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Subject: Re: [PATCH v2] ACPI/IORT: Do not blindly trust DMA masks from
 firmware
To:     Moritz Fischer <mdf@kernel.org>
Cc:     lorenzo.pieralisi@arm.com, guohanjun@huawei.com, rjw@rjwysocki.net,
        linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org,
        moritzf@google.com, sudeep.holla@arm.com, will@kernel.org,
        linux-arm-kernel@lists.infradead.org
References: <20210122012419.95010-1-mdf@kernel.org>
 <e01e2fd6-7f78-354e-374c-f93a5d1b8fd6@arm.com> <YAsQcLqvi0Lh8PVv@archbook>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <29575ef5-a1c1-16d7-5fed-7fc34d772a7a@arm.com>
Date:   Fri, 22 Jan 2021 19:17:59 +0000
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On 2021-01-22 17:50, Moritz Fischer wrote:
> Hi Robin,
> 
> On Fri, Jan 22, 2021 at 02:42:05PM +0000, Robin Murphy wrote:
>> On 2021-01-22 01:24, Moritz Fischer wrote:
>>> Address issue observed on real world system with suboptimal IORT table
>>> where DMA masks of PCI devices would get set to 0 as result.
>>>
>>> iort_dma_setup() would query the root complex'/named component IORT
>>> entry for a DMA mask, and use that over the one the device has been
>>> configured with earlier.
>>>
>>> Ideally we want to use the minimum mask of what the IORT contains for
>>> the root complex and what the device was configured with.
>>>
>>> Fixes: 5ac65e8c8941 ("ACPI/IORT: Support address size limit for root complexes")
>>> Signed-off-by: Moritz Fischer <mdf@kernel.org>
>>> ---
>>>
>>> Changes from v1:
>>> - Changed warning to FW_BUG
>>> - Warn for both Named Component or Root Complex
>>> - Replaced min_not_zero() with min()
>>>
>>> ---
>>>    drivers/acpi/arm64/iort.c | 14 ++++++++++++--
>>>    1 file changed, 12 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
>>> index d4eac6d7e9fb..2494138a6905 100644
>>> --- a/drivers/acpi/arm64/iort.c
>>> +++ b/drivers/acpi/arm64/iort.c
>>> @@ -1107,6 +1107,11 @@ static int nc_dma_get_range(struct device *dev, u64 *size)
>>>    	ncomp = (struct acpi_iort_named_component *)node->node_data;
>>> +	if (!ncomp->memory_address_limit) {
>>> +		pr_warn(FW_BUG "Named component missing memory address limit\n");
>>> +		return -EINVAL;
>>> +	}
>>> +
>>>    	*size = ncomp->memory_address_limit >= 64 ? U64_MAX :
>>>    			1ULL<<ncomp->memory_address_limit;
>>> @@ -1126,6 +1131,11 @@ static int rc_dma_get_range(struct device *dev, u64 *size)
>>>    	rc = (struct acpi_iort_root_complex *)node->node_data;
>>> +	if (!rc->memory_address_limit) {
>>> +		pr_warn(FW_BUG "Root complex missing memory address limit\n");
>>> +		return -EINVAL;
>>> +	}
>>> +
>>>    	*size = rc->memory_address_limit >= 64 ? U64_MAX :
>>>    			1ULL<<rc->memory_address_limit;
>>> @@ -1173,8 +1183,8 @@ void iort_dma_setup(struct device *dev, u64 *dma_addr, u64 *dma_size)
>>>    		end = dmaaddr + size - 1;
>>>    		mask = DMA_BIT_MASK(ilog2(end) + 1);
>>>    		dev->bus_dma_limit = end;
>>> -		dev->coherent_dma_mask = mask;
>>> -		*dev->dma_mask = mask;
>>> +		dev->coherent_dma_mask = min(dev->coherent_dma_mask, mask);
>>> +		*dev->dma_mask = min(*dev->dma_mask, mask);
>>
>> Oops, I got so distracted by the "not_zero" aspect in v1 that I ended up
>> thinking purely about smaller-than-default masks, but of course this *does*
>> matter the other way round. And it is what we've always done on the DT side,
>> so at least it makes us consistent.
>>
>> FWIW I've already started writing up a patch to kill off this bit entirely,
>> but either way we still can't meaningfully interpret a supposed DMA limit of
>> 0 bits in a table describing DMA-capable devices, so for this patch as a
>> fix,
>>
>> Reviewed-by: Robin Murphy <robin.murphy@arm.com>
> 
> I think there's another issue the comparisons for revision should be
> against < 2 not < 1.
> 
>  From what I could find DEN0049D (IORT) spec introduced the fields
> (curiously the C doc seems to be missing).

I guess it got lost in the documentation system move. FWIW I still have 
a copy of issue C, and root complex nodes are unchanged at revision 0 there.

> DEN0049B specifies revision as '0', DEN0049C (missing?), DEN0049D
> specifies new fields for memory_size_limit and both Named Component and
> Root Complex nodes set revision to 2.

My copy of issue D says Root Complex nodes are at revision 1, with 
memory address size limit added.

(Note that Named Component nodes did bump to rev. 1 in issue C, then to 
rev. 2 in issue D)

Issue E bumped Root Complex nodes to revision 2 with the addition of the 
PRI flag, then E.a made a mess of everything by deprecating the revision 
numbers for individual tables - we probably need to deal with *that*, 
since otherwise we'll think new tables are back at rev. 0 again, but 
AFAICS the current check is correct for anything written against the 
first 5 releases.

Robin.

> so I think it should be:
> 
> if (!node || node->revision < 2)
> 	return -ENODEV;
> 
> Only if we go past this and there is no address limit is it really a
> firmware bug.
>>
>> Thanks,
>> Robin.
>>
>>>    	}
>>>    	*dma_addr = dmaaddr;
>>>
> 
> - Moritz
>