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 1C73A27AC3D; Fri, 5 Sep 2025 16:24:32 +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=1757089474; cv=none; b=IwPtbG2Vxl/fKNoaO4clM0zGLB9d5eUmoLQCE/IUxvzVkpBYbXjt7KBv4WennMPIx36ABDrrVDfQnzeZKSDPxuJH6rZuoBmbARbMhXkjOcmCR0cWWnXeqnFfSzQ0EVse+FIgQxWyemfM3iB62tSUhkJQ2jOMWxl30y/UwS4LwKc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757089474; c=relaxed/simple; bh=WvIHFzUl3Av8I2RQ/GFgqZySs7WZ+xyp12KqrfwTLy0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=DCvuPebblciM8Sgjj6jWBVzwT+y5c11PXWztBBtfuzfE2HCS0UWhtv0NAK2KAZtee3CbIAcigXf6vNyztUsVj9HA46LrpqzfS8+OBSDJLslR66wMULqs2+R/I9RVoCIdTHXvHe3F6/iBt4CY1fSyUO0+slvWZDH+0da84u+WrJo= 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 DCB75152B; Fri, 5 Sep 2025 09:24:23 -0700 (PDT) Received: from e133380.arm.com (e133380.arm.com [10.1.197.68]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 873AA3F63F; Fri, 5 Sep 2025 09:24:26 -0700 (PDT) Date: Fri, 5 Sep 2025 17:24:23 +0100 From: Dave Martin To: James Morse Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-acpi@vger.kernel.org, devicetree@vger.kernel.org, D Scott Phillips OS , carl@os.amperecomputing.com, 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 , Koba Ko , Shanker Donthineni , fenghuay@nvidia.com, baisheng.gao@unisoc.com, Jonathan Cameron , Rob Herring , Rohit Mathew , Rafael Wysocki , Len Brown , Lorenzo Pieralisi , Hanjun Guo , Sudeep Holla , Krzysztof Kozlowski , Conor Dooley , Catalin Marinas , Will Deacon , Greg Kroah-Hartman , Danilo Krummrich Subject: Re: [PATCH 03/33] ACPI / PPTT: Add a helper to fill a cpumask from a processor container Message-ID: References: <20250822153048.2287-1-james.morse@arm.com> <20250822153048.2287-4-james.morse@arm.com> <29d0a34d-71d3-42ac-ba66-b5536f576f3a@arm.com> Precedence: bulk X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <29d0a34d-71d3-42ac-ba66-b5536f576f3a@arm.com> Hi James, On Thu, Aug 28, 2025 at 04:57:06PM +0100, James Morse wrote: > Hi Dave, > > On 27/08/2025 11:48, Dave Martin wrote: > > On Fri, Aug 22, 2025 at 03:29:44PM +0000, James Morse wrote: > >> The PPTT describes CPUs and caches, as well as processor containers. > >> The ACPI table for MPAM describes the set of CPUs that can access an MSC > >> with the UID of a processor container. > >> > >> Add a helper to find the processor container by its id, then walk > >> the possible CPUs to fill a cpumask with the CPUs that have this > >> processor container as a parent. > > > Nit: The motivation for the change is not clear here. > > > > I guess this boils down to the need to map the MSC topology information > > in the the ACPI MPAM table to a cpumask for each MSC. > > > > If so, a possible rearrangement and rewording might be, say: > > > > --8<-- > > > > The ACPI MPAM table uses the UID of a processor container specified in > > the PPTT, to indicate the subset of CPUs and upstream cache topology > > that can access each MPAM Memory System Component (MSC). > > > > This information is not directly useful to the kernel. The equivalent > > cpumask is needed instead. > > > > Add a helper to find the processor container by its id, then [...] > > > > -->8-- > > Thanks, that is clearer! Thanks > >> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c [...] > >> @@ -298,6 +298,92 @@ static struct acpi_pptt_processor *acpi_find_processor_node(struct acpi_table_he [...] > >> +static void acpi_pptt_get_child_cpus(struct acpi_table_header *table_hdr, > >> + struct acpi_pptt_processor *parent_node, > >> + cpumask_t *cpus) > >> +{ > >> + struct acpi_pptt_processor *cpu_node; > >> + u32 acpi_id; > >> + int cpu; > >> + > >> + cpumask_clear(cpus); > >> + > >> + for_each_possible_cpu(cpu) { > >> + acpi_id = get_acpi_id_for_cpu(cpu); > > > ^ Presumably this can't fail? > > It'll return something! This could only be a problem if this raced with a CPU becoming > impossible, and there is no mechanism