netdev - Re: [PATCH net-next v3 4/9] gve: Add support for dma

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Date:   Wed, 9 Sep 2020 15:12:31 -0700
From:   David Awogbemila <awogbemila@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, Catherine Sullivan <csully@...gle.com>,
        Yangchun Fu <yangchun@...gle.com>
Subject: Re: [PATCH net-next v3 4/9] gve: Add support for dma_mask register

On Tue, Sep 8, 2020 at 12:15 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Tue,  8 Sep 2020 11:39:04 -0700 David Awogbemila wrote:
> > +     dma_mask = readb(&reg_bar->dma_mask);
> > +     // Default to 64 if the register isn't set
> > +     if (!dma_mask)
> > +             dma_mask = 64;
> >       gve_write_version(&reg_bar->driver_version);
> >       /* Get max queues to alloc etherdev */
> >       max_rx_queues = ioread32be(&reg_bar->max_tx_queues);
> >       max_tx_queues = ioread32be(&reg_bar->max_rx_queues);
> > +
> > +     err = pci_set_dma_mask(pdev, DMA_BIT_MASK(64));
>
> You use the constant 64, not dma_mask?
>
> You jump through hoops to get GFP_DMA allocations yet you don't set the
> right DMA mask. Why would swiotlb become an issue to you if there never
> was any reasonable mask set?
>
> > +     if (err) {
> > +             dev_err(&pdev->dev, "Failed to set dma mask: err=%d\n", err);
> > +             goto abort_with_reg_bar;
> > +     }
> > +
> > +     err = pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(64));
>
> dma_set_mask_and_coherent()
>
> > +     if (err) {
> > +             dev_err(&pdev->dev,
> > +                     "Failed to set consistent dma mask: err=%d\n", err);
> > +             goto abort_with_reg_bar;
> > +     }

I think you're right. Setting dma_set_mask_and_coherent(dev, 64)
should mean we no longer have the swiotlb issue. I will drop this
patch altogether.