* Re: [PATCH] powerpc/64s: Clarify copy_and_flush() cache sync loop comment
2026-07-01 18:27 [PATCH] powerpc/64s: Clarify copy_and_flush() cache sync loop comment Nikhil Kumar Singh
@ 2026-07-01 18:38 ` Aditya Gupta
2026-07-02 5:28 ` Mukesh Kumar Chaurasiya
2026-07-06 8:27 ` [PATCH v2] " Nikhil Kumar Singh
2 siblings, 0 replies; 5+ messages in thread
From: Aditya Gupta @ 2026-07-01 18:38 UTC (permalink / raw)
To: Nikhil Kumar Singh, linuxppc-dev
Cc: linux-kernel, maddy, mpe, npiggin, chleroy, mahesh
On 01/07/26 23:57, Nikhil Kumar Singh wrote:
> The value loaded into r0 in copy_and_flush() represents the number of
> 8-byte words processed between cache synchronization operations.
>
> The existing comment refers to cache line size, which can make it appear
> that the value is a cache line size in bytes rather than a loop count.
> Clarify the comment to explain that the loop processes 8 words (64 bytes)
> per cache synchronization iteration, and that increasing the value would
> skip cache maintenance for intermediate cache lines.
>
> This is a comment-only change with no functional impact.
>
> Signed-off-by: Nikhil Kumar Singh <nikhilks@linux.ibm.com>
> ---
> arch/powerpc/kernel/head_64.S | 16 ++++++++--------
> 1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
> index 63432a33ec49..e21c2bce8f7e 100644
> --- a/arch/powerpc/kernel/head_64.S
> +++ b/arch/powerpc/kernel/head_64.S
> @@ -713,14 +713,14 @@ p_end: .8byte _end - copy_to_here
> _GLOBAL(copy_and_flush)
> addi r5,r5,-8
> addi r6,r6,-8
> -4: li r0,8 /* Use the smallest common */
> - /* denominator cache line */
> - /* size. This results in */
> - /* extra cache line flushes */
> - /* but operation is correct. */
> - /* Can't get cache line size */
> - /* from NACA as it is being */
> - /* moved too. */
> +4: li r0,8 /* r0 is the number of 8-byte words */
> + /* to copy per cache sync iteration. */
> + /* 8 words * 8 bytes = 64 bytes. 64B is */
> + /* the current default cache line size. */
> + /* This is a loop count, not a byte */
> + /* count. Increasing it will skip */
> + /* dcbst/icbi for lines in between and */
> + /* leave stale instructions in icache. */
Looks good to me, previous comment stating cache line size may make it
look like
r0 is number of bytes being copied which isn't the case when the code is
read.
Hence:
Reviewed-by: Aditya Gupta <adityag@linux.ibm.com>
Thanks,
- Aditya G
>
> mtctr r0 /* put # words/line in ctr */
> 3: addi r6,r6,8 /* copy a cache line */
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] powerpc/64s: Clarify copy_and_flush() cache sync loop comment
2026-07-01 18:27 [PATCH] powerpc/64s: Clarify copy_and_flush() cache sync loop comment Nikhil Kumar Singh
2026-07-01 18:38 ` Aditya Gupta
@ 2026-07-02 5:28 ` Mukesh Kumar Chaurasiya
2026-07-06 8:27 ` [PATCH v2] " Nikhil Kumar Singh
2 siblings, 0 replies; 5+ messages in thread
From: Mukesh Kumar Chaurasiya @ 2026-07-02 5:28 UTC (permalink / raw)
To: Nikhil Kumar Singh
Cc: linuxppc-dev, linux-kernel, maddy, mpe, npiggin, chleroy, adityag,
mahesh
On Wed, Jul 01, 2026 at 11:57:56PM +0530, Nikhil Kumar Singh wrote:
> The value loaded into r0 in copy_and_flush() represents the number of
> 8-byte words processed between cache synchronization operations.
>
> The existing comment refers to cache line size, which can make it appear
> that the value is a cache line size in bytes rather than a loop count.
> Clarify the comment to explain that the loop processes 8 words (64 bytes)
> per cache synchronization iteration, and that increasing the value would
> skip cache maintenance for intermediate cache lines.
>
> This is a comment-only change with no functional impact.
>
> Signed-off-by: Nikhil Kumar Singh <nikhilks@linux.ibm.com>
> ---
> arch/powerpc/kernel/head_64.S | 16 ++++++++--------
> 1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
> index 63432a33ec49..e21c2bce8f7e 100644
> --- a/arch/powerpc/kernel/head_64.S
> +++ b/arch/powerpc/kernel/head_64.S
> @@ -713,14 +713,14 @@ p_end: .8byte _end - copy_to_here
> _GLOBAL(copy_and_flush)
> addi r5,r5,-8
> addi r6,r6,-8
> -4: li r0,8 /* Use the smallest common */
> - /* denominator cache line */
> - /* size. This results in */
> - /* extra cache line flushes */
> - /* but operation is correct. */
> - /* Can't get cache line size */
> - /* from NACA as it is being */
> - /* moved too. */
> +4: li r0,8 /* r0 is the number of 8-byte words */
> + /* to copy per cache sync iteration. */
> + /* 8 words * 8 bytes = 64 bytes. 64B is */
> + /* the current default cache line size. */
> + /* This is a loop count, not a byte */
> + /* count. Increasing it will skip */
> + /* dcbst/icbi for lines in between and */
> + /* leave stale instructions in icache. */
>
You dropped 2 pieces of information here, That we can't use NACA and
that we are doing an extra cache line flush(atleast in case of P10).
Also use tab instead of spaces before commment close per line "*/" can
you keep the last character aligned to rest of the file. It looks
little nicer.
