Re: [PATCH] arm64: Add support to pass earlyprintk argument via device tree

[Grant Likely <grant.likely@secretlab.ca>]

On Mon,  3 Jun 2013 21:21:11 +0530, Pranavkumar Sawargaonkar <pranavkumar@linaro.org> wrote:
> This patch adds support for defining and passing earlyprintk
> related information i.e. device and address information via
> device tree by adding it inside "chosen" node.
> 
> This will help user to just specify "earlyprintk" from bootargs
> without actually knowing the address and device to enable
> earlyprintk.
> 
> Mechanism:
> 
> One can just append earlyprintk=device-type,address (same as we pass
> through command line) in "/chosen" node to notify kernel which is the
> earlyprintk device and what is its address.
> 
> Backward Compatibility:
> 
> This patch also allows existing method of specifying earlyprintk
> parameter via bootargs.
> 
> Existing method i.e. passing via bootargs will still have precedence
> over device tree i.e. if one specifies earlyprintk=device-type,address
> in bootargs then kernel will use information from bootargs instead of
> device tree.
> 
> If user just specifies earlyprintk (without =...) then kernel will
> look for device tree earlyprintk parameter.
> 
> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
> Signed-off-by: Anup Patel <anup.patel@linaro.org>

I'm not a big fan of this. It seems to be short-circuiting around
existing properties. The kernel /should/ be able to use the
linux,stdout-path property to determine what the earlyprintk device to
use is.

g.

> ---
>  arch/arm64/kernel/early_printk.c |    7 +++++++
>  arch/arm64/kernel/setup.c        |   21 ++++++++++++++++++++-
>  2 files changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/early_printk.c b/arch/arm64/kernel/early_printk.c
> index fbb6e18..4e6f845 100644
> --- a/arch/arm64/kernel/early_printk.c
> +++ b/arch/arm64/kernel/early_printk.c
> @@ -29,6 +29,8 @@
>  static void __iomem *early_base;
>  static void (*printch)(char ch);
>  
> +extern char *earlyprintk_dt_args;
> +
>  /*
>   * PL011 single character TX.
>   */
> @@ -116,6 +118,11 @@ static int __init setup_early_printk(char *buf)
>  	phys_addr_t paddr = 0;
>  
>  	if (!buf) {
> +		/* Try to check if Device Tree has this argument or not ? */
> +		buf = earlyprintk_dt_args;
> +	}
> +
> +	if (!buf) {
>  		pr_warning("No earlyprintk arguments passed.\n");
>  		return 0;
>  	}
> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index 6a9a532..fb2d56f 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -60,6 +60,8 @@ EXPORT_SYMBOL(processor_id);
>  unsigned int elf_hwcap __read_mostly;
>  EXPORT_SYMBOL_GPL(elf_hwcap);
>  
> +char *earlyprintk_dt_args;
> +
>  static const char *cpu_name;
>  static const char *machine_name;
>  phys_addr_t __fdt_pointer __initdata;
> @@ -122,6 +124,23 @@ static void __init setup_processor(void)
>  	elf_hwcap = 0;
>  }
>  
> +int __init early_init_dt_scan_chosen_arm64(unsigned long node,
> +					   const char *uname,
> +					   int depth, void *data)
> +{
> +	char *prop;
> +
> +	/* Check if this is chosen node */
> +	if (early_init_dt_scan_chosen(node, uname, depth, data) == 0)
> +		return 0;
> +
> +	prop = of_get_flat_dt_prop(node, "earlyprintk", NULL);
> +	if (prop)
> +		earlyprintk_dt_args = prop;
> +
> +	return 1;
> +}
> +
>  static void __init setup_machine_fdt(phys_addr_t dt_phys)
>  {
>  	struct boot_param_header *devtree;
> @@ -165,7 +184,7 @@ static void __init setup_machine_fdt(phys_addr_t dt_phys)
>  	pr_info("Machine: %s\n", machine_name);
>  
>  	/* Retrieve various information from the /chosen node */
> -	of_scan_flat_dt(early_init_dt_scan_chosen, boot_command_line);
> +	of_scan_flat_dt(early_init_dt_scan_chosen_arm64, boot_command_line);
>  	/* Initialize {size,address}-cells info */
>  	of_scan_flat_dt(early_init_dt_scan_root, NULL);
>  	/* Setup memory, calling early_init_dt_add_memory_arch */
> -- 
> 1.7.9.5
> 
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-- 
Grant Likely, B.Sc, P.Eng.
Secret Lab Technologies, Ltd.