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From: Hanjun Guo <guohanjun@huawei.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
	<devicetree@vger.kernel.org>, Will Deacon <will@kernel.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Jeremy Linton <jeremy.linton@arm.com>,
	Linuxarm <linuxarm@huawei.com>,
	ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
	Linux IOMMU <iommu@lists.linux-foundation.org>,
	Rob Herring <robh+dt@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Sudeep Holla <sudeep.holla@arm.com>, Len Brown <lenb@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Christoph Hellwig <hch@lst.de>,
	"moderated list:BROADCOM BCM2835 ARM ARCHITECTURE"
	<linux-rpi-kernel@lists.infradead.org>
Subject: Re: [PATCH v3 7/8] arm64: mm: Set ZONE_DMA size based on early IORT scan
Date: Fri, 16 Oct 2020 15:27:21 +0800
Message-ID: <89ed58a5-b3ca-e361-94d8-b6754ce5eb34@huawei.com> (raw)
In-Reply-To: <CAMj1kXFzYbr_mYm-zhsio2XV+KGgDBjtgy_NWNYnanyfU-U-Nw@mail.gmail.com>

Hi Ard,

On 2020/10/16 14:54, Ard Biesheuvel wrote:
> On Fri, 16 Oct 2020 at 08:51, Hanjun Guo <guohanjun@huawei.com> wrote:
>>
>> On 2020/10/16 2:03, Catalin Marinas wrote:
>>> On Thu, Oct 15, 2020 at 10:26:18PM +0800, Hanjun Guo wrote:
>>>> On 2020/10/15 3:12, Nicolas Saenz Julienne wrote:
>>>>> From: Ard Biesheuvel <ardb@kernel.org>
>>>>>
>>>>> We recently introduced a 1 GB sized ZONE_DMA to cater for platforms
>>>>> incorporating masters that can address less than 32 bits of DMA, in
>>>>> particular the Raspberry Pi 4, which has 4 or 8 GB of DRAM, but has
>>>>> peripherals that can only address up to 1 GB (and its PCIe host
>>>>> bridge can only access the bottom 3 GB)
>>>>>
>>>>> Instructing the DMA layer about these limitations is straight-forward,
>>>>> even though we had to fix some issues regarding memory limits set in
>>>>> the IORT for named components, and regarding the handling of ACPI _DMA
>>>>> methods. However, the DMA layer also needs to be able to allocate
>>>>> memory that is guaranteed to meet those DMA constraints, for bounce
>>>>> buffering as well as allocating the backing for consistent mappings.
>>>>>
>>>>> This is why the 1 GB ZONE_DMA was introduced recently. Unfortunately,
>>>>> it turns out the having a 1 GB ZONE_DMA as well as a ZONE_DMA32 causes
>>>>> problems with kdump, and potentially in other places where allocations
>>>>> cannot cross zone boundaries. Therefore, we should avoid having two
>>>>> separate DMA zones when possible.
>>>>>
>>>>> So let's do an early scan of the IORT, and only create the ZONE_DMA
>>>>> if we encounter any devices that need it. This puts the burden on
>>>>> the firmware to describe such limitations in the IORT, which may be
>>>>> redundant (and less precise) if _DMA methods are also being provided.
>>>>> However, it should be noted that this situation is highly unusual for
>>>>> arm64 ACPI machines. Also, the DMA subsystem still gives precedence to
>>>>> the _DMA method if implemented, and so we will not lose the ability to
>>>>> perform streaming DMA outside the ZONE_DMA if the _DMA method permits
>>>>> it.
>>>>
>>>> Sorry, I'm still a little bit confused. With this patch, if we have
>>>> a device which set the right _DMA method (DMA size >= 32), but with the
>>>> wrong DMA size in IORT, we still have the ZONE_DMA created which
>>>> is actually not needed?
>>>
>>> With the current kernel, we get a ZONE_DMA already with an arbitrary
>>> size of 1GB that matches what RPi4 needs. We are trying to eliminate
>>> such unnecessary ZONE_DMA based on some heuristics (well, something that
>>> looks "better" than a OEM ID based quirk). Now, if we learn that IORT
>>> for platforms in the field is that broken as to describe few bits-wide
>>> DMA masks, we may have to go back to the OEM ID quirk.
>>
>> Some platforms using 0 as the memory size limit, for example D05 [0] and
>> D06 [1], I think we need to go back to the OEM ID quirk.
>>
>> For D05/D06, there are multi interrupt controllers named as mbigen,
>> mbigen is using the named component to describe the mappings with
>> the ITS controller, and mbigen is using 0 as the memory size limit.
>>
>> Also since the memory size limit for PCI RC was introduced by later
>> IORT revision, so firmware people may think it's fine to set that
>> as 0 because the system works without it.
>>
> 
> Hello Hanjun,
> 
> The patch only takes the address limit field into account if its value > 0.

