Re: [RFC PATCH 1/4] usb: xhci: Synopsys xHC consolidate TRBs

[Mathias Nyman <mathias.nyman@linux.intel.com>]

>> As this is extremely rare (never reported in a real use case) 
>> I'd keep this as simple as possible. Allocate one bounce buffer when needed, and copy all.
>> If memory allocation fails (unlikely) print a warning, then we immediately know what to fix.
>>
>> We are talking about sizes where 16 sg entries have in total 1024 bytes of data.
>>
> Ok, I agree.
>> ehci-tegra.c does something related to what you want. It replaces
>> urb->transfer_buffer with a dma aligned bounce buffer. 
>>
> I tried this (in hcd.c) and it seems to be working with my tests. I want to override the 
> unmap_urb_for_dma() and map_urb_for_dma() and keep all other things same as xhci driver.
> I thought to use xhci_driver_override for doing this instead of creating a whole new glue
> driver. Would that be ok ?

There is already support that allows host controller drivers to override the default
usb_hcd_map_urb_for_dma() and usb_hcd_unmap_urb_for_dma() functions.
see comments in include/linux/usb/hcd.h struct hc_driver

We recently added xhci_map_urb_for_dma() override to support Immediate Data Transfers (IDT),
I think your quirk will fit nicely there. It can be after the IDT check as your quirk is not
needed if IDT is used (one trb with up to 8 bytes of data).

you will probably need to add a xhci_unmap_urb_for_dma() callback.

-Mathias