linux-kernel - Re: [PATCH v3 2/2] ARC: Dynamically determine BASE

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Date:	Tue, 06 Jan 2015 07:29:26 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	gregkh@...uxfoundation.org
CC:	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	robh@...nel.org, Jiri Slaby <jslaby@...e.cz>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v3 2/2] ARC: Dynamically determine BASE_BAUD from DeviceTree

On 01/06/2015 06:29 AM, Vineet Gupta wrote:
> This resolves the BASE_BAUD issue in Multiplatform images

This commit log could be improved. Something like:

8250 earlycon is broken on multi-platform ARC because the UART clk
value (BASE_BAUD) is fixed at build time.

Instead, determine the appropriate UART clk at runtime; parse the
devicetree early for platforms requiring alternate UART clk values
(currently only the TB10X platform).

Regards,
Peter Hurley

> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Jiri Slaby <jslaby@...e.cz>
> Cc: linux-serial@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: Rob Herring <robh@...nel.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Peter Hurley <peter@...leysoftware.com>
> Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
> ---
>  arch/arc/include/asm/serial.h | 23 +++++------------------
>  arch/arc/kernel/devtree.c     | 24 ++++++++++++++++++++++++
>  2 files changed, 29 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/arc/include/asm/serial.h b/arch/arc/include/asm/serial.h
> index 602b0970a764..744a6ae15754 100644
> --- a/arch/arc/include/asm/serial.h
> +++ b/arch/arc/include/asm/serial.h
> @@ -10,26 +10,13 @@
>  #define _ASM_ARC_SERIAL_H
>  
>  /*
> - * early-8250 requires BASE_BAUD to be defined and includes this header.
> - * We put in a typical value:
> - * 	(core clk / 16) - i.e. UART samples 16 times per sec.
> - * Athough in multi-platform-image this might not work, specially if the
> - * clk driving the UART is different.
> - * We can't use DeviceTree as this is typically for early serial.
> + * early 8250 (now earlycon) requires BASE_BAUD to be defined in this header.
> + * However to still determine it dynamically (for multi-platform images)
> + * we do this in a helper by parsing the FDT early
>   */
>  
> -#include <asm/clk.h>
> +extern unsigned int __init arc_early_base_baud(void);
>  
> -#define BASE_BAUD	(arc_get_core_freq() / 16)
> -
> -/*
> - * This is definitely going to break early 8250 consoles on multi-platform
> - * images but hey, it won't add any code complexity for a debug feature of
> - * one broken driver.
> - */
> -#ifdef CONFIG_ARC_PLAT_TB10X
> -#undef BASE_BAUD
> -#define BASE_BAUD	(arc_get_core_freq() / 16 / 3)
> -#endif
> +#define BASE_BAUD	arc_early_base_baud()
>  
>  #endif /* _ASM_ARC_SERIAL_H */
> diff --git a/arch/arc/kernel/devtree.c b/arch/arc/kernel/devtree.c
> index fffdb5e41b20..69a790cd9b6e 100644
> --- a/arch/arc/kernel/devtree.c
> +++ b/arch/arc/kernel/devtree.c
> @@ -17,6 +17,28 @@
>  #include <asm/clk.h>
>  #include <asm/mach_desc.h>
>  
> +#ifdef CONFIG_SERIAL_8250_CONSOLE
> +
> +static unsigned int arc_base_baud;
> +
> +unsigned int __init arc_early_base_baud(void)
> +{
> +	return arc_base_baud/16;
> +}
> +
> +static void __init arc_set_early_base_baud(unsigned long dt_root)
> +{
> +	unsigned int core_clk = arc_get_core_freq();
> +
> +	if (of_flat_dt_is_compatible(dt_root, "abilis,arc-tb10x"))
> +		arc_base_baud = core_clk/3;
> +	else
> +		arc_base_baud = core_clk;
> +}
> +#else
> +#define arc_set_early_base_baud(dt_root)
> +#endif
> +
>  static const void * __init arch_get_next_mach(const char *const **match)
>  {
>  	static const struct machine_desc *mdesc = __arch_info_begin;
> @@ -56,5 +78,7 @@ const struct machine_desc * __init setup_machine_fdt(void *dt)
>  	if (clk)
>  		arc_set_core_freq(of_read_ulong(clk, len/4));
>  
> +	arc_set_early_base_baud(dt_root);
> +
>  	return mdesc;
>  }
> 

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