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Re: [PATCH] serial driver PMC MSP71xx, kernel linux-mips.git mast er

To: Marc St-Jean <>
Subject: Re: [PATCH] serial driver PMC MSP71xx, kernel linux-mips.git mast er
From: Sergei Shtylyov <>
Date: Mon, 22 Jan 2007 23:23:56 +0300
In-reply-to: <>
Organization: MontaVista Software Inc.
Original-recipient: rfc822;
References: <>
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Marc St-Jean wrote:

   Your maile is strange: it line-wraps quotes but not your own text... :-/

Here is a serial driver patch for the PMC-Sierra MSP71xx device.

There are three different fixes:
1. Fix for THRE errata
2. Fix for Busy Detect on LCR write
3. Workaround for interrupt/data concurrency issue

The first fix is handled cleanly using a UART_BUG_* flag.
   Hm, I wouldn't call it clean...

Relative to the other two. Any recommended improvements on this one?

   I already told: runtime check.


Signed-off-by: Marc St-Jean <>

Index: linux_2_6/drivers/serial/8250.c =================================================================== RCS file: linux_2_6/drivers/serial/8250.c,v retrieving revision retrieving revision 1.9 diff -u -r1.1.1.7 -r1.9 --- linux_2_6/drivers/serial/8250.c 19 Oct 2006 21:00:58


+++ linux_2_6/drivers/serial/8250.c 19 Oct 2006 22:08:15

-0000   1.9

@@ -44,6 +44,10 @@
 #include <asm/io.h>
 #include <asm/irq.h>

+#include <msp_regs.h>
 #include "8250.h"

@@ -130,6 +134,9 @@
        unsigned char           mcr_mask;       /* mask of user bits */
        unsigned char           mcr_force;      /* mask of forced bits */

        unsigned char           lsr_break_flag;
+       int                                     dwapb_lcr;      

/* saved LCR for DW APB WAR */


   There was already 'lcr' field there, couldn't it be used?

Possibly but the current driver doesn't always save the LCR value before trying 
to write it. For example see serial8250_set_termios()
        serial_outp(up, UART_LCR, cval);                /* reset DLAB */
        up->lcr = cval;                                      /* Save LCR */

   This certainly may be fixed.

It's impossible to ensure that going foward the driver will always save the 
value before writing it.
We need to have it saved before since the write will cause an interrupt, hence 
the save in serial_out.

You may rework driver to always save it on write to 'lcr' field (in case of your UART). I don't think it's going to spoil something there...

If that's not acceptable I can audit the driver and ensure all LCR writes are 
first saved.

   What's *certainly* undesirable is #ifdef mess. :-)

@@ -333,6 +340,10 @@
 static void
 serial_out(struct uart_8250_port *up, int offset, int value)
+       /* Save the offset before it's remapped */
+       int dwapb_offset = offset;
        offset = map_8250_out_reg(up, offset) << up->port.regshift;

        switch (up->port.iotype) {
@@ -342,7 +353,19 @@

        case UPIO_MEM:
+               /* Save the LCR value so it can be re-written when a
+                * Busy Detect interrupt occurs. */
+               if (dwapb_offset == UART_LCR)
+                       up->dwapb_lcr = value;
                writeb(value, up->port.membase + offset);
+               /* Re-read the IER to ensure any interrupt disabling has
+                * completed before proceeding with ISR. */
+               if (dwapb_offset == UART_IER)
+                       value = serial_in(up, dwapb_offset); #endif

   Hm, was there really a need for #ifdef mess here?
I'd vote for introducing new UPIO_* here, like was done for TSi10x UARTs just for the same reason.

I'm willing to do this, however IIRC there was a thread in late Sept. which came
to the conclusion that UPIO were to be used for different access methods  and 
not UART types.

AFAIR, almost every participant was left with his own opinion. I myself changed it twice during the discussion, to finalyl mostly agree with Russell. :-D

Do you see this as an exception?

Well, there's already an incident of using this to work around the register access errata (that UPIO_TSI I mentioned) -- and I must note that this is certainly cleaner than #ifdef's, which should be avoided at all costs. :-)

In the second #ifdef CONFIG_PMC_MSP above the workaround is required because of 
> interrupt design which is out-of-band with respect to r/w of UART registers, it's not
specific to the DesignWare UART.

   Erm... so why is this under #ifdef?

-1141,6 +1175,12 @@
                        iir = serial_in(up, UART_IIR);
if (lsr & UART_LSR_TEMT && iir &


+               } else if (up->bugs & UART_BUG_DWTHRE) {
+                       unsigned char lsr, iir;
+                       lsr = serial_in(up, UART_LSR);
+                       iir = serial_in(up, UART_IIR);
+                       if (lsr & UART_LSR_THRE)
+                               transmit_chars(up);

   I don't see how this *really* differs from the UART_BUG_TXEN case.
Have you tried *that* workaround?

