Re: [RFC] ARM: sa1100/assabet: move dmabounce hack to ohci driver

From: Robin Murphy
Date: Tue Feb 01 2022 - 12:10:51 EST

On 2022-02-01 15:02, Arnd Bergmann wrote:
From: Arnd Bergmann <arnd@xxxxxxxx>

The sa1111 platform is one of the two remaining users of the old Arm
specific "dmabounce" code, which is an earlier implementation of the
generic swiotlb.

Linus Walleij submitted a patch that removes dmabounce support from
the ixp4xx, and I had a look at the other user, which is the sa1111
companion chip.

Looking at how dmabounce is used, I could narrow it down to one driver
one one machine:

- dmabounce is only initialized on assabet and pfs168, but not on
any other sa1100 or pxa platform using sa1111.

Hmm, my reading of it was different. AFAICS it should affect all platforms with CONFIG_ARCH_SA1100 + CONFIG_SA1111 - the bus notifier from sa1111_init() will initialise dmabounce for everything where sa1111_configure_smc() has punched a hole in the DMA mask to handle the addressing erratum. sa1111_needs_bounce() looks to be a further consideration for platforms where DMA *additionally* cannot target an entire bank of memory at all.

- pfs168 is not supported in mainline Linux.

- only the OHCI and audio devices on sa1111 support DMA

- There is no audio driver for this hardware

In the OHCI code, I noticed that two other platforms already have
a local bounce buffer support in the form of the "local_mem"
allocator. Specifically, TMIO and SM501 use this on a few other ARM
boards with 16KB or 128KB of local SRAM that can be accessed from the
OHCI and from the CPU.

While this is not the same problem as on sa1111, I could not find a
reason why we can't re-use the existing implementation but replace the
physical SRAM address mapping with a locally allocated DMA buffer.

There are two main downsides:

- rather than using a dynamically sized pool, this buffer needs
to be allocated at probe time using a fixed size. Without
having any idea of what it should be, I picked a size of
64KB, which is between what the other two OHCI front-ends use
in their SRAM. If anyone has a better idea what that size
is reasonable, this can be trivially changed.

- Previously, only USB transfers to the second memory bank
on Assabet needed to go through the bounce buffer, now all
of them do, which may impact runtime performance, depending
on what type of device is attached.

As above, bouncing should also happen for every other 1/8/16/32MB (depending on configuration) of memory, see the "Figure out if we need to bounce from the DMA mask" case in needs_bounce().

Thanks,
Robin.

On the upside, the local_mem support uses write-combining
buffers, which should be a bit faster for transfers to the device
compared to normal uncached coherent memory as used in dmabounce.

Cc: Linus Walleij <linus.walleij@xxxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Laurentiu Tudor <laurentiu.tudor@xxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: linux-usb@xxxxxxxxxxxxxxx
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
I don't have this hardware, so the patch is not tested at all.
---
arch/arm/common/Kconfig | 1 -
arch/arm/common/sa1111.c | 64 ----------------------------------
drivers/usb/core/hcd.c | 17 +++++++--
drivers/usb/host/ohci-sa1111.c | 16 +++++++++
4 files changed, 30 insertions(+), 68 deletions(-)

