[Skiboot] [PATCH] cpu: supply ibm,dec-bits via devicetree

[Vaidyanathan Srinivasan]

* Oliver O'Halloran <oohall at gmail.com> [2016-06-27 15:07:43]:

> ISAv3 adds a mode to increase the size of the decrementer from 32 bits.
> The enlarged decrementer can be between 32 and 64 bits wide with the
> exact value being implementation dependent. This patch adds support for
> detecting the size of the large decrementer and populating each CPU node
> with the "ibm,dec-bits" property.
> 
> Signed-off-by: Oliver O'Halloran <oohall at gmail.com>

Reviewed-by: Vaidyanathan Srinivasan <svaidy at linux.vnet.ibm.com>

> ---
>  core/cpu.c          | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/processor.h |  1 +
>  2 files changed, 49 insertions(+)
> 
> diff --git a/core/cpu.c b/core/cpu.c
> index ffc264f90907..32b70e0c3713 100644
> --- a/core/cpu.c
> +++ b/core/cpu.c
> @@ -524,10 +524,55 @@ void init_boot_cpu(void)
>  	list_head_init(&global_job_queue);
>  }
>  
> +static void enable_ld(bool on)
> +{
> +	u64 lpcr = mfspr(SPR_LPCR);
> +
> +	if (on)
> +		lpcr |= SPR_LPCR_P9_LD;
> +	else
> +		lpcr &= ~SPR_LPCR_P9_LD;
> +
> +	mtspr(SPR_LPCR, lpcr);
> +}

How about expanding 'ld' in the function name and also maybe have
enable/disable for better readability instead of taking a parameter.

static inline void enable_long_dec()
static inline void disable_long_dec()

> +
> +#define HIGH_BIT (1ull << 63)
> +
> +static int find_dec_bits(void)
> +{
> +	int bits = 65; /* we always decrement once */
> +	u64 mask = ~0ull;
> +
> +	if (proc_gen < proc_gen_p9)
> +		return 32;
> +
> +	/* The ISA doesn't specify the width of the decrementer register so we
> +	 * need to discover it. When in large mode (LPCR.LD = 1) reads from the
> +	 * DEC SPR are sign extended to 64 bits and writes are truncated to the
> +	 * physical register width. We can use this behaviour to detect the
> +	 * width by starting from an all 1s value and left shifting until we
> +	 * read a value from the DEC with it's high bit cleared.
> +	 */
> +
> +	enable_ld(true);
> +
> +	do {
> +		bits--;
> +		mask = mask >> 1;
> +		mtspr(SPR_DEC, mask);
> +	} while (mfspr(SPR_DEC) & HIGH_BIT);
> +
> +	enable_ld(false);
> +
> +	prlog(PR_DEBUG, "CPU: decrementer bits %d\n", bits);
> +	return bits;

Number of bits supported by the decrementer is one more than the
number of bits set in the mask at the time when sign extension stops
and HIGH_BIT is not set.  Will this be true always?

Also the decrementer would count down from the mask value before we
compare, but that will not toggle the higher order bits and the loop
will work correctly :)


> +}
> +
>  void init_all_cpus(void)
>  {
>  	struct dt_node *cpus, *cpu;
>  	unsigned int thread, new_max_pir = 0;
> +	int dec_bits = find_dec_bits();

You already test function of large decrementer and only then export
ibm,dec-bits > 32.

You leave LD disabled in LPCR and let the Linux kernel check and
enable.

Why do we have to test the number of bits again in kernel?  Do you
have a reason to believe that test/check in skiboot is not sufficient?

You have check_large_decrementer() in kernel that will again validate
the number of bits that you have just discovered.

  
>  	cpus = dt_find_by_path(dt_root, "/cpus");
>  	assert(cpus);
> @@ -582,6 +627,9 @@ void init_all_cpus(void)
>  		/* Add associativity properties */
>  		add_core_associativity(t);
>  
> +		/* Add the decrementer width property */
> +		dt_add_property_cells(cpu, "ibm,dec-bits", dec_bits);
> +
>  		/* Adjust max PIR */
>  		if (new_max_pir < (pir + cpu_thread_count - 1))
>  			new_max_pir = pir + cpu_thread_count - 1;
> diff --git a/include/processor.h b/include/processor.h
> index 1236c779aac8..48bbf903df58 100644
> --- a/include/processor.h
> +++ b/include/processor.h
> @@ -96,6 +96,7 @@
>  #define SPR_LPCR_P8_PECE2	PPC_BIT(49)   /* Wake on external interrupts */
>  #define SPR_LPCR_P8_PECE3	PPC_BIT(50)   /* Wake on decrementer */
>  #define SPR_LPCR_P8_PECE4	PPC_BIT(51)   /* Wake on MCs, HMIs, etc... */
> +#define SPR_LPCR_P9_LD		PPC_BIT(46)   /* Large decrementer mode bit */
>  
>  
>  /* Bits in TFMR - control bits */
> -- 
> 2.5.5
> 
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