Re: [Patch 01/12] Prepare the code for Hardware Breakpoint interfaces

K.Prasad wrote:
On Thu, May 21, 2009 at 09:16:38AM -0700, David Daney wrote:
K.Prasad wrote:
[...]
+struct hw_breakpoint {
+ void (*triggered)(struct hw_breakpoint *, struct pt_regs *);
+ struct arch_hw_breakpoint info;
+};
+/*
+ * len and type values are defined in include/asm/hw_breakpoint.h.
+ * Available values vary according to the architecture. On i386 the
+ * possibilities are:
+ *
+ * HW_BREAKPOINT_LEN_1
+ * HW_BREAKPOINT_LEN_2
+ * HW_BREAKPOINT_LEN_4
+ * HW_BREAKPOINT_RW
+ * HW_BREAKPOINT_READ
+ *
+ * On other architectures HW_BREAKPOINT_LEN_8 may be available, and the
+ * 1-, 2-, and 4-byte lengths may be unavailable. There also may be
+ * HW_BREAKPOINT_WRITE. You can use #ifdef to check at compile time.
+ */
+
I question weather having all these symbols for lengths is the proper approach.

On mips we would currently have:

HW_BREAKPOINT_LEN_8
HW_BREAKPOINT_LEN_16
HW_BREAKPOINT_LEN_32
HW_BREAKPOINT_LEN_64
HW_BREAKPOINT_LEN_128
HW_BREAKPOINT_LEN_256
HW_BREAKPOINT_LEN_512
HW_BREAKPOINT_LEN_1024
HW_BREAKPOINT_LEN_2048

If we were to use a debug agent hooked into the MIPS EJTAG debugger
support registers, 63 different even powers of 2 are valid lengths.

Determining the range of allowed breakpoint lengths, converting back
and forth between numeric values that are likely to be used in a
debugger, and these symbolic values that the proposed kernel interface
would use, could be a little ugly.

Have you thought about passing just the raw length? And perhaps
having:

HW_BREAKPOINT_LEN_MASK that would have a bit set for each log2 of a
legal length?


As explained to you here: http://lkml.org/lkml/2008/12/16/553/, defining
every possible length of the HW Breakpoint works for x86, but may not be
suitable for MIPS.


Sorry, I missed it the first time.

As you might have seen, the HW_BREAKPOINT_LEN_* values are defined in
x86-specific files and will be compared against 'len' field in
arch-specific 'struct arch_hw_breakpoint', for the reason that these
attributes are not valid for all architectures and have to be defined
for each processor in their own way.


But the comment mentioning all of this is in a generic non-architecture specific file. I do see where it says '...On i386...', so that does clarify it somewhat. Adding something similar to your following explanation ...:

Defining a HW_BREAKPOINT_LEN_MASK mask and validation of the input
length to check if it is a valid power of 2 can still be done for MIPS
in the arch-specific files and I don't see any part of the generic
interface being a hurdle during its implementation. Let me know if you
think there's any.

... might be worthwhile.

Thanks,
David Daney

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