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Wed, 1 Jul 2026 18:38:37 +0000 (GMT) Message-ID: <2baaed1b-38aa-4764-8249-34e688d747e2@linux.ibm.com> Date: Thu, 2 Jul 2026 00:08:36 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] powerpc/64s: Clarify copy_and_flush() cache sync loop comment To: Nikhil Kumar Singh , linuxppc-dev@lists.ozlabs.org Cc: linux-kernel@vger.kernel.org, maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com, chleroy@kernel.org, mahesh@linux.ibm.com References: <20260701182756.273019-1-nikhilks@linux.ibm.com> Content-Language: en-US From: Aditya Gupta In-Reply-To: <20260701182756.273019-1-nikhilks@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Authority-Analysis: v=2.4 cv=a4kAM0SF c=1 sm=1 tr=0 ts=6a455eb5 cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=IkcTkHD0fZMA:10 a=RAioF0-LDSMA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=VnNF1IyMAAAA:8 a=k39Bn-rZO4FkpeY6yEkA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-ORIG-GUID: RtZ3zGaBmHq6FEZveBFhBtPHAheOzE_u X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNzAxMDE5NiBTYWx0ZWRfX2tNYp9dnIP10 ETD40uCqS4MJoZx+YWUMX3P2p5S7S1xoZtr/AO7/COLt7rFUSl/LFSPd30BpzUBjG750ZLx0keB ViijkVIE2lP0w1swI3AHZp9uL7ml4MUSkDqmfH6NoRUgF5tVIe17u26qH5HcqN/xCpOInSzsEV9 2EIdutRuEQYf2QsZKGupRbBmK5zsxJ6Plx9XMoDYL2f0qt/9JhiRj9TgkhZtVXFlvjk183jM/hv vQj6HEwo1eGoV1XALrEW98DKHqDM/umUevL5IUvnjoy21UIHk/qkgHScdCCF2jsh13QRsboawkv ayvk3OlpfnEnM60OD5tfqcYphfBqsURykXlf8xAPujmz3pSk1rkSoANurljs50bznPyBZ560FKE wS6Jr3riVymMD5Bg7LjQpjtfBYXyQCPy5j+XVI6UZTKWbhNQz5WeTv3vkKCJ1IyB7IVDMZ/87bJ FpM3MrTmYayku029NBw== X-Proofpoint-GUID: g8IJoDVlQcspC1TxsofXWBIfrt22jHK_ X-Proofpoint-Spam-Info: AW1haW4tMjYwNzAxMDE5NiBTYWx0ZWRfX+/hH/lZQ72XJ TT+YyYYI5929m2Q4jnnpjP4WtIotxNlrW1A0PsMcb0zvAolT3pW55IulwqEgmbmFjxJm8770071 eMdxokOgSjjRhME2lSazBwd2cLOaA4g= X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.125,FMLib:17.12.100.49 definitions=2026-07-01_04,2026-06-26_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 bulkscore=0 suspectscore=0 lowpriorityscore=0 impostorscore=0 spamscore=0 priorityscore=1501 adultscore=0 malwarescore=0 phishscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2607010196 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 > --- > 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 Thanks, - Aditya G > > mtctr r0 /* put # words/line in ctr */ > 3: addi r6,r6,8 /* copy a cache line */