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Date:   Fri, 14 Jun 2019 09:46:48 +0200
From:   Christoph Hellwig <hch@....de>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] dma-mapping: truncate dma masks to what dma_addr_t
 can hold

If I don't hear anything back in the next days I will just merge
these patches, please comment.

On Wed, May 29, 2019 at 02:22:19PM +0200, Christoph Hellwig wrote:
> Russell,
> 
> any additional comments on this series?
> 
> On Tue, May 21, 2019 at 03:15:03PM +0200, Christoph Hellwig wrote:
> > On Tue, May 21, 2019 at 02:04:37PM +0100, Russell King - ARM Linux admin wrote:
> > > So how does the driver negotiation for >32bit addresses work if we don't
> > > fail for large masks?
> > > 
> > > I'm thinking about all those PCI drivers that need DAC cycles for >32bit
> > > addresses, such as e1000, which negotiate via (eg):
> > > 
> > >         /* there is a workaround being applied below that limits
> > >          * 64-bit DMA addresses to 64-bit hardware.  There are some
> > >          * 32-bit adapters that Tx hang when given 64-bit DMA addresses
> > >          */
> > >         pci_using_dac = 0;
> > >         if ((hw->bus_type == e1000_bus_type_pcix) &&
> > >             !dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64))) {
> > >                 pci_using_dac = 1;
> > >         } else {
> > >                 err = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
> > >                 if (err) {
> > >                         pr_err("No usable DMA config, aborting\n");
> > >                         goto err_dma;
> > >                 }
> > >         }
> > > 
> > > and similar.  If we blindly trunate the 64-bit to 32-bit, aren't we
> > > going to end up with PCI cards using DAC cycles to a host bridge that
> > > do not support DAC cycles?
> > 
> > In general PCI devices just use DAC cycles when they need it.  I only
> > know of about a handful of devices that need to negotiate their
> > addressing mode, and those already use the proper API for that, which
> > is dma_get_required_mask.
> > 
> > The e1000 example is a good case of how the old API confused people.
> > First it only sets the 64-bit mask for devices which can support it,
> > which is good, but then it sets the NETIF_F_HIGHDMA flag only if we
> > set a 64-bit mask, which is completely unrelated to the DMA mask,
> > it just means the driver can handle sk_buff fragments that do not
> > have a kernel mapping, which really is a driver and not a hardware
> > issue.
> > 
> > So what this driver really should do is something like:
> > 
> > 
> > diff --git a/drivers/net/ethernet/intel/e1000/e1000_main.c b/drivers/net/ethernet/intel/e1000/e1000_main.c
> > index 551de8c2fef2..d9236083da94 100644
> > --- a/drivers/net/ethernet/intel/e1000/e1000_main.c
> > +++ b/drivers/net/ethernet/intel/e1000/e1000_main.c
> > @@ -925,7 +925,7 @@ static int e1000_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
> >  
> >  	static int cards_found;
> >  	static int global_quad_port_a; /* global ksp3 port a indication */
> > -	int i, err, pci_using_dac;
> > +	int i, err;
> >  	u16 eeprom_data = 0;
> >  	u16 tmp = 0;
> >  	u16 eeprom_apme_mask = E1000_EEPROM_APME;
> > @@ -996,16 +996,11 @@ static int e1000_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
> >  	 * 64-bit DMA addresses to 64-bit hardware.  There are some
> >  	 * 32-bit adapters that Tx hang when given 64-bit DMA addresses
> >  	 */
> > -	pci_using_dac = 0;
> > -	if ((hw->bus_type == e1000_bus_type_pcix) &&
> > -	    !dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64))) {
> > -		pci_using_dac = 1;
> > -	} else {
> > -		err = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
> > -		if (err) {
> > -			pr_err("No usable DMA config, aborting\n");
> > -			goto err_dma;
> > -		}
> > +	err = dma_set_mask_and_coherent(&pdev->dev,
> > +		DMA_BIT_MASK(hw->bus_type == e1000_bus_type_pcix ? 64 : 32));
> > +	if (err) {
> > +		pr_err("No usable DMA config, aborting\n");
> > +		goto err_dma;
> >  	}
> >  
> >  	netdev->netdev_ops = &e1000_netdev_ops;
> > @@ -1047,19 +1042,15 @@ static int e1000_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
> >  
> >  	netdev->priv_flags |= IFF_SUPP_NOFCS;
> >  
> > -	netdev->features |= netdev->hw_features;
> > +	netdev->features |= netdev->hw_features | NETIF_F_HIGHDMA;
> >  	netdev->hw_features |= (NETIF_F_RXCSUM |
> >  				NETIF_F_RXALL |
> >  				NETIF_F_RXFCS);
> >  
> > -	if (pci_using_dac) {
> > -		netdev->features |= NETIF_F_HIGHDMA;
> > -		netdev->vlan_features |= NETIF_F_HIGHDMA;
> > -	}
> > -
> >  	netdev->vlan_features |= (NETIF_F_TSO |
> >  				  NETIF_F_HW_CSUM |
> > -				  NETIF_F_SG);
> > +				  NETIF_F_SG |
> > +				  NETIF_F_HIGHDMA);
> >  
> >  	/* Do not set IFF_UNICAST_FLT for VMWare's 82545EM */
> >  	if (hw->device_id != E1000_DEV_ID_82545EM_COPPER ||
> > 
> ---end quoted text---
---end quoted text---

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