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SubjectRe: Exar ST16C2550 rev A2 bug
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Russell,

On Sun, 2004-11-28 at 09:26 -0700, Alex Williamson wrote:
> On Sun, 2004-11-28 at 11:10 +0000, Russell King wrote:
> > On Wed, Nov 17, 2004 at 11:26:47AM -0700, Alex Williamson wrote:
> >
> > Can you check whether this patch solves it? I'd rather not rely on
> > size_fifo() to do the right thing in every circumstance.
>
> I have a system on order with one of these bad UARTs. I'll be happy
> to check the patch when it arrives (hopefully this week).

No system yet, hopefully soon :^(

> > Essentially, if we find an EFR, we check whether the UART reports
> > DVID/DREV values corresponding to the known problem scenario, and
> > if so, we essentially ignore the EFR.
> >
> > What I don't know is whether these DVID/DREV values correspond to
> > a real device which does have an EFR. Maybe Exar people can shed
> > some light on this?
>
> I'll see if I can get an engineering contact from Exar to confirm.

Michael Knight from Exar is CC'd. Unfortunately it sounds like the
DVID/DREV registers are the same as another Exar product that does have
a valid EFR. So this would restrict functionality on that part as well.
Perhaps a hybrid test, something like:

static int broken_efr(struct uart_8250_port *p)
{
/*
* Exar ST16C2550 "A2" devices incorrectly detect as
* having an EFR, and report an ID of 0x0201. See
* http://www.exar.com/info.php?pdf=dan180_oct2004.pdf
*/
if (autoconfig_read_divisor_id(p) == 0x0201 && size_fifo(p) == 16)
return 1;

return 0;
}

...

if (!broken_efr(up)) {
autoconfig_has_efr(up);
return;
}
...

I think we know size_fifo() works on these parts, so we'd limit our
reliance on size_fifo(). If any more UARTs come alone with this issue
we could add them to the test. Thoughts? I can incorporate this into
your patch and test it when I get a system with these parts. Thanks,

Alex


> > ===== drivers/serial/8250.c 1.92 vs edited =====
> > --- 1.92/drivers/serial/8250.c 2004-11-19 07:03:10 +00:00
> > +++ edited/drivers/serial/8250.c 2004-11-28 11:02:32 +00:00
> > @@ -479,6 +479,34 @@
> > }
> >
> > /*
> > + * Read UART ID using the divisor method - set DLL and DLM to zero
> > + * and the revision will be in DLL and device type in DLM. We
> > + * preserve the device state across this.
> > + */
> > +static unsigned int autoconfig_read_divisor_id(struct uart_8250_port *p)
> > +{
> > + unsigned char old_dll, old_dlm, old_lcr;
> > + unsigned int id;
> > +
> > + old_lcr = serial_inp(p, UART_LCR);
> > + serial_outp(p, UART_LCR, UART_LCR_DLAB);
> > +
> > + old_dll = serial_inp(p, UART_DLL);
> > + old_dlm = serial_inp(p, UART_DLM);
> > +
> > + serial_outp(p, UART_DLL, 0);
> > + serial_outp(p, UART_DLM, 0);
> > +
> > + id = serial_inp(p, UART_DLL) | serial_inp(p, UART_DLM) << 8;
> > +
> > + serial_outp(p, UART_DLL, old_dll);
> > + serial_outp(p, UART_DLM, old_dlm);
> > + serial_outp(p, UART_LCR, old_lcr);
> > +
> > + return id;
> > +}
> > +
> > +/*
> > * This is a helper routine to autodetect StarTech/Exar/Oxsemi UART's.
> > * When this function is called we know it is at least a StarTech
> > * 16650 V2, but it might be one of several StarTech UARTs, or one of
> > @@ -490,7 +518,7 @@
> > */
> > static void autoconfig_has_efr(struct uart_8250_port *up)
> > {
> > - unsigned char id1, id2, id3, rev, saved_dll, saved_dlm;
> > + unsigned int id1, id2, id3, rev;
> >
> > /*
> > * Everything with an EFR has SLEEP
> > @@ -540,21 +568,13 @@
> > * 0x12 - XR16C2850.
> > * 0x14 - XR16C854.
> > */
> > - serial_outp(up, UART_LCR, UART_LCR_DLAB);
> > - saved_dll = serial_inp(up, UART_DLL);
> > - saved_dlm = serial_inp(up, UART_DLM);
> > - serial_outp(up, UART_DLL, 0);
> > - serial_outp(up, UART_DLM, 0);
> > - id2 = serial_inp(up, UART_DLL);
> > - id1 = serial_inp(up, UART_DLM);
> > - serial_outp(up, UART_DLL, saved_dll);
> > - serial_outp(up, UART_DLM, saved_dlm);
> > -
> > - DEBUG_AUTOCONF("850id=%02x:%02x ", id1, id2);
> > -
> > - if (id1 == 0x10 || id1 == 0x12 || id1 == 0x14) {
> > - if (id1 == 0x10)
> > - up->rev = id2;
> > + id1 = autoconfig_read_divisor_id(up);
> > + DEBUG_AUTOCONF("850id=%04x ", id1);
> > +
> > + id2 = id1 >> 8;
> > + if (id2 == 0x10 || id2 == 0x12 || id2 == 0x14) {
> > + if (id2 == 0x10)
> > + up->rev = id1 & 255;
> > up->port.type = PORT_16850;
> > return;
> > }
> > @@ -634,8 +654,16 @@
> > serial_outp(up, UART_LCR, 0xBF);
> > if (serial_in(up, UART_EFR) == 0) {
> > DEBUG_AUTOCONF("EFRv2 ");
> > - autoconfig_has_efr(up);
> > - return;
> > +
> > + /*
> > + * Exar ST16C2550 "A2" devices incorrectly detect as
> > + * having an EFR, and report an ID of 0x0201. See
> > + * http://www.exar.com/info.php?pdf=dan180_oct2004.pdf
> > + */
> > + if (autoconfig_read_divisor_id(up) != 0x0201) {
> > + autoconfig_has_efr(up);
> > + return;
> > + }
> > }
> >
> > /*
> >
>
--
Alex Williamson HP Linux & Open Source Lab

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