Re: [PATCH] tty: 8250: handle USR for DesignWare 8250 with correct accessors

[Ralf Baechle <ralf@linux-mips.org>]

On Thu, Jun 09, 2011 at 11:48:45AM +0100, Jamie Iles wrote:

> Don't pass a pointer to the USR register through the private_data field
> of the platform data.  This isn't type safe and it's not clear what is
> happening.  Add the USR offset to serial_reg.h and use an explicit
> serial_in() to read it with the correct accessor.
> 
> Fix up the only in-tree user to not pass anything through private_data.
> 
> Cc: Ralf Baechle <ralf@linux-mips.org>
> Cc: Greg Kroah-Hartman <gregkh@suse.de>
> Signed-off-by: Jamie Iles <jamie@jamieiles.com>
> ---
>  arch/mips/pmc-sierra/msp71xx/msp_serial.c |    1 -
>  drivers/tty/serial/8250.c                 |    3 +--
>  include/linux/serial_reg.h                |    3 +++
>  3 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/mips/pmc-sierra/msp71xx/msp_serial.c b/arch/mips/pmc-sierra/msp71xx/msp_serial.c
> index f726162..5ccfdcc 100644
> --- a/arch/mips/pmc-sierra/msp71xx/msp_serial.c
> +++ b/arch/mips/pmc-sierra/msp71xx/msp_serial.c
> @@ -63,7 +63,6 @@ void __init msp_serial_setup(void)
>  	up.flags        = ASYNC_BOOT_AUTOCONF | ASYNC_SKIP_TEST;
>  	up.type         = PORT_16550A;
>  	up.line         = 0;
> -	up.private_data		= (void*)UART0_STATUS_REG;
>  	if (early_serial_setup(&up))
>  		printk(KERN_ERR "Early serial init of port 0 failed\n");
>  
> diff --git a/drivers/tty/serial/8250.c b/drivers/tty/serial/8250.c
> index b40f7b9..0596caa 100644
> --- a/drivers/tty/serial/8250.c
> +++ b/drivers/tty/serial/8250.c
> @@ -1666,8 +1666,7 @@ static irqreturn_t serial8250_interrupt(int irq, void *dev_id)
>  			 * interrupt meaning an LCR write attempt occurred while the
>  			 * UART was busy. The interrupt must be cleared by reading
>  			 * the UART status register (USR) and the LCR re-written. */
> -			unsigned int status;
> -			status = *(volatile u32 *)up->port.private_data;
> +			(void)serial_in(up, UART_DWAPB_USR);
>  			serial_out(up, UART_LCR, up->lcr);
>  
>  			handled = 1;
> diff --git a/include/linux/serial_reg.h b/include/linux/serial_reg.h
> index c75bda3..abfd8ea 100644
> --- a/include/linux/serial_reg.h
> +++ b/include/linux/serial_reg.h
> @@ -350,6 +350,9 @@
>  #define UART_OMAP_SYSS		0x16	/* System status register */
>  #define UART_OMAP_WER		0x17	/* Wake-up enable register */
>  
> +/* Extra serial register definitions for the Synopsys DesignWare UART. */
> +#define UART_DWAPB_USR		0x1F	/* UART status register */
> +
>  /*
>   * These are the definitions for the MDR1 register
>   */

NAck.

The original read access was for a read access at offset 0xc0 from the
base address.  Your patch changes this to offset 0x1f * 4 = 0x7c.

If you look at arch/mips/include/asm/pmc-sierra/msp71xx/msp_regs.h there's

#define MSP_UART0_BASE          (MSP_SLP_BASE + 0x100)
                                        /* UART0 controller base        */
#define MSP_BCPY_CTRL_BASE      (MSP_SLP_BASE + 0x120)
                                        /* Block Copy controller base   */

So there are just 0x20 of address space reserved for that UART.  Me thinks
that PMC-Sierra clamped the 256 byte address space of the DesignWare APB
UART to what is standard for 16550 class UARTs, 8 registers which at a
shift of 4 is 0x20 bytes and the status register being accesses is really
something else.  I'd guess PMC-Sierra just remapped the register to
another address.

A more proper cleanup would probably be passing something like

struct serial_private {
	void (*dw_abp_int_callback)(void);
	unsigned long private;
};

then in the 8250 interrupt handler something like:

                } else if ((up->port.iotype == UPIO_DWAPB ||
                            up->port.iotype == UPIO_DWAPB32) &&
                          (iir & UART_IIR_BUSY) == UART_IIR_BUSY) {
			struct serial_private *sp;

			sp = (void *) up->port.private_data;
			if (sp->dw_abp_int_callback)
				sp->dw_abp_int_callback(sp->private);
                        serial_out(up, UART_LCR, up->lcr);

                        handled = 1;

                        end = NULL;
		}

On a 2nd thought I wonder if the restricted address space of the PMC-Sierra
variant and the strange remapping would justify treating it as a subvariant
of the DW APB UART, rename it to UPIO_PMC_MSP_DWAPB, hardcode the access to
the remapped status register.  And get rid of the unused UPIO_DWAPB32 ...

I've cced a few people who should know more about this.

  Ralf