linux-kernel - Re: [RFC PATCH 1/3] iommu: Add support for out of band flushing

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Date:	Tue, 29 Sep 2015 20:56:12 +0900
From:	Tomasz Figa <tfiga@...omium.org>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	"open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
	Joerg Roedel <joro@...tes.org>,
	Hiroshi Doyu <hdoyu@...dia.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Alexandre Courbot <gnurou@...il.com>,
	Vince Hsu <vince.h@...dia.com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Paul Walmsley <paul@...an.com>,
	Mikko Perttunen <mperttunen@...dia.com>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	Will Deacon <will.deacon@....com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Arnd Bergmann <arnd@...db.de>,
	Antonios Motakis <a.motakis@...tualopensystems.com>,
	Olav Haugan <ohaugan@...eaurora.org>,
	Nicolas Iooss <nicolas.iooss_linux@....org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-tegra@...r.kernel.org, Rob Clark <robdclark@...il.com>
Subject: Re: [RFC PATCH 1/3] iommu: Add support for out of band flushing

On Tue, Sep 29, 2015 at 6:32 PM, Thierry Reding
<thierry.reding@...il.com> wrote:
> On Tue, Sep 29, 2015 at 02:25:24PM +0900, Tomasz Figa wrote:
>> This patch adds a new "flush" callback to iommu_ops, which is supposed
>> to perform any necessary flushes within given IOMMU domain to make any
>> changes to mappings of given area [iova; iova + size) be reflected to
>> IOMMU clients.
>>
>> The purpose is to let IOMMU drivers skip page-by-page flushes and
>> replace it with one flush of full address range on devices which support
>> it.
>>
>> Signed-off-by: Tomasz Figa <tfiga@...omium.org>
>> ---
>>  drivers/iommu/iommu.c | 33 ++++++++++++++++++++++++++++++---
>>  include/linux/iommu.h |  2 ++
>>  2 files changed, 32 insertions(+), 3 deletions(-)
>
> I seem to remember that Rob Clark had proposed something like this back
> before it was decided to introduce the ->map_sg() callback instead. I
> can't find a reference to the discussion, so perhaps I'm misremembering
> but adding Rob Clark in case he has any recollection of why the outcome
> was what it was.

Oops, I was supposed to add Rob, but forgot in the end. Thanks for remembering.

>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> [...]
>> @@ -1389,6 +1410,7 @@ size_t iommu_unmap(struct iommu_domain *domain, unsigned long iova, size_t size)
>>               unmapped += unmapped_page;
>>       }
>>
>> +     iommu_flush(domain, orig_iova, unmapped);
>>       trace_unmap(orig_iova, size, unmapped);
>>       return unmapped;
>>  }
>> @@ -1419,19 +1441,24 @@ size_t default_iommu_map_sg(struct iommu_domain *domain, unsigned long iova,
>>               if (!IS_ALIGNED(s->offset, min_pagesz))
>>                       goto out_err;
>>
>> -             ret = iommu_map(domain, iova + mapped, phys, s->length, prot);
>> +             ret = __iommu_map(domain, iova + mapped, phys, s->length, prot);
>>               if (ret)
>>                       goto out_err;
>>
>>               mapped += s->length;
>>       }
>>
>> +     iommu_flush(domain, iova, mapped);
>> +
>>       return mapped;
>>
>>  out_err:
>>       /* undo mappings already done */
>>       iommu_unmap(domain, iova, mapped);
>>
>> +     /* flush in case part of our mapping already got cached */
>> +     iommu_flush(domain, iova, mapped);
>> +
>>       return 0;
>>
>>  }
>
> iommu_unmap() already does an iommu_flush(), so why flush again after
> iommu_unmap()?

Right, my mistake. This should be removed. Thanks for catching.

Best regards,
Tomasz
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