[PATCH 5/6] usb: don't inherity DMA properties for USB devices

Christoph Hellwig
2020-09-14T07:47:56Z

As the comment in usb_alloc_dev correctly states, drivers can't use
the DMA API on usb device, and at least calling dma_set_mask on them
is highly dangerous. Unlike what the comment states upper level drivers
also can't really use the presence of a dma mask to check for DMA
support, as the dma_mask is set by default for most busses.

Setting the dma_mask comes from "[PATCH] usbcore dma updates (and doc)"
in BitKeeper times, as it seems like it was primarily for setting the
NETIF_F_HIGHDMA flag in USB drivers, something that has long been
fixed up since.

Setting the dma_pfn_offset comes from commit b44bbc46a8bb
("usb: core: setup dma_pfn_offset for USB devices and, interfaces"),
which worked around the fact that the scsi_calculate_bounce_limits
functions wasn't going through the proper driver interface to query
DMA information, but that function was removed in commit 21e07dba9fb1
("scsi: reduce use of block bounce buffers") years ago.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
drivers/usb/core/message.c | 6 ------
drivers/usb/core/usb.c | 12 ------------
2 files changed, 18 deletions(-)

diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index 6197938dcc2d8f..9e45732dc1d1d1 100644
--- a/drivers/usb/core/message.c
+++ b/drivers/usb/core/message.c
@@ -1954,12 +1954,6 @@ int usb_set_configuration(struct usb_device *dev, int configuration)
intf->dev.bus = &usb_bus_type;
intf->dev.type = &usb_if_device_type;
intf->dev.groups = usb_interface_groups;
- /*
- * Please refer to usb_alloc_dev() to see why we set
- * dma_mask and dma_pfn_offset.
- */
- intf->dev.dma_mask = dev->dev.dma_mask;
- intf->dev.dma_pfn_offset = dev->dev.dma_pfn_offset;
INIT_WORK(&intf->reset_ws, __usb_queue_reset_device);
intf->minor = -1;
device_initialize(&intf->dev);
diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
index bafc113f2b3ef3..9b4ac4415f1a47 100644
--- a/drivers/usb/core/usb.c
+++ b/drivers/usb/core/usb.c
@@ -599,18 +599,6 @@ struct usb_device *usb_alloc_dev(struct usb_device *parent,
dev->dev.bus = &usb_bus_type;
dev->dev.type = &usb_device_type;
dev->dev.groups = usb_device_groups;
- /*
- * Fake a dma_mask/offset for the USB device:
- * We cannot really use the dma-mapping API (dma_alloc_* and
- * dma_map_*) for USB devices but instead need to use
- * usb_alloc_coherent and pass data in 'urb's, but some subsystems
- * manually look into the mask/offset pair to determine whether
- * they need bounce buffers.
- * Note: calling dma_set_mask() on a USB device would set the
- * mask for the entire HCD, so don't do that.
- */
- dev->dev.dma_mask = bus->sysdev->dma_mask;
- dev->dev.dma_pfn_offset = bus->sysdev->dma_pfn_offset;
set_dev_node(&dev->dev, dev_to_node(bus->sysdev));
dev->state = USB_STATE_ATTACHED;
dev->lpm_disable_count = 1;
--
2.28.0