I didn't write the code so I haven't tried it personally but I believe it was 

It looks like the results of the bugs are similar but the causes are different. 
In the UART_BUG_TXEN case,
if I understand the comment correctly, NO interrupt is generated on enabling 
This is verified by looking for Transmitter Empty (0x40) and no interrupt.

I must note that "transmitter empty" condition in *not* the cause of the THRI interrupt -- it signifies that the TX shift register is empty, not the TX holding register.

In the UART_BUG_DWTHRE case an interrupt IS generated on enabling THRI.

   If there was no prior sent characters, how it's generated?

> However no new THRE interupts will occur until a new character is written to the THR and it's sent.
This is verified by looking for Transmit-Hold-Register Empty (0x20).

Again, THRE condition doesn't mean that the charecter is actually sent. I only means that it's been stored to the shift register and so, the transmission started...

        In any case, looks like this errata is auto-detectable just like 

I don't know how we would be able to test this without sending junk test 
to the console port.

   IIUC, you must *not* send characters to detect it. :-)

We currently enable the bug flag in our platform setup code when we know the 
bug is present from the SoC version.
Is that not acceptable?

Well, if that errata is indeed not readily detectable at runtime (or even effectively the same as already handled),
this seems like the only option indeed...

@@ -1366,6 +1406,31 @@
                        handled = 1;

                        end = NULL;
+               } else if ((iir & UART_IER_BUSY) == UART_IER_BUSY) {

   Hm, masking IIR with IER mask, is this correct? Doubt it.

I agree, that was badly named. I've changed it to UART_IIR_BUSY for the next 

+                       /*
+                        * The MSP (DesignWare APB UART) serial subsystem has a
+                        * non-standard interrupt condition (0x7) which means
+                        * that the LCR was written while the UART was busy, so
+                        * the LCR was not actually written.  It is cleared by
+                        * reading the special non-standard extended UART status
+                        * register.
+                        */
+                       unsigned int tmp;
+                       if( up->port.line == 0 )
+                               tmp = *UART0_STATUS_REG;
+                       else
+                               tmp = *UART1_STATUS_REG;
+                       /* Check if saved on LCR write */
+                       if( up->dwapb_lcr != -1 )
+                               serial_outp(up, UART_LCR, up->dwapb_lcr);
+                       else
+                               printk(KERN_ERR "serial8250: UART BUSY, no LCR 
write!\n" );
+                       handled = 1;
+                       end = NULL;

Not sure if this also shouldn't be handled in other places which check for interrupt status, like serial8250_timeout()...

I can move the code to a function but I still see two issues:
A) We could move the code the a new function. We could also check for a new 
UART_BUG_* flag before calling the function,
but in reality this is a feature of the UART not a bug.

   May also check for certain UPIO_* once it's introduced...

B) How to eliminate the platform #ifdef. How to pass in the address of the 
UARTx_STATUS_REG in a platform independent way?

I'd consider defining the UART status reg. in serial_reg.h and also doing reads via serial_in(),
again under a new UPIO_* case...

We could add it to one of the data structures like uart_port, but it would need 
an #ifdef in the header file.

   Add what, register?

Index: linux_2_6/include/linux/serial_reg.h
RCS file: linux_2_6/include/linux/serial_reg.h,v
retrieving revision
retrieving revision 1.3
diff -u -r1.1.1.2 -r1.3
--- linux_2_6/include/linux/serial_reg.h        19 Oct 2006 18:29:50 -0000
+++ linux_2_6/include/linux/serial_reg.h        19 Oct 2006 19:45:04 -0000      
@@ -218,6 +218,10 @@
 #define UART_FCR_PXAR16        0x80    /* receive FIFO treshold = 16 */
 #define UART_FCR_PXAR32        0xc0    /* receive FIFO treshold = 32 */

+ * DesignWare APB UART
+ */
+#define UART_IER_BUSY          0x07    /* Busy Detect */

Are you sure it's not *IIR* value? Doesn't look like interrupt mask for IER. And IIR value of 7 already means something else, namely, no interrupt and receiver status. Hm...

As mentioned earlier I have now renamed this UART_IIR_BUSY.

From serial_reg.h, the receiver status is 0x06 and no interrupt is 0x01. I 
thought these couldn't both be set at the same time?
In any case this is how DesignWare implemented the status so we can't change it 

Well, bits 1-2 have no meaning when bit 0 is set. Probably, chip designers decided to abuse this. :-)

Thanks for the feedback,

MBR, Sergei

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