Date:   Mon, 7 May 2018 00:19:01 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Robin Murphy <robin.murphy@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     linux-tegra@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        Jonathan Hunter <jonathanh@...dia.com>
Subject: Re: [PATCH v1 4/4] iommu/tegra: gart: Optimize map/unmap

On 27.04.2018 15:36, Robin Murphy wrote:
> Hi Thierry,
> 
> On 27/04/18 11:02, Thierry Reding wrote:
>> On Mon, Apr 09, 2018 at 11:07:22PM +0300, Dmitry Osipenko wrote:
>>> Currently GART writes one page entry at a time. More optimal would be to
>>> aggregate the writes and flush BUS buffer in the end, this gives map/unmap
>>> 10-40% (depending on size of mapping) performance boost compared to a
>>> flushing after each entry update.
>>>
>>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>>> ---
>>>   drivers/iommu/tegra-gart.c | 63 +++++++++++++++++++++++++++++++++++-----------
>>>   1 file changed, 48 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/drivers/iommu/tegra-gart.c b/drivers/iommu/tegra-gart.c
>>> index 4a0607669d34..9f59f5f17661 100644
>>> --- a/drivers/iommu/tegra-gart.c
>>> +++ b/drivers/iommu/tegra-gart.c
>>> @@ -36,7 +36,7 @@
>>>   #define GART_APERTURE_SIZE    SZ_32M
>>>     /* bitmap of the page sizes currently supported */
>>> -#define GART_IOMMU_PGSIZES    (SZ_4K)
>>> +#define GART_IOMMU_PGSIZES    GENMASK(24, 12)
>>
>> That doesn't look right. The GART really only supports 4 KiB pages. You
>> seem to be "emulating" more page sizes here in order to improve mapping
>> performance. That seems wrong to me. I'm wondering if this couldn't be
>> improved by a similar factor by simply moving the flushing into an
>> implementation of ->iotlb_sync().
>>
>> That said, it seems like ->iotlb_sync() is only used for unmapping, but
>> I don't see a reason why iommu_map() wouldn't need to call it as well
>> after going through several calls to ->map(). It seems to me like a
>> driver that implements ->iotlb_sync() would want to use it to optimize
>> for both the mapping and unmapping cases.
>>
>> Joerg, I've gone over the git log and header files and I see no mention
>> of why the TLB flush interface isn't used for mapping. Do you recall any
>> special reasons why the same shouldn't be applied for mapping? Would you
>> accept any patches doing this?
> 
> In general, requiring TLB maintenance when transitioning from an invalid entry
> to a valid one tends to be the exception rather than the norm, and I think we
> ended up at the consensus that it wasn't worth the complication of trying to
> cater for this in the generic iotlb API.
> 
> To be fair, on simple hardware which doesn't implement multiple page sizes with
> associated walk depth/TLB pressure benefits for larger ones, there's no need for
> the IOMMU API (and/or the owner of the domain) to try harder to use them, so
> handling "compound" page sizes within the driver is a more reasonable thing to
> do. There is already some precedent for this in other drivers (e.g. mtk_iommu_v1).
Probably the best variant would be to give an explicit control over syncing to a
user of the IOMMU API, like for example device driver may perform multiple
mappings / unmappings and then sync/flush in the end. I'm not sure that it's
really worth the hassle to shuffle the API right now, maybe we can implement it
later if needed. Joerg, do you have objections to a 'compound page' approach?

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