diff --git a/arch/arm/common/Kconfig b/arch/arm/common/Kconfig
index c8e198631d41..286ea014c015 100644
--- a/arch/arm/common/Kconfig
+++ b/arch/arm/common/Kconfig
@@ -1,7 +1,6 @@
# SPDX-License-Identifier: GPL-2.0
config SA1111
bool
- select DMABOUNCE if !ARCH_PXA
config DMABOUNCE
bool
diff --git a/arch/arm/common/sa1111.c b/arch/arm/common/sa1111.c
index 7df003b149c6..a00915883f78 100644
--- a/arch/arm/common/sa1111.c
+++ b/arch/arm/common/sa1111.c
@@ -1391,70 +1391,9 @@ void sa1111_driver_unregister(struct sa1111_driver *driver)
}
EXPORT_SYMBOL(sa1111_driver_unregister);
-#ifdef CONFIG_DMABOUNCE
-/*
- * According to the "Intel StrongARM SA-1111 Microprocessor Companion
- * Chip Specification Update" (June 2000), erratum #7, there is a
- * significant bug in the SA1111 SDRAM shared memory controller. If
- * an access to a region of memory above 1MB relative to the bank base,
- * it is important that address bit 10 _NOT_ be asserted. Depending
- * on the configuration of the RAM, bit 10 may correspond to one
- * of several different (processor-relative) address bits.
- *
- * This routine only identifies whether or not a given DMA address
- * is susceptible to the bug.
- *
- * This should only get called for sa1111_device types due to the
- * way we configure our device dma_masks.
- */
-static int sa1111_needs_bounce(struct device *dev, dma_addr_t addr, size_t size)
-{
- /*
- * Section 4.6 of the "Intel StrongARM SA-1111 Development Module
- * User's Guide" mentions that jumpers R51 and R52 control the
- * target of SA-1111 DMA (either SDRAM bank 0 on Assabet, or
- * SDRAM bank 1 on Neponset). The default configuration selects
- * Assabet, so any address in bank 1 is necessarily invalid.
- */
- return (machine_is_assabet() || machine_is_pfs168()) &&
- (addr >= 0xc8000000 || (addr + size) >= 0xc8000000);
-}
-
-static int sa1111_notifier_call(struct notifier_block *n, unsigned long action,
- void *data)
-{
- struct sa1111_dev *dev = to_sa1111_device(data);
-
- switch (action) {
- case BUS_NOTIFY_ADD_DEVICE:
- if (dev->dev.dma_mask && dev->dma_mask < 0xffffffffUL) {
- int ret = dmabounce_register_dev(&dev->dev, 1024, 4096,
- sa1111_needs_bounce);
- if (ret)
- dev_err(&dev->dev, "failed to register with dmabounce: %d\n", ret);
- }
- break;
-
- case BUS_NOTIFY_DEL_DEVICE:
- if (dev->dev.dma_mask && dev->dma_mask < 0xffffffffUL)
- dmabounce_unregister_dev(&dev->dev);
- break;
- }
- return NOTIFY_OK;
-}
-
-static struct notifier_block sa1111_bus_notifier = {
- .notifier_call = sa1111_notifier_call,
-};
-#endif
-
static int __init sa1111_init(void)
{
int ret = bus_register(&sa1111_bus_type);
-#ifdef CONFIG_DMABOUNCE
- if (ret == 0)
- bus_register_notifier(&sa1111_bus_type, &sa1111_bus_notifier);
-#endif
if (ret == 0)
platform_driver_register(&sa1111_device_driver);
return ret;
@@ -1463,9 +1402,6 @@ static int __init sa1111_init(void)
static void __exit sa1111_exit(void)
{
platform_driver_unregister(&sa1111_device_driver);
-#ifdef CONFIG_DMABOUNCE
- bus_unregister_notifier(&sa1111_bus_type, &sa1111_bus_notifier);
-#endif
bus_unregister(&sa1111_bus_type);
}
diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
index 3c7c64ff3c0a..5f2fa46c7958 100644
--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -1260,7 +1260,8 @@ void usb_hcd_unlink_urb_from_ep(struct usb_hcd *hcd, struct urb *urb)
EXPORT_SYMBOL_GPL(usb_hcd_unlink_urb_from_ep);
/*
- * Some usb host controllers can only perform dma using a small SRAM area.
+ * Some usb host controllers can only perform dma using a small SRAM area,
+ * or that have restrictions in addressable DRAM.
* The usb core itself is however optimized for host controllers that can dma
* using regular system memory - like pci devices doing bus mastering.
*
@@ -3095,8 +3096,18 @@ int usb_hcd_setup_local_mem(struct usb_hcd *hcd, phys_addr_t phys_addr,
if (IS_ERR(hcd->localmem_pool))
return PTR_ERR(hcd->localmem_pool);
- local_mem = devm_memremap(hcd->self.sysdev, phys_addr,
- size, MEMREMAP_WC);
+ /*
+ * if a physical SRAM address was passed, map it, otherwise
+ * allocate system memory as a buffer.
+ */
+ if (phys_addr)
+ local_mem = devm_memremap(hcd->self.sysdev, phys_addr,
+ size, MEMREMAP_WC);
+ else
+ local_mem = dmam_alloc_attrs(hcd->self.sysdev, size, &dma,
+ GFP_KERNEL,
+ DMA_ATTR_WRITE_COMBINE);
+
if (IS_ERR(local_mem))
return PTR_ERR(local_mem);
diff --git a/drivers/usb/host/ohci-sa1111.c b/drivers/usb/host/ohci-sa1111.c
index 137f66f6977f..488033f2e144 100644
--- a/drivers/usb/host/ohci-sa1111.c
+++ b/drivers/usb/host/ohci-sa1111.c
@@ -206,6 +206,22 @@ static int ohci_hcd_sa1111_probe(struct sa1111_dev *dev)
goto err1;
}
+ /*
+ * Section 4.6 of the "Intel StrongARM SA-1111 Development Module
+ * User's Guide" mentions that jumpers R51 and R52 control the
+ * target of SA-1111 DMA (either SDRAM bank 0 on Assabet, or
+ * SDRAM bank 1 on Neponset). The default configuration selects
+ * Assabet, so any address in bank 1 is necessarily invalid.
+ *
+ * As a workaround, use a bounce buffer in addressable memory
+ * as local_mem.
+ */
+ if (machine_is_assabet()) {
+ ret = usb_hcd_setup_local_mem(hcd, 0, 0, SZ_64K);
+ if (ret)
+ goto out_err1;
+ }
+
if (!request_mem_region(hcd->rsrc_start, hcd->rsrc_len, hcd_name)) {
dev_dbg(&dev->dev, "request_mem_region failed\n");
ret = -EBUSY;