Subject: [PATCH 2/3] [v4] ARM64: TTY: hvc_dcc: Add support for ARM64 dcc

From: [email-masked] (Will Deacon)
Date: Mon, 10 Aug 2015 10:40:08 +0100

On Sat, Aug 08, 2015 at 01:16:34AM +0100, Timur Tabi wrote:
> From: Abhimanyu Kapur <[email-masked]>
> 
> Add support for debug communications channel based
> hvc console for arm64 cpus.
> 
> Signed-off-by: Abhimanyu Kapur <[email-masked]>
> Signed-off-by: Timur Tabi <[email-masked]>
> ---
>  arch/arm64/include/asm/dcc.h | 52 ++++++++++++++++++++++++++++++++++++++++++++
>  drivers/tty/hvc/Kconfig      |  2 +-
>  2 files changed, 53 insertions(+), 1 deletion(-)
>  create mode 100644 arch/arm64/include/asm/dcc.h
> 
> diff --git a/arch/arm64/include/asm/dcc.h b/arch/arm64/include/asm/dcc.h
> new file mode 100644
> index 0000000..fcb8d7d
> --- /dev/null
> +++ b/arch/arm64/include/asm/dcc.h
> @@ -0,0 +1,52 @@
> +/* Copyright (c) 2014-2015 The Linux Foundation. All rights reserved.
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 and
> + * only version 2 as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * A call to __dcc_getchar() or __dcc_putchar() is typically followed by
> + * a call to __dcc_getstatus().  We want to make sure that the CPU does
> + * not speculative read the DCC status before executing the read or write
> + * instruction.  That's what the ISBs are for.
> + *
> + * The 'volatile' ensures that the compiler does not cache the status bits,
> + * and instead reads the DCC register every time.
> + */
> +#ifndef __ASM_DCC_H
> +#define __ASM_DCC_H
> +
> +#include <asm/barrier.h>
> +
> +static inline u32 __dcc_getstatus(void)
> +{
> +	u32 ret;
> +
> +	asm volatile("mrs %0, mdccsr_el0" : "=r" (ret));
> +
> +	return ret;
> +}
> +
> +static inline char __dcc_getchar(void)
> +{
> +	char c;
> +
> +	asm volatile("mrs %0, dbgdtrrx_el0" : "=r" (c));
> +	isb();
> +
> +	return c;
> +}
> +
> +static inline void __dcc_putchar(char c)
> +{
> +	asm volatile("msr dbgdtrtx_el0, %0"
> +			: /* No output register */
> +			: "r" (c));
> +	isb();

I think we should be masking out the upper bits of c before the msr
(the compiler probably expects a uxtb).

Other than that, this looks ok.

Will