linux-kernel - Re: [PATCH v6 2/3] uacce: add uacce driver

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Message-ID: <20191024141759.GA4793@redhat.com>
Date:   Thu, 24 Oct 2019 10:17:59 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Kenneth Lee <Kenneth-Lee-2012@...mail.com>
Cc:     Zhangfei Gao <zhangfei.gao@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        jonathan.cameron@...wei.com, grant.likely@....com,
        jean-philippe <jean-philippe@...aro.org>,
        ilias.apalodimas@...aro.org, francois.ozog@...aro.org,
        Wangzhou <wangzhou1@...ilicon.com>,
        "haojian . zhuang" <haojian.zhuang@...aro.org>,
        Zaibo Xu <xuzaibo@...wei.com>, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org, Kenneth Lee <liguozhu@...ilicon.com>,
        linux-accelerators@...ts.ozlabs.org
Subject: Re: [PATCH v6 2/3] uacce: add uacce driver

On Thu, Oct 24, 2019 at 02:41:29PM +0800, Kenneth Lee wrote:
> On Tue, Oct 22, 2019 at 02:49:29PM -0400, Jerome Glisse wrote:
> > Date: Tue, 22 Oct 2019 14:49:29 -0400
> > From: Jerome Glisse <jglisse@...hat.com>
> > To: Zhangfei Gao <zhangfei.gao@...aro.org>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arnd Bergmann
> >  <arnd@...db.de>, Herbert Xu <herbert@...dor.apana.org.au>,
> >  jonathan.cameron@...wei.com, grant.likely@....com, jean-philippe
> >  <jean-philippe@...aro.org>, ilias.apalodimas@...aro.org,
> >  francois.ozog@...aro.org, kenneth-lee-2012@...mail.com, Wangzhou
> >  <wangzhou1@...ilicon.com>, "haojian . zhuang" <haojian.zhuang@...aro.org>,
> >  Zaibo Xu <xuzaibo@...wei.com>, linux-kernel@...r.kernel.org,
> >  linux-crypto@...r.kernel.org, Kenneth Lee <liguozhu@...ilicon.com>,
> >  linux-accelerators@...ts.ozlabs.org
> > Subject: Re: [PATCH v6 2/3] uacce: add uacce driver
> > Message-ID: <20191022184929.GC5169@...hat.com>
> > 
> > On Wed, Oct 16, 2019 at 04:34:32PM +0800, Zhangfei Gao wrote:
> > > From: Kenneth Lee <liguozhu@...ilicon.com>
> > > 
> > > Uacce (Unified/User-space-access-intended Accelerator Framework) targets to
> > > provide Shared Virtual Addressing (SVA) between accelerators and processes.
> > > So accelerator can access any data structure of the main cpu.
> > > This differs from the data sharing between cpu and io device, which share
> > > data content rather than address.
> > > Since unified address, hardware and user space of process can share the
> > > same virtual address in the communication.
> > > 
> > > Uacce create a chrdev for every registration, the queue is allocated to
> > > the process when the chrdev is opened. Then the process can access the
> > > hardware resource by interact with the queue file. By mmap the queue
> > > file space to user space, the process can directly put requests to the
> > > hardware without syscall to the kernel space.
> > 
> > You need to remove all API that is not use by your first driver as
> > it will most likely bit rot without users. It is way better to add
> > things when a driver start to make use of it.
> 
> Yes. Good point. Thank you:)
> 
> > 
> > I am still not convince of the value of adding a new framework here
> > with only a single device as an example. It looks similar to some of
> > the fpga devices. Saddly because framework layering is not something
> > that exist i guess inventing a new framework is the only answer when
> > you can not quite fit into an existing one.
> > 
> > More fundamental question is why do you need to change the IOMMU
> > domain of the device ? I do not see any reason for that unless the
> > PASID has some restriction on ARM that i do not know of.
> 
> But I think this is the only way. As my understanding, by default, the
> system creates a DMA IOMMU domain for each device behine an IOMMU. If
> you want to call iommu interface directly, we have to rebind the device
> to an unmanaged domain.

Why would you need to call iommu directly ? On some GPUs we do use
PASID and we do not rebind to different domain, we just don't mess
with that. So i do not see any reason to change the domain.

Cheers,
Jérôme