From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id F24052EF9D5 for ; Fri, 25 Jul 2025 17:06:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753463163; cv=none; b=WV0xcpeqYDzOmBnQZJWZSV0l05oWLpgTjUxBsfSL1C6zMiao8Hfz1kmbdkX2TKIDn0/uimt7RFwuDVwz4EUVxRIvDfezaCaPVZC2QwcwgdSdXhKaiz+cUVhKD0+K1jlflu/3w7zVVbEuPQFCxI3ZI+fiSyGK+m69Le46pqDha34= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753463163; c=relaxed/simple; bh=u/bhy8xxIZ0ARS/nMRQKUMG+kwfHMUzFQ72YHJNJbHk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=j2NjTGkavsHFCONf7hbSyigJpGFdUfw4Z7m/Fh8d6tJl9Oo2cQKteUs/H5KHbb1A+h3tMtZRSzmxYDSdGQf+QnMn+bcUFXeKz0Qt76ocjTQOQ/lyOYZehTMj27VdL8ZP6iTsZQ093CvYwjOAPBfCRrZo8iZ9OtlK1rdvguil7wI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 556171A00; Fri, 25 Jul 2025 10:05:53 -0700 (PDT) Received: from [10.1.197.43] (eglon.cambridge.arm.com [10.1.197.43]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 17E913F66E; Fri, 25 Jul 2025 10:05:55 -0700 (PDT) Message-ID: <1422ff0e-6aa3-42de-ba12-4dff32e7505d@arm.com> Date: Fri, 25 Jul 2025 18:05:50 +0100 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: [RFC PATCH 06/36] ACPI / PPTT: Stop acpi_count_levels() expecting callers to clear levels To: Jonathan Cameron Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Rob Herring , Ben Horgan , Rohit Mathew , Shanker Donthineni , Zeng Heng , Lecopzer Chen , Carl Worth , shameerali.kolothum.thodi@huawei.com, D Scott Phillips OS , lcherian@marvell.com, bobo.shaobowang@huawei.com, tan.shaopeng@fujitsu.com, baolin.wang@linux.alibaba.com, Jamie Iles , Xin Hao , peternewman@google.com, dfustini@baylibre.com, amitsinght@marvell.com, David Hildenbrand , Rex Nie , Dave Martin , Koba Ko , Sudeep Holla References: <20250711183648.30766-1-james.morse@arm.com> <20250711183648.30766-7-james.morse@arm.com> <20250716165142.00002c46@huawei.com> Content-Language: en-GB From: James Morse In-Reply-To: <20250716165142.00002c46@huawei.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Jonathan, On 16/07/2025 16:51, Jonathan Cameron wrote: > On Fri, 11 Jul 2025 18:36:18 +0000 > James Morse wrote: > >> acpi_count_levels() passes the number of levels back via a pointer argument. >> It also passes this to acpi_find_cache_level() as the starting_level, and >> preserves this value as it walks up the cpu_node tree counting the levels. >> >> The only caller acpi_get_cache_info() happens to have already initialised >> levels to zero, which acpi_count_levels() depends on to get the correct >> result. >> >> Explicitly zero the levels variable, so the count always starts at zero. >> This saves any additional callers having to work out they need to do this. > This is all a bit fiddly as we now end up with that initialized in various > different places. I've debugged this one a few times, (turns out I'm forgetful) ... I figured doing this was better than adding a comment to warn others. As its static, I figured it was something the compiler can optimise out if there is a duplicate. (I couldn't find any initialisation I could remove because of this) > Perhaps simpler to have acpi_count_levels() return the > number of levels rather than void. Then return number of levels rather > than 0 on success from acpi_get_cache_info(). Negative error codes used > for failure just like now. > > That would leave only a local variable in acpi_count_levels being > initialized to 0 and passed to acpi_find_cache_level() before being > returned when the loop terminates. > > I think that sequence then makes it such that we can't fail to > initialize it at without the compiler noticing and screaming. > > Requires a few changes from if (ret) to if (ret < 0) at callers > of acpi_get_cache_info() but looks simple (says the person who > hasn't actually coded it!) Breaking the symmetry between levels and split_levels is an argument against this. I think within pptt.c this is fine, because 'level's is used internally as 'starting_level', and this expectation it was initialised to zero is a nasty surprise. But exposing that from acpi_get_cache_info() looks stranger - and would need to touch users in cacheinfo, arm64, riscv. I've updated acpi_count_levels() to look as you describe - that at least makes it harder to miss this in future. (not sure whether it saves anything) >> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c >> index 13619b1b821b..13ca2eee3b98 100644 >> --- a/drivers/acpi/pptt.c >> +++ b/drivers/acpi/pptt.c >> @@ -183,7 +183,7 @@ acpi_find_cache_level(struct acpi_table_header *table_hdr, >> * @cpu_node: processor node we wish to count caches for >> * @levels: Number of levels if success. >> * @split_levels: Number of split cache levels (data/instruction) if >> - * success. Can by NULL. >> + * success. Can be NULL. > > Grumpy reviewer hat. Unrelated cleanup up - good to have but not in this patch where > it's a distraction. I was hoping diff would keep it as one hunk. Happy to leave it tyopd! Thanks, James