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SubjectRe: [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---

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