On Tue, Feb 01, 2022 at 04:02:48PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <[email protected]>
>
> 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.
>
> - 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.
>
> 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 <[email protected]>
> Cc: Russell King <[email protected]>
> Cc: Christoph Hellwig <[email protected]>
> Cc: Laurentiu Tudor <[email protected]>
> Cc: Alan Stern <[email protected]>
> Cc: [email protected]
> Signed-off-by: Arnd Bergmann <[email protected]>
> ---
> I don't have this hardware, so the patch is not tested at all.
> 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.
s/that //
s/in/on/
Otherwise the USB parts of this look okay to me. I don't have suitable
hardware to test either. (I wonder if anyone is still using this
platform...)
Acked-by: Alan Stern <[email protected]>
Alan Stern
On Tue, Feb 1, 2022 at 4:31 PM Alan Stern <[email protected]> wrote:
> > 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.
>
> s/that //
> s/in/on/
Fixed now.
> Otherwise the USB parts of this look okay to me. I don't have suitable
> hardware to test either. (I wonder if anyone is still using this
> platform...)
I assumed Russell was still using the Assabet, but his last upstream
commits for sa1100 are from 2016 (merged in 2019), so that may have
changed in the meantime.
> Acked-by: Alan Stern <[email protected]>
Thanks!
Arnd