to do that. Yep, now I go and look more closely at that function, my question looks misguided. [...] > >> +void acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id, cpumask_t *cpus) > >> +{ > >> + struct acpi_pptt_processor *cpu_node; > >> + struct acpi_table_header *table_hdr; > >> + struct acpi_subtable_header *entry; > >> + unsigned long table_end; > >> + acpi_status status; > >> + bool leaf_flag; > >> + u32 proc_sz; > >> + > >> + cpumask_clear(cpus); > >> + > >> + status = acpi_get_table(ACPI_SIG_PPTT, 0, &table_hdr); > >> + if (ACPI_FAILURE(status)) > >> + return; > > > Is acpi_get_pptt() applicable here? > > Oh, that is new, and would let me chuck the reference counting. > I guess this replaces Jonthan's magic table free'ing cleanup thing! Ah, rightho. > > (That function is not thread-safe, but then, perhaps most/all of these > > functions are not thread safe. If we are still on the boot CPU at this > > point (?) then this wouldn't be a concern.) > > I think that relies on the first caller being from somewhere that can't race. > In this case its the architecture's smp_prepare_cpus() call to setup the acpi topology. > That is sufficiently early its not a concern. I guess so. [...] > >> + cpu_node = (struct acpi_pptt_processor *)entry; > >> + if (entry->type == ACPI_PPTT_TYPE_PROCESSOR && > >> + cpu_node->flags & ACPI_PPTT_ACPI_PROCESSOR_ID_VALID) { > >> + leaf_flag = acpi_pptt_leaf_node(table_hdr, cpu_node); > >> + if (!leaf_flag) { > >> + if (cpu_node->acpi_processor_id == acpi_cpu_id) > > > > Is there any need to distinguish processor containers from (leaf) CPU > > nodes, here? If not, dropping the distinction might simplify the code > > here (even if callers do not care). > > In the namespace the object types are different, so I assumed they have their own UID > space. The PPTT holds both - hence the check for which kind of thing it is. The risk is > looking for processor-container-4 and finding CPU-4 instead... > > The relevant ACPI bit is "8.4.2.1 Processor Container Device", its says: > | A processor container declaration must supply a _UID method returning an ID that is > | unique in the processor container hierarchy. > > Which doesn't quite let me combine them here. I was going by the PPTT spec, where the types are not distinct -- you're probably right, though. According to that, isn't it the "ACPI Processor ID valid" flag, not the "Node is a Leaf" flag, that says whether this field is meaningful? It's reasonable not to bother to try to enumerate the children of a node that claims to be a leaf (even if there actually are children), but I wonder what happens if acpi_processor_id is not declared to be valid and matches by accident. That's probably not a valid table (?) but does anything bad happen on the kernel side? > > Otherwise, maybe eliminate leaf_flag and collapse these into a single > > if(), as suggested by Ben [1]. > > > >> + acpi_pptt_get_child_cpus(table_hdr, cpu_node, cpus); > > > > Can there ever be multiple matches? > > > > The possibility of duplicate processor IDs in the PPTT sounds weird to > > me, but then I'm not an ACPI expert. > > Multiple processor-containers with the same ID? That would be a corrupt table. > acpi_pptt_get_child_cpus() then walks the tree again to find the CPUs below this > processor-container - those have a different kind of id. Does anything bad happen if we encounter duplicates? (Other then the MPAM driver never getting enabled, or not working as advertised, that is.) I haven't tried to think through all the implications, here. > > If there can only be a single match, though, then we may as well break > > out of the loop here, unless we want to be paranoid and report > > duplicates as an error -- but that would require extra implementation, > > so I'm not sure that would be worth it. > > Hmmm, the PPTT node should map to only one processor or processor-container. > I'll chuck the break in. Ack Cheers ---Dave