RE: [PATCH 0/5] Implement SMMU passthrough using the default domain

["Sricharan" <sricharan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

Hi Rob,

>-----Original Message-----
>From: linux-arm-kernel [mailto:linux-arm-kernel-bounces-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org] On Behalf Of Rob Clark
>Sent: Thursday, February 02, 2017 8:33 PM
>To: Joerg Roedel <joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org>
>Cc: Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>; iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org; Sricharan <sricharan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>; linux-arm-
>kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org
>Subject: Re: [PATCH 0/5] Implement SMMU passthrough using the default domain
>
>On Thu, Jan 26, 2017 at 12:18 PM, Joerg Roedel <joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org> wrote:
>> On Tue, Jan 24, 2017 at 08:42:23PM +0530, Sricharan wrote:
>>> Thanks for this series. We had a case with the GPU.
>>> The GPU's iommu was setup by kernel and the GPU
>>> also does dynamic updates for on-the-fly switching between
>>> process pagetables.  GPU driver was not using DMA domain and
>>> the GPU's firmware was always expecting to run out  of contextbank
>>>  '0' (although not correct) , which was not the case after the DMA domain
>>> was made default  as '0' was getting allocated for DMA domain and
>>> there were concerns about reusing the DMA domain as well.
>>> Now with this series, looks there is an way out of that that can be tried.
>>>
>>> So should the default domain not be per device specific selectable ?
>>
>> Note that iommu-drivers can request direct-mapping for any given device
>> on its initializtion. This is used on x86 for devices that need a 1-1
>> mapping for some reason.
>>
>> Also device drivers can use the iommu-api and assign their own domain to
>> a device, which allows them to manage the dma address space on their
>> own.
>
>Part of the problem is that dev->archdata.dma_ops gets wired up to
>iommu_dma_ops.  Which isn't so bad on it's own, except that cache ops
>are not exposed to drivers, forcing us to use dma-mapping API
>(dma_map_sg, etc) for cache operations.
>
>Possibly we should just expose cache op's to drivers bypass this abuse
>of dma-mapping.
>
[1], with this, when the default domain in not DOMAIN_DMA, then
dev->archdata.dma_ops is not set to iommu_dma_ops , instead remains
to be swiotlb_ops. Is that not correct for gpu's unmanaged domain case ?

https://www.spinics.net/lists/arm-kernel/msg556209.html

>btw, Will, we definitely want this to *not* rely on kcmdline for the
>gpu with it's own private iommu case..

Ya, that changes behavior for all devices and some might want 
DMA_DOMAIN and some UNMANAGED.

Regards,
 Sricharan

>
>BR,
>-R
>
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