Sorry I missed the if (*->memory_address_limit) check, thanks
for the reminding.

> 
> Also, before commit 7fb89e1d44cb6aec ("ACPI/IORT: take _DMA methods
> into account for named components"), the _DMA method was not taken
> into account for named components at all, and only the IORT limit was
> used, so I do not anticipate any problems with that.

Then this patch is fine to me.

Thanks
Hanjun
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Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-14 19:12 [PATCH v3 0/8] arm64: Default to 32-bit wide ZONE_DMA Nicolas Saenz Julienne
2020-10-14 19:12 ` [PATCH v3 1/8] arm64: mm: Move reserve_crashkernel() into mem_init() Nicolas Saenz Julienne
2020-10-15  8:40   ` Will Deacon
2020-10-15  8:55     ` Nicolas Saenz Julienne
2020-10-14 19:12 ` [PATCH v3 2/8] arm64: mm: Move zone_dma_bits initialization into zone_sizes_init() Nicolas Saenz Julienne
2020-10-14 19:12 ` [PATCH v3 3/8] of/address: Introduce of_dma_get_max_cpu_address() Nicolas Saenz Julienne
2020-10-14 22:02   ` Rob Herring
2020-10-15  6:56     ` Ard Biesheuvel
2020-10-15  9:16       ` Nicolas Saenz Julienne
2020-10-15  9:18         ` Ard Biesheuvel
2020-10-15  8:54     ` Nicolas Saenz Julienne
2020-10-15  5:42   ` Christoph Hellwig
2020-10-15 10:03     ` Nicolas Saenz Julienne
2020-10-16 13:19     ` Rob Herring
2020-10-14 19:12 ` [PATCH v3 4/8] of: unittest: Add test for of_dma_get_max_cpu_address() Nicolas Saenz Julienne
2020-10-14 22:04   ` Rob Herring
2020-10-15  9:51     ` Nicolas Saenz Julienne
2020-10-14 19:12 ` [PATCH v3 5/8] dma-direct: Turn zone_dma_bits default value into a define Nicolas Saenz Julienne
2020-10-15  5:38   ` Christoph Hellwig
2020-10-15 10:05     ` Nicolas Saenz Julienne
2020-10-14 19:12 ` [PATCH v3 6/8] arm64: mm: Set ZONE_DMA size based on devicetree's dma-ranges Nicolas Saenz Julienne
2020-10-15  5:39   ` Christoph Hellwig
2020-10-15 10:05     ` Nicolas Saenz Julienne
2020-10-14 19:12 ` [PATCH v3 7/8] arm64: mm: Set ZONE_DMA size based on early IORT scan Nicolas Saenz Julienne
2020-10-15 10:31   ` Lorenzo Pieralisi
2020-10-16  6:56     ` Ard Biesheuvel
2020-10-15 14:26   ` Hanjun Guo
2020-10-15 15:15     ` Nicolas Saenz Julienne
2020-10-15 18:03     ` Catalin Marinas
2020-10-16  6:51       ` Hanjun Guo
2020-10-16  6:54         ` Ard Biesheuvel
2020-10-16  7:27           ` Hanjun Guo [this message]
2020-10-16  7:34             ` Hanjun Guo
2020-10-14 19:12 ` [PATCH v3 8/8] mm: Update DMA zones description Nicolas Saenz Julienne
2020-10-15  5:40   ` Christoph Hellwig

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