Hardware Flow Control on 16C950/954 (kernel 2.6.11)

[Ben Papps <ben.papps@oxsemi.com>]

Hi all,

I have found an issue concerning hardware flow control on the 16C950/954 
(specifically I am working on the OX16PCI954/952 devices) in Linux 2.6 
(Specifically the 2.6.11 kernel source (Fedora Core 3)). I have been 
looking at how I can get this small fix made in the kernel, and thought 
that I'd post the issue here first to see what other people think about it.

The short version...: In file 8250.c, in uart_config[], lines 223-229.

[PORT_16C950] = {
.name = “16C950/954”,
.fifo_size = 128,
.tx_loadsz = 128,
.fcr = UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_10,
.flags = UART_CAP_FIFO,
}

They should be*:*

[PORT_16C950] = {
.name = “16C950/954”,
.fifo_size = 128,
.tx_loadsz = 128,
.fcr = UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_10,
.flags = UART_CAP_FIFO *| UART_CAP_EFR | UART_CAP_SLEEP*,
}

These flags allow the enhanced register to be used, allowing flow 
control amongst other things.

The long version... also in 8250.c, considering the OX16PCI954 device.
The autoconfig routine calls the autoconfig_16550a routine, in which we 
find that the C95x chip has an EFR register. The autoconfig_16550a 
routine then calls the autoconfig_has_efr routine where up->capabilities 
is correctly set with UART_CAP_EFR and UART_CAP_SLEEP. On recognising 
the Oxford Semiconductor 16C950 devise autoconfig_has_efr returns to 
autoconfig_16550a which in turn returns to autoconfig..

 From line 879 to 888 in autoconfig the driver the checks the 
up->capabilities against the devices information in uart_config[]. This 
of course doesn't match, as shown above in the short version, and 
up->capabilities is reset with the values that are in uart_config[].

Without the EFR settings the set_termios routine is unable to assert the 
hardware flow control feature of the C950 uart.

The 16C954/950 entry in uart_config[] needs to have the UART_CAP_EFR and 
UART_CAP_SLEEP flags added to fix this bug.

I would appreciate any feedback on this.

Thanks,

Ben Papps


 
_____________________________
 
Ben Papps

Software Design Engineer
Oxford Semiconductor Ltd
25 Milton Park, Oxfordshire, OX14 4SH
 
Tel: 01235 824887


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