kolibrios/kernel/trunk/bus/usb/pipe.inc

; Functions for USB pipe manipulation: opening/closing, sending data etc.
;
USB_STDCALL_VERIFY = 1
macro stdcall_verify [arg]
{
common
if USB_STDCALL_VERIFY
        pushad
        stdcall arg
        call    verify_regs
        popad
else
        stdcall arg
end if
}

; Initialization of usb_static_ep structure,
; called from controller-specific initialization; edi -> usb_static_ep
proc usb_init_static_endpoint
        mov     [edi+usb_static_ep.NextVirt], edi
        mov     [edi+usb_static_ep.PrevVirt], edi
        ret
endp

; Part of API for drivers, see documentation for USBOpenPipe.
proc usb_open_pipe stdcall uses ebx esi edi,\
 config_pipe:dword, endpoint:dword, maxpacket:dword, type:dword, interval:dword
locals
tt_vars         rd      24      ; should be enough for ehci_select_tt_interrupt_list
targetsmask     dd      ?       ; S-Mask for USB2
bandwidth       dd      ?
target          dd      ?
endl
; 1. Verify type of pipe: it must be one of *_PIPE constants.
; Isochronous pipes are not supported yet.
        mov     eax, [type]
        cmp     eax, INTERRUPT_PIPE
        ja      .badtype
        cmp     al, ISOCHRONOUS_PIPE
        jnz     .goodtype
.badtype:
        dbgstr 'unsupported type of USB pipe'
        jmp     .return0
.goodtype:
; 2. Allocate memory for pipe and transfer queue.
; Empty transfer queue consists of one inactive TD.
        mov     ebx, [config_pipe]
        mov     esi, [ebx+usb_pipe.Controller]
        mov     edx, [esi+usb_controller.HardwareFunc]
        call    [edx+usb_hardware_func.AllocPipe]