linux-kernel - Re: [PATCH 02/11] driver core: Set DMA ownership during driver bind/unbind

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Date:   Mon, 15 Nov 2021 10:15:07 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Ashok Raj <ashok.raj@...el.com>, kvm@...r.kernel.org,
        rafael@...nel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Cornelia Huck <cohuck@...hat.com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org,
        Alex Williamson <alex.williamson@...hat.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        Diana Craciun <diana.craciun@....nxp.com>
Subject: Re: [PATCH 02/11] driver core: Set DMA ownership during driver
 bind/unbind

On Mon, Nov 15, 2021 at 11:56:13AM -0400, Jason Gunthorpe wrote:
> drivers/base/platform.c:        .dma_configure  = platform_dma_configure,
> drivers/bus/fsl-mc/fsl-mc-bus.c:        .dma_configure  = fsl_mc_dma_configure,
> drivers/pci/pci-driver.c:       .dma_configure  = pci_dma_configure,
> drivers/gpu/host1x/bus.c:       .dma_configure = host1x_dma_configure,
> 
> Other than host1x they all work with VFIO.
> 
> Also, there is no bus->dma_unconfigure() which would be needed to
> restore the device as well.
> 
> So, would you rather see duplicated code into the 4 drivers, and a new
> bus op to 'unconfigure dma'

The tend to mostly call into common helpers eventually.

> 
> Or, a 'dev_configure_dma()' function that is roughly:
> 
>         if (dev->bus->dma_configure) {
>                 ret = dev->bus->dma_configure(dev);
>                 if (ret)
>                         return ret;
>                 if (!drv->suppress_auto_claim_dma_owner) {
>                        ret = iommu_device_set_dma_owner(dev, DMA_OWNER_KERNEL,
>                                                         NULL);
>                        if (ret)
>                                ret;
>                 }
>          }
> 
> And a pair'd undo.

But that seems like an even better idea to me.  Even better with an
early return and avoiding the pointless indentation.