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

[Jerome Glisse <jglisse@redhat.com>]

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@redhat.com>
> > To: Zhangfei Gao <zhangfei.gao@linaro.org>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Arnd Bergmann
> >  <arnd@arndb.de>, Herbert Xu <herbert@gondor.apana.org.au>,
> >  jonathan.cameron@huawei.com, grant.likely@arm.com, jean-philippe
> >  <jean-philippe@linaro.org>, ilias.apalodimas@linaro.org,
> >  francois.ozog@linaro.org, kenneth-lee-2012@foxmail.com, Wangzhou
> >  <wangzhou1@hisilicon.com>, "haojian . zhuang" <haojian.zhuang@linaro.org>,
> >  Zaibo Xu <xuzaibo@huawei.com>, linux-kernel@vger.kernel.org,
> >  linux-crypto@vger.kernel.org, Kenneth Lee <liguozhu@hisilicon.com>,
> >  linux-accelerators@lists.ozlabs.org
> > Subject: Re: [PATCH v6 2/3] uacce: add uacce driver
> > Message-ID: <20191022184929.GC5169@redhat.com>
> > 
> > On Wed, Oct 16, 2019 at 04:34:32PM +0800, Zhangfei Gao wrote:
> > > From: Kenneth Lee <liguozhu@hisilicon.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