linux-kernel - RE: [PATCH V2 3/4] misc: vop: simply return the saved dma address instead of virt_to

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Date:   Tue, 29 Sep 2020 13:10:12 +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>,
        "kishon@...com" <kishon@...com>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: RE: [PATCH V2 3/4] misc: vop: simply return the saved dma address
 instead of virt_to_phys

Hi Christoph,

> On Tue, Sep 29, 2020 at 04:44:24PM +0800, Sherry Sun wrote:
> > The device page and vring should use consistent memory which are
> > allocated by dma_alloc_coherent api, when user space wants to get its
> > physical address, virt_to_phys cannot be used, should simply return
> > the saved device page dma address by get_dp_dma callback and the vring
> > dma address saved in mic_vqconfig.
> 
> More importantly you can't just all virt_to_phys on a return value from
> dma_alloc_coherent, so this needs to be folded into patch 1.

Okay, will move this change into patch 1.
> 
> >  	if (!offset) {
> > -		*pa = virt_to_phys(vpdev->hw_ops->get_dp(vpdev));
> > +		if (vpdev->hw_ops->get_dp_dma)
> > +			*pa = vpdev->hw_ops->get_dp_dma(vpdev);
> > +		else {
> > +			dev_err(vop_dev(vdev), "can't get device page
> physical address\n");
> > +			return -EINVAL;
> > +		}
> 
> I don't think we need the NULL check here.  Wouldn't it also make sense to
> return the virtual and DMA address from ->get_dp instead of adding another
> method?

Do you mean that we should only change the original ->get_dp callback to return virtual
and DMA address at the same time, instead of adding the ->get_dp_dma callback?

Regards
Sherry