Date:   Thu, 3 Feb 2022 09:47:56 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        linux-usb@...r.kernel.org, Alan Stern <stern@...land.harvard.edu>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [v2] ARM: sa1100/assabet: move dmabounce hack to ohci driver

On Thu, 3 Feb 2022 at 09:38, Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.de>
>
> 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 three machines:
>
>  - dmabounce is only initialized on assabet/neponset, jornada720 and
>    badge4, which are the platforms that have an sa1111 and support
>    DMA on it.
>
>  - All three of these suffer from "erratum #7" that requires only
>    doing DMA to half the memory sections based on one of the address
>    lines, in addition, the neponset also can't DMA to the RAM that
>    is connected to sa1111 itself.
>
>  - the pxa lubbock machine also has sa1111, but does not support DMA
>    on it and does not set dmabounce.
>
>  - only the OHCI and audio devices on sa1111 support DMA, but as
>    there is no audio driver for this hardware, only OHCI remains.
>
> 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.
>

I suppose this is a problem if the driver falls back to ordinary DRAM
once the allocation runs out?

>  - Previously, only USB transfers to unaddressable memory needed
>    to go through the bounce buffer, now all of them do, which may
>    impact runtime performance for USB endpoints that do a lot of
>    transfers.
>
> 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.
>

Talking from past experience using this trick on a NXP ARM9 SoC ~10
years ago, using on-chip SRAM for USB DMA likely results in a
significant performance boost, even without write combining, although
the exact scenario obviously matters.


> Cc: Linus Walleij <linus.walleij@...aro.org>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Christoph Hellwig <hch@...radead.org>
> Cc: Laurentiu Tudor <laurentiu.tudor@....com>
> Cc: linux-usb@...r.kernel.org
> Acked-by: Alan Stern <stern@...land.harvard.edu>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> Changes in v2:
>
>  - drop check for assabet, as bounce buffers are required on
>    all sa1100 machines
>  - select CONFIG_ZONE_DMA again
>  - update comments and changelog text based on discussion
> ---
>  arch/arm/common/Kconfig        |  2 +-
>  arch/arm/common/sa1111.c       | 64 ----------------------------------
>  drivers/usb/core/hcd.c         | 17 +++++++--
>  drivers/usb/host/ohci-sa1111.c | 25 +++++++++++++
>  4 files changed, 40 insertions(+), 68 deletions(-)
>
> diff --git a/arch/arm/common/Kconfig b/arch/arm/common/Kconfig
> index c8e198631d41..bc158fd227e1 100644
> --- a/arch/arm/common/Kconfig
> +++ b/arch/arm/common/Kconfig
> @@ -1,7 +1,7 @@
>  # SPDX-License-Identifier: GPL-2.0
>  config SA1111
>         bool
> -       select DMABOUNCE if !ARCH_PXA
> +       select ZONE_DMA if ARCH_SA1100
>
>  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..8417baedc89c 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 have restrictions on 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..0da2badf0658 100644
> --- a/drivers/usb/host/ohci-sa1111.c
> +++ b/drivers/usb/host/ohci-sa1111.c
> @@ -206,6 +206,31 @@ static int ohci_hcd_sa1111_probe(struct sa1111_dev *dev)
>                 goto err1;
>         }
>
> +       /*
> +        * 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.
> +        *
> +        * 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, relying on ZONE_DMA to provide an area that
> +        * fits within the above constraints.
> +        *
> +        * SZ_64K is an estimate for what size this might need.
> +        */
> +       ret = usb_hcd_setup_local_mem(hcd, 0, 0, SZ_64K);
> +       if (ret)
> +               goto err1;
> +
>         if (!request_mem_region(hcd->rsrc_start, hcd->rsrc_len, hcd_name)) {
>                 dev_dbg(&dev->dev, "request_mem_region failed\n");
>                 ret = -EBUSY;
> --
> 2.29.2
>
>
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