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 63108307AC6; Mon, 3 Aug 2026 14:53:47 +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=1785768829; cv=none; b=qkB2Ej+VYsJa4oC3EdWfQ1JSQJ/WTJAdLQz7gflFG0XozhCAuQzKevw8/krsaZse+i+A6kAUUJ0Omgpwu4BH/28A7KPfjjqxtzN2+kMR+A1zbm8GmqU7uDydRpeMDdt6pB8PBIH5SC8A298LxTxV2GmieDHKRkuyszsv9ahd+WE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785768829; c=relaxed/simple; bh=22b24nKVspIIerMyJPhz4Lk+/zJYhUSjkwYys92cSQM=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=NSGnTya5ccRtb+faz4zXEvxghLGuFTzspfXEfaNfah7XsKDbIaxKNhW/OXw60vBo5HjaW426ZH+ZGrN+TLHjLp74LNOTkeRYqAWI5jxj0hZuMQ+Y6Vb34Ethovc3NmrUX8TaB4vyM8gH2F37RkUh82OK7UDdwmfCSCDTNlyRXms= 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=lIRqUf+b; 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="lIRqUf+b" 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 A702D143D; Mon, 3 Aug 2026 07:53:42 -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 B95EF3F632; Mon, 3 Aug 2026 07:53:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1785768826; bh=22b24nKVspIIerMyJPhz4Lk+/zJYhUSjkwYys92cSQM=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=lIRqUf+bRhghUJq+UNbfYsV6sy8wAfOUrtNwwmcHyowc0tWFr2zJpNqBNTxrVlRTR /LifKsbx1oc1UceaSso5mGQqkFZguD143ddzqogbOQef4pqOwAWqog+UQfxIuVzeTz oFUEgmviRB1fOovP0rTMsMh/zqnrp+89Z7D+1Lp4= Message-ID: Date: Mon, 3 Aug 2026 16:53:40 +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 v7 11/11] arm_mpam: detect and enable MPAM-Fb PCC support From: Andre Przywara 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 , Ritwick Sharma , linux-acpi@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20260731170324.2927758-1-andre.przywara@arm.com> <20260731170324.2927758-12-andre.przywara@arm.com> <21d9c35b-58e8-4b99-a764-58fe45160cf6@arm.com> Content-Language: en-GB In-Reply-To: <21d9c35b-58e8-4b99-a764-58fe45160cf6@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Hi, On 8/3/26 15:37, Andre Przywara wrote: > Hi Srivathsa, > > On 8/3/26 12:49, Srivathsa L Rao wrote: >> Hi Andre, >> >> On 7/31/2026 10:33 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 >>> Tested-by: Ritwick Sharma >>> Reviewed-by: Jonathan Cameron >>> --- >>>   drivers/resctrl/mpam_devices.c  | 113 +++++++++++++++++++++++++++++++- >>>   drivers/resctrl/mpam_fb.c       |  41 ++++++++++++ >>>   drivers/resctrl/mpam_internal.h |   2 + >>>   3 files changed, 154 insertions(+), 2 deletions(-) >>> >>> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/ >>> mpam_devices.c >>> index b238feb559d4..40b247a89f8e 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 +54,88 @@ 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); >>> +    mutex_destroy(&cur->pcc_chan_lock); >>> +    kfree(cur); >>> +} >>> + >>> +static struct mpam_pcc_chan *mpam_pcc_chan_get(struct device *dev, >>> +                           int subspace_id) >>> +{ >>> +    struct mpam_pcc_chan *cur; >>> + >>> +    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; >> >> While testing v7 with multiple PCC MSCs sharing a single PCC channel, I >> noticed that pcc_cl.dev is set once at channel creation time (from the >> first MSC's device) and never updated when subsequent MSCs reuse the >> channel.In mpam_pcc_chan_get(), the first call sets: >>   cur->pcc_cl.dev = dev; >> >> Subsequent calls return the existing channel without updating >> pcc_cl.dev. Here dev_err() calls through the channel report errors >> against the first MSC's device, even when a different MSC triggered them. > > Mmh, that's true, but I don't think there is much we can do about it? > Ultimately it's a channel shared between multiple devices. > And I am less concerned about our own dev_err() (we can easily - and > actually