Re: [PATCH v2 1/2] serial: 8250: Mask out floating 16/32-bit bus bits

From: Philippe Mathieu-Daudé
Date: Sat Jun 26 2021 - 12:43:12 EST

On Sat, Jun 26, 2021 at 6:11 AM Maciej W. Rozycki <macro@xxxxxxxxxxx> 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 ................
> 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://>
> 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@xxxxxxxxxxx>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@xxxxxxxxxxxxxxx # v2.6.12+
> ---
> Changes from v1:
> - Comments added as to truncation of bits above 7 required.
> ---
> drivers/tty/serial/8250/8250_port.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)

Reviewed-by: Philippe Mathieu-Daudé <f4bug@xxxxxxxxx>