Regards,
Mukesh
> mtctr r0 /* put # words/line in ctr */
> 3: addi r6,r6,8 /* copy a cache line */
> --
> 2.43.5
>
^ permalink raw reply [flat|nested] 5+ messages in thread
* [PATCH v2] powerpc/64s: Clarify copy_and_flush() cache sync loop comment
2026-07-01 18:27 [PATCH] powerpc/64s: Clarify copy_and_flush() cache sync loop comment Nikhil Kumar Singh
2026-07-01 18:38 ` Aditya Gupta
2026-07-02 5:28 ` Mukesh Kumar Chaurasiya
@ 2026-07-06 8:27 ` Nikhil Kumar Singh
2026-07-07 5:38 ` Mukesh Kumar Chaurasiya
2 siblings, 1 reply; 5+ messages in thread
From: Nikhil Kumar Singh @ 2026-07-06 8:27 UTC (permalink / raw)
To: linuxppc-dev, mkchauras
Cc: linux-kernel, maddy, mpe, npiggin, chleroy, adityag, mahesh,
Nikhil Kumar Singh
The value loaded into r0 in copy_and_flush() represents the number of
8-byte words processed between cache synchronization operations.
The existing comment refers to cache line size, which can make it appear
that the value is a cache line size in bytes rather than a loop count.
Clarify the comment to explain that the loop processes 8 words (64 bytes)
per cache synchronization iteration, and that increasing the value would
skip cache maintenance for intermediate cache lines.
This is a comment-only change with no functional impact.
Signed-off-by: Nikhil Kumar Singh <nikhilks@linux.ibm.com>
---
arch/powerpc/kernel/head_64.S | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 63432a33ec49..a54f6f979173 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -713,14 +713,18 @@ p_end: .8byte _end - copy_to_here
_GLOBAL(copy_and_flush)
addi r5,r5,-8
addi r6,r6,-8
-4: li r0,8 /* Use the smallest common */
- /* denominator cache line */
- /* size. This results in */
- /* extra cache line flushes */
- /* but operation is correct. */
- /* Can't get cache line size */
- /* from NACA as it is being */
- /* moved too. */
+4: li r0,8 /* r0 is the number of 8-byte words */
+ /* to copy per cache sync iteration. */
+ /* 8 words * 8 bytes = 64 bytes. 64B is */
+ /* the current default cache line size. */
+ /* This is a loop count, not a byte */
+ /* count. Increasing it may skip */
+ /* dcbst/icbi for lines in between and */
+ /* leave stale instructions in icache. */
+ /* This results in extra cache line */
+ /* flushes but operation is correct. */
+ /* Can't get cache line size from NACA */
+ /* as it is being moved too. */
mtctr r0 /* put # words/line in ctr */
3: addi r6,r6,8 /* copy a cache line */
--
2.43.5
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH v2] powerpc/64s: Clarify copy_and_flush() cache sync loop comment
2026-07-06 8:27 ` [PATCH v2] " Nikhil Kumar Singh
@ 2026-07-07 5:38 ` Mukesh Kumar Chaurasiya
0 siblings, 0 replies; 5+ messages in thread
From: Mukesh Kumar Chaurasiya @ 2026-07-07 5:38 UTC (permalink / raw)
To: Nikhil Kumar Singh
Cc: linuxppc-dev, linux-kernel, maddy, mpe, npiggin, chleroy, adityag,
mahesh
On Mon, Jul 06, 2026 at 01:57:08PM +0530, Nikhil Kumar Singh wrote:
> The value loaded into r0 in copy_and_flush() represents the number of
> 8-byte words processed between cache synchronization operations.
>
> The existing comment refers to cache line size, which can make it appear
> that the value is a cache line size in bytes rather than a loop count.
> Clarify the comment to explain that the loop processes 8 words (64 bytes)
> per cache synchronization iteration, and that increasing the value would
> skip cache maintenance for intermediate cache lines.
>
> This is a comment-only change with no functional impact.
>
> Signed-off-by: Nikhil Kumar Singh <nikhilks@linux.ibm.com>
> ---
> arch/powerpc/kernel/head_64.S | 20 ++++++++++++--------
> 1 file changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
> index 63432a33ec49..a54f6f979173 100644
> --- a/arch/powerpc/kernel/head_64.S
> +++ b/arch/powerpc/kernel/head_64.S
> @@ -713,14 +713,18 @@ p_end: .8byte _end - copy_to_here
> _GLOBAL(copy_and_flush)
> addi r5,r5,-8
> addi r6,r6,-8
> -4: li r0,8 /* Use the smallest common */
> - /* denominator cache line */
> - /* size. This results in */
> - /* extra cache line flushes */
> - /* but operation is correct. */
> - /* Can't get cache line size */
> - /* from NACA as it is being */
> - /* moved too. */
> +4: li r0,8 /* r0 is the number of 8-byte words */
> + /* to copy per cache sync iteration. */
> + /* 8 words * 8 bytes = 64 bytes. 64B is */
> + /* the current default cache line size. */
> + /* This is a loop count, not a byte */
> + /* count. Increasing it may skip */
> + /* dcbst/icbi for lines in between and */
> + /* leave stale instructions in icache. */
> + /* This results in extra cache line */
> + /* flushes but operation is correct. */
> + /* Can't get cache line size from NACA */
> + /* as it is being moved too. */
The comment looks fine. But the alignement of the comment is still
different from earlier. Keep the alignment same as earlier, it looks a
little nicer.
Regards,
Mukesh
>
> mtctr r0 /* put # words/line in ctr */
> 3: addi r6,r6,8 /* copy a cache line */
> --
> 2.43.5
>
^ permalink raw reply [flat|nested] 5+ messages in thread