linux-kernel - RE: [PATCH 1/5] misc: vop: change the way of allocating vring for noncoherent platform

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Date:   Mon, 28 Sep 2020 10:10:58 +0000
From:   Sherry Sun <sherry.sun@....com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     "sudeep.dutt@...el.com" <sudeep.dutt@...el.com>,
        "ashutosh.dixit@...el.com" <ashutosh.dixit@...el.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "wang.yi59@....com.cn" <wang.yi59@....com.cn>,
        "huang.zijiang@....com.cn" <huang.zijiang@....com.cn>,
        "rikard.falkeborn@...il.com" <rikard.falkeborn@...il.com>,
        "lee.jones@...aro.org" <lee.jones@...aro.org>,
        "mst@...hat.com" <mst@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: RE: [PATCH 1/5] misc: vop: change the way of allocating vring for
 noncoherent platform

Hi Christoph,

> On Sun, Sep 27, 2020 at 07:58:29AM +0000, Sherry Sun wrote:
> > Thanks for your reply.
> > Can you explain why we cannot use the API and header above in drivers?
> > And do you know if there are any APIs that could replace this to check the
> device hardware dma coherent support?
> 
> If your treat the memory as if it is coherent with device access you should
> always use dma_alloc_coherent.  The whole point of the DMA API is to
> abstract such differences away.

Well, for vring and device pages in vop driver, dma coherent memory maybe more reasonable.
Will use dma_alloc_coherent to replace the orginal memory allocate method in V2.

> 
> > >
> > > > +			vdev->vvr[i].vrh.vring.used =
> > > > +				(void __force *)vpdev->hw_ops->remap(
> > >
> > > And you must not use __force casts without a comment explanation why
> > > they are safe.
> >
> > Here is the original code, I moved the original code into the if() without
> change it.
> > But I think  this is because vpdev->hw_ops->remap() return type is void
> __iomem *, but vring.used need type void *.
> > Maybe this is a workaround for Intel MIC platform, as it reassigns the used
> ring on the EP side.
> 
> Well, we'll need to figure out what is going on here, and blind casts to and
> from __iomem are dangerous.

Yes, you are right. 

Regards
Sherry