Re: [PATCH v4 09/10] powerpc/watchpoint: Return available watchpoints dynamically

On Tue, Jul 21, 2020 at 1:57 PM Ravi Bangoria
[off-list ref] wrote:

On 7/20/20 9:12 AM, Jordan Niethe wrote:

quoted

On Fri, Jul 17, 2020 at 2:11 PM Ravi Bangoria
[off-list ref] wrote:

quoted

So far Book3S Powerpc supported only one watchpoint. Power10 is
introducing 2nd DAWR. Enable 2nd DAWR support for Power10.
Availability of 2nd DAWR will depend on CPU_FTR_DAWR1.

Signed-off-by: Ravi Bangoria <redacted>
---
  arch/powerpc/include/asm/cputable.h      | 4 +++-
  arch/powerpc/include/asm/hw_breakpoint.h | 5 +++--
  2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/cputable.h b/arch/powerpc/include/asm/cputable.h
index 3445c86e1f6f..36a0851a7a9b 100644
--- a/arch/powerpc/include/asm/cputable.h
+++ b/arch/powerpc/include/asm/cputable.h

@@ -633,7 +633,9 @@ enum {
   * Maximum number of hw breakpoint supported on powerpc. Number of
   * breakpoints supported by actual hw might be less than this.
   */
-#define HBP_NUM_MAX    1
+#define HBP_NUM_MAX    2
+#define HBP_NUM_ONE    1
+#define HBP_NUM_TWO    2

I wonder if these defines are necessary - has it any advantage over
just using the literal?

No, not really. Initially I had something like:

#define HBP_NUM_MAX    2
#define HBP_NUM_P8_P9  1
#define HBP_NUM_P10    2

But then I thought it's also not right. So I made it _ONE and _TWO.
Now the function that decides nr watchpoints dynamically (nr_wp_slots)
is in different file, I thought to keep it like this so it would be
easier to figure out why _MAX is 2.

quoted

quoted

  #endif /* !__ASSEMBLY__ */

diff --git a/arch/powerpc/include/asm/hw_breakpoint.h b/arch/powerpc/include/asm/hw_breakpoint.h
index cb424799da0d..d4eab1694bcd 100644
--- a/arch/powerpc/include/asm/hw_breakpoint.h
+++ b/arch/powerpc/include/asm/hw_breakpoint.h

@@ -5,10 +5,11 @@
   * Copyright 2010, IBM Corporation.
   * Author: K.Prasad <prasad@linux.vnet.ibm.com>
   */
-

Was removing this line deliberate?

Nah. Will remove that hunk.

quoted

quoted

  #ifndef _PPC_BOOK3S_64_HW_BREAKPOINT_H
  #define _PPC_BOOK3S_64_HW_BREAKPOINT_H

+#include <asm/cpu_has_feature.h>
+
  #ifdef __KERNEL__
  struct arch_hw_breakpoint {
         unsigned long   address;

@@ -46,7 +47,7 @@ struct arch_hw_breakpoint {

  static inline int nr_wp_slots(void)
  {
-       return HBP_NUM_MAX;
+       return cpu_has_feature(CPU_FTR_DAWR1) ? HBP_NUM_TWO : HBP_NUM_ONE;

So it'd be something like:
+       return cpu_has_feature(CPU_FTR_DAWR1) ? HBP_NUM_MAX : 1;
But thinking that there might be more slots added in the future, it
may be better to make the number of slots a variable that is set
during the init and then have this function return that.

Not sure I follow. What do you mean by setting number of slots a
variable that is set during the init?

Sorry I was unclear there.
I was just looking and saw arm also has a variable number of hw breakpoints.
If we did something like how they handle it, it might look something like:

static int num_wp_slots __ro_after_init;

int nr_wp_slots(void) {
    return num_wp_slots;
}

static int __init arch_hw_breakpoint_init(void) {
    num_wp_slots = work out how many wp_slots
}
arch_initcall(arch_hw_breakpoint_init);

Then we wouldn't have to calculate everytime nr_wp_slots() is called.
In the future if more wp's are added nr_wp_slots() will get more complicated.
But just an idea, feel free to ignore.

Thanks,
Ravi