should - replace this with pr_err()), but more about the mbox > uses of the dev pointer. Did you by any chance find more critical users > of the dev pointer other than dev_err() prints, in the mailbox code? To answer my own question: I could only find code using the dev pointer during initialisation, to attribute the error messages to the right device. Which means when we drop the single usage in mpam_fb.c, there should be no more problems. Fixed that accordingly. Cheers, Andre > I don't know if the correct MSC attribution really matters, or if we can > live with just a valid reference to some "related" device, but ... > >> I reproduced this with four PCC MSCs sharing subspace 0. After unbinding >> the first MSC (kref 4→3), triggering dev_err() via a test hook showed >> the device prefix change from "mpam_msc mpam_msc.1:" to "platform >> mpam_msc.1:" > > Mmmh, interesting, where does the change come from? I would think that > would result in a use after free? > Which means we should make sure that some other MSC donates their dev > pointer if the first MSC goes away? I will have a look into that > direction, though it still feels a bit odd. > >> A simple fix maybe to use the MSC's own device for error messages in >> mpam_fb_send_request() instead of pcc_chan->pcc_cl.dev > > But we don't have that pointer at this time, do we?  But as mentioned, > we could just revert to a simple pr_err() instead. > > Thanks for the heads up anyway! > Cheers, > Andre > > >> >>> +    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" in us, from the >>> +     * PCC ACPI table. tx_tout is in ms. >>> +     * Add some margin here to be on the safe side. >>> +     */ >>> +    cur->pcc_cl.tx_tout = DIV_ROUND_UP(cur->pcc_chan->latency * 5, >>> 1000); >>> + >>> +    mutex_init(&cur->pcc_chan_lock); >>> + >>> +    cur->subspace_id = subspace_id; >>> +    kref_init(&cur->refcount); >>> + >>> +    list_add_tail(&cur->pcc_chans, &pcc_chan_list); >>> + >>> +    return cur; >>> +} >>> + >>> +static int mpam_pcc_chan_put(struct mpam_pcc_chan *pcc_chan) >>> +{ >>> +    struct mpam_pcc_chan *cur, *tmp; >>> + >>> +    if (!pcc_chan) >>> +        return 0; >>> + >>> +    guard(mutex)(&pcc_chan_list_lock); >>> + >>> +    list_for_each_entry_safe(cur, tmp, &pcc_chan_list, pcc_chans) { >>> +        if (cur == pcc_chan) { >>> +            kref_put(&cur->refcount, mpam_pcc_chan_release); >>> + >>> +            return 0; >>> +        } >>> +    } >>> + >>> +    return -ENOENT; >>> +} >>> + >>>   /* >>>    * Number of MSCs that have been probed. Once all MSCs have been >>> probed MPAM >>>    * can be enabled. >>> @@ -2274,6 +2361,8 @@ static void mpam_msc_drv_remove(struct >>> platform_device *pdev) >>>   { >>>       struct mpam_msc *msc = platform_get_drvdata(pdev); >>> +    mpam_pcc_chan_put(msc->pcc_chan); >>> + >>>       mutex_lock(&mpam_list_lock); >>>       mpam_msc_destroy(msc); >>>       mutex_unlock(&mpam_list_lock); >>> @@ -2284,7 +2373,7 @@ static void mpam_msc_drv_remove(struct >>> platform_device *pdev) >>>   static struct mpam_msc *do_mpam_msc_drv_probe(struct >>> platform_device *pdev) >>>   { >>>       int err; >>> -    u32 tmp; >>> +    u32 pcc_subspace_id; >>>       struct mpam_msc *msc; >>>       struct resource *msc_res; >>>       struct device *dev = &pdev->dev; >>> @@ -2328,7 +2417,7 @@ static struct mpam_msc >>> *do_mpam_msc_drv_probe(struct platform_device *pdev) >>>       if (err) >>>           return ERR_PTR(err); >>> -    if (device_property_read_u32(&pdev->dev, "pcc-channel", &tmp)) >>> +    if (device_property_read_u32(dev, "pcc-channel", &pcc_subspace_id)) >>>           msc->iface = MPAM_IFACE_MMIO; >>>       else >>>           msc->iface = MPAM_IFACE_PCC; >>> @@ -2349,6 +2438,26 @@ static struct mpam_msc >>> *do_mpam_msc_drv_probe(struct platform_device *pdev) >>>           } >>>           msc->mapped_hwpage_sz = msc_res->end - msc_res->start; >>>           msc->mapped_hwpage = io; >>> +    } else if (msc->iface == MPAM_IFACE_PCC) { >>> +        msc->pcc_chan = mpam_pcc_chan_get(dev, pcc_subspace_id); >>> +        if (IS_ERR(msc->pcc_chan)) { >>> +            pr_err("Failed to request MSC PCC channel\n"); >>> +            return ERR_CAST(msc->pcc_chan); >>> +        } >>> + >>> +        err = mpam_fb_check_shared_buffer_size(msc); >>> +        if (err) { >>> +            mpam_pcc_chan_put(msc->pcc_chan); >>> + >>> +            return ERR_PTR(err); >>> +        } >>> + >>> +        err = mpam_fb_check_protocol_version(msc); >>> +        if (err) { >>> +            mpam_pcc_chan_put(msc->pcc_chan); >>> + >>> +            return ERR_PTR(err); >>> +        } >>>       } else { >>>           return ERR_PTR(-EINVAL); >>>       } >>> diff --git a/drivers/resctrl/mpam_fb.c b/drivers/resctrl/mpam_fb.c >>> index 61a91c6cec0a..7a7fb6d067ba 100644 >>> --- a/drivers/resctrl/mpam_fb.c >>> +++ b/drivers/resctrl/mpam_fb.c >>> @@ -37,6 +37,11 @@ >>>   #define MPAM_MSC_TOKEN_MASK        GENMASK(27, 18) >>>   #define MPAM_FB_PROT_HEADER_LEN        sizeof(u32) >>> +/* The longest message is MPAM_MSC_WRITE, with 4 parameters. */ >>> +#define MPAM_FB_MAX_MSG_SIZE        (4 * sizeof(u32)) >>> + >>> +#define MPAM_FB_VERSION_MAJOR_MASK    GENMASK(31, 16) >>> +#define MPAM_FB_VERSION_MINOR_MASK    GENMASK(15, 0) >>>   static atomic_t mpam_fb_token = ATOMIC_INIT(0); >>> @@ -207,3 +212,39 @@ int mpam_fb_send_write_request(struct mpam_msc >>> *msc, u16 reg, u32 value) >>>       return mpam_fb_send_request(msc->pcc_chan, msc->id, reg, &value, >>>                       MPAM_MSC_WRITE_CMD); >>>   } >>> + >>> +/* We only support MPAM-Fb protocol version 1.x */ >>> +int mpam_fb_check_protocol_version(struct mpam_msc *msc) >>> +{ >>> +    u32 version; >>> +    int ret; >>> + >>> +    ret = mpam_fb_send_request(msc->pcc_chan, 0, >>> +                   0, &version, MPAM_PROTOCOL_VERSION_CMD); >>> +    if (ret) >>> +        return ret; >>> + >>> +    if (FIELD_GET(MPAM_FB_VERSION_MAJOR_MASK, version) != 1) { >>> +        pr_err("Incompatible MPAM-Fb protocol version %ld.%ld\n", >>> +               FIELD_GET(MPAM_FB_VERSION_MAJOR_MASK, version), >>> +               FIELD_GET(MPAM_FB_VERSION_MINOR_MASK, version)); >>> + >>> +        return -EINVAL; >>> +    } >>> + >>> +    return 0; >>> +} >>> + >>> +int mpam_fb_check_shared_buffer_size(struct mpam_msc *msc) >>> +{ >>> +    int min_buffer_size = MPAM_FB_MAX_MSG_SIZE + >>> +                  sizeof(struct acpi_pcct_ext_pcc_shared_memory); >>> + >>> +    if (msc->pcc_chan->pcc_chan->shmem_size < min_buffer_size) { >>> +        pr_err("MPAM-Fb PCC channel size too small.\n"); >>> + >>> +        return -ENOMEM; >>> +    } >>> + >>> +    return 0; >>> +} >>> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/ >>> mpam_internal.h >>> index f13a79a84c58..8859189088f3 100644 >>> --- a/drivers/resctrl/mpam_internal.h >>> +++ b/drivers/resctrl/mpam_internal.h >>> @@ -528,6 +528,8 @@ static inline void >>> mpam_resctrl_teardown_class(struct mpam_class *class) { } >>>   /* MPAM-Fb Firmware-backed protocol wrappers */ >>>   int mpam_fb_send_read_request(struct mpam_msc *msc, u16 reg, u32 >>> *result); >>>   int mpam_fb_send_write_request(struct mpam_msc *msc, u16 reg, u32 >>> value); >>> +int mpam_fb_check_protocol_version(struct mpam_msc *msc); >>> +int mpam_fb_check_shared_buffer_size(struct mpam_msc *msc); >>>   /* >>>    * MPAM MSCs have the following register layout. See: >> >> -- >> Best Regards, >> Srivathsa >> >