.  Fix the same issue in `serial_dl_read'.
>
> The problem first appeared with Linux 2.6.9-rc3 which predates our repo
> history, but the origin could be identified with the old MIPS/Linux repo
> also at: <git://git.kernel.org/pub/scm/linux/kernel/git/ralf/linux.git>
> as commit e0d2356c0777 ("Merge with Linux 2.6.9-rc3."), where code in
> `serial_in' was updated with this case:
>
> +       case UPIO_MEM32:
> +               return readl(up->port.membase + offset);
> +
>
> which made it produce results outside the unsigned 8-bit range for the
> first time, though obviously it is system dependent what actual values
> appear in the high order bits retrieved and it may well have been zeros
> in the relevant positions with the system the change originally was
> intended for.  It is at that point that code in `autoconf' should have
> been updated accordingly, but clearly it was overlooked.
>
> Signed-off-by: Maciej W. Rozycki <macro@...am.me.uk>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> ---
>  drivers/tty/serial/8250/8250_port.c |    9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)

Reviewed-by: Philippe Mathieu-Daudé <f4bug@...at.org>