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 EB53F44605E; Wed, 29 Jul 2026 11:53:41 +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=1785326024; cv=none; b=kGOoNi0bc3b2lS1Si+64Y9G3yGuVK4YOfBb+Z99HHLRUm5O/lcfC+4ANmb64DjrpHXmnFHtVWXZayRs6ovWBHCRGxOGotPRJoWsB8rhiaRaN+J3B+omcXRiqnamY9MEf2nViS2uCs4iCPtYcQeeuVPRaa2j0er5Ud7cemj5sBRA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785326024; c=relaxed/simple; bh=/RDp9rxsJQEOfIipnO7msV7vkcBVlVpqDy3nZBDHhvg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YBQNZxNQVwp6FQvr3kn+alDJO+giMXwWJZv+a2Gu3MBRq4rSPRXITH7ZwubdsHR+Owe7E+HmphJ+puLZbF+CZPw2e3hJ14Je0Ved/pE2Qjd8qRJpYZatUcxsUOdLcp1Qpg3X66NatkiGTd+3kRLd1lyi2kL8OnWTzBLZKw+aJnQ= 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; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=t+/4zVBV; 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 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="t+/4zVBV" 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 C25F21688; Wed, 29 Jul 2026 04:53:36 -0700 (PDT) Received: from [192.168.178.24] (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3B41B3F86F; Wed, 29 Jul 2026 04:53:37 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1785326020; bh=/RDp9rxsJQEOfIipnO7msV7vkcBVlVpqDy3nZBDHhvg=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=t+/4zVBVxO/NbyfabjrcVpPdppift/tgVo9jfeQMflVzg4VXrYGEf8DLzclawgYSG 81m0m2g+1nJGzeZUD9Hj7gH+l0ku1le3imZBg0VbcuXGZbVGzx96Rf/wa5c+ImNZAM hhB+Ke/O3+Xc2mlHwyfLS01LiIZDDrE1b87Q1X8I= Message-ID: <0813a57c-85ed-45b7-8cd1-933d99b31d8f@arm.com> Date: Wed, 29 Jul 2026 13:53:36 +0200 Precedence: bulk X-Mailing-List: linux-acpi@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 10/10] arm_mpam: detect and enable MPAM-Fb PCC support To: Srivathsa L Rao , Lorenzo Pieralisi , Hanjun Guo , Sudeep Holla , Catalin Marinas , Will Deacon , "Rafael J . Wysocki" , Len Brown , James Morse , Ben Horgan , Reinette Chatre , Fenghua Yu Cc: Jonathan Cameron , Ganapatrao Kulkarni , Trilok Soni , Srinivas Ramana , Niyas Sait , Lee Trager , linux-acpi@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20260723155454.1760823-1-andre.przywara@arm.com> <20260723155454.1760823-11-andre.przywara@arm.com> <4faf0089-4615-4fb1-bc07-e52b4d43f073@oss.qualcomm.com> Content-Language: en-GB From: Andre Przywara In-Reply-To: <4faf0089-4615-4fb1-bc07-e52b4d43f073@oss.qualcomm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Hi Srivathsa, On 7/27/26 18:27, Srivathsa L Rao wrote: > > > On 7/23/2026 9:24 PM, Andre Przywara wrote: >> The Arm MPAM-Fb specification [1] describes a protocol to access MSC >> registers through a firmware interface. This requires a shared memory >> region to hold the message, and a mailbox to trigger the access. >> For ACPI this is wrapped as a PCC channel, described using existing >> ACPI abstractions. >> >> Add code to parse those PCC table descriptions associated with an MSC, >> and store the parsed information in the MSC struct. >> There can be multiple PCC channels, and each channel can serve multiple >> MSCs, so we need to keep track of the channel usage, using a list and >> a refcount. >> This will be used by the MPAM-Fb access wrapper code. >> >> [1] https://developer.arm.com/documentation/den0144/latest >> >> Signed-off-by: Andre Przywara >> --- >>   drivers/acpi/arm64/mpam.c      |   6 +- >>   drivers/resctrl/mpam_devices.c | 127 ++++++++++++++++++++++++++++++++- >>   2 files changed, 129 insertions(+), 4 deletions(-) >> >> diff --git a/drivers/acpi/arm64/mpam.c b/drivers/acpi/arm64/mpam.c >> index 84963a20c3e7..ca9b8754ae5f 100644 >> --- a/drivers/acpi/arm64/mpam.c >> +++ b/drivers/acpi/arm64/mpam.c >> @@ -220,8 +220,8 @@ static struct platform_device * __init >> acpi_mpam_parse_msc(struct acpi_mpam_msc_ >>       struct platform_device *pdev __free(platform_device_put) = >>           platform_device_alloc("mpam_msc", tbl_msc->identifier); >>       int next_res = 0, next_prop = 0, err; >> -    /* pcc, nrdy, affinity and a sentinel */ >> -    struct property_entry props[4] = { 0 }; >> +    /* pcc, msc-id, nrdy, affinity and a sentinel */ >> +    struct property_entry props[5] = { 0 }; >>       /* mmio, 2xirq, no sentinel. */ >>       struct resource res[3] = { 0 }; >>       struct acpi_device *companion; >> @@ -256,6 +256,8 @@ static struct platform_device * __init >> acpi_mpam_parse_msc(struct acpi_mpam_msc_ >>       } else if (iface == MPAM_IFACE_PCC) { >>           props[next_prop++] = PROPERTY_ENTRY_U32("pcc-channel", >>                               tbl_msc->base_address); >> +        props[next_prop++] = PROPERTY_ENTRY_U32("msc-id", >> +                            tbl_msc->identifier); >>       } >>       acpi_mpam_parse_irqs(pdev, tbl_msc, res, &next_res); >> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/ >> mpam_devices.c >> index fea3161ffbe9..fc4b5f87fbe9 100644 >> --- a/drivers/resctrl/mpam_devices.c >> +++ b/drivers/resctrl/mpam_devices.c >> @@ -19,14 +19,19 @@ >>   #include >>   #include >>   #include >> +#include >>   #include >>   #include >>   #include >> +#include >>   #include >>   #include >>   #include >>   #include >> +#include >> +#include >> + >>   #include "mpam_internal.h" >>   /* Values for the T241 errata workaround */ >> @@ -49,6 +54,86 @@ static LIST_HEAD(mpam_all_msc); >>   struct srcu_struct mpam_srcu; >> +/* PCC channels might be serving multiple MSCs, so keep a refcounted >> list. */ >> +static DEFINE_MUTEX(pcc_chan_list_lock); >> +static LIST_HEAD(pcc_chan_list); >> + >> +static void mpam_pcc_chan_release(struct kref *ref) >> +{ >> +    struct mpam_pcc_chan *cur = container_of(ref, struct mpam_pcc_chan, >> +                         refcount); >> + >> +    pcc_mbox_free_channel(cur->pcc_chan); >> +    list_del(&cur->pcc_chans); >> +    kfree(cur); >> +} >> + >> +static struct mpam_pcc_chan *mpam_pcc_chan_get(struct device *dev, >> +                           int subspace_id) >> +{ >> +    struct mpam_pcc_chan *cur; >> +    int ret; >> + >> +    guard(mutex)(&pcc_chan_list_lock); >> + >> +    list_for_each_entry(cur, &pcc_chan_list, pcc_chans) { >> +        if (cur->subspace_id == subspace_id) { >> +            kref_get(&cur->refcount); >> + >> +            return cur; >> +        } >> +    } >> + >> +    cur = kzalloc_obj(*cur); >> +    if (!cur) >> +        return ERR_PTR(-ENOMEM); >> + >> +    cur->pcc_cl.dev = dev; >> +    cur->pcc_cl.tx_block = true; >> + >> +    cur->pcc_chan = pcc_mbox_request_channel(&cur->pcc_cl, subspace_id); >> +    if (IS_ERR(cur->pcc_chan)) { >> +        long err = PTR_ERR(cur->pcc_chan); >> + >> +        kfree(cur); >> +        return ERR_PTR(err); >> +    } >> + >> +    /* Timeout based on the "nominal latency" from the PCC ACPI >> table. */ >> +    cur->pcc_cl.tx_tout = cur->pcc_chan->latency * 5; >> + >> +    ret = devm_mutex_init(dev, &cur->pcc_chan_lock); >> +    if (ret) >> +        return ERR_PTR(ret); >> + > > Here, devm_mutex_init() ties the lifetime of pcc_chan_lock to dev, which > is &pdev->dev of the first MSC to call mpam_pcc_chan_get(). But > mpam_pcc_chan is shared across multiple MSCs via kref, so it can > outlive that first device. Ah, that's a good point, thanks for catching that. It was too tempting to just replace the original mutex_init() with the devm_ version, but indeed the lifetime is not the same. > If a second MSC has incremented the refcount and is still active when > the first MSC's device is removed, the devm cleanup on the first device > would call mutex_destroy() on pcc_chan_lock while the second MSC may > still be inside: > >     guard(mutex)(&pcc_chan->pcc_chan_lock); > > I am not 100% sure this is reachable in practice — it may depend on > the order in which platform devices are unbound, and whether that can > happen with MSCs sharing a channel. I also note that devm_mutex_init() > is a no-op on non-debug kernels, so this would only be observable with > CONFIG_DEBUG_MUTEXES=y. But it felt worth raising in case > it is a real path. > > If my understanding is correct, would it make sense to use a plain > mutex_init() here instead, and move mutex_destroy() into > mpam_pcc_chan_release() where the kref guarantees no concurrent users > remain? Yes, that's a good idea, since we free the data structure in the release function anyway, we can surely destroy the mutex there. And thanks for the heads up about this being a no-op mostly! Cheers, Andre