linux-kernel - Re: [PATCH 1/2] dma-mapping: truncate dma masks to what dma_addr

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Date:   Wed, 29 May 2019 14:22:19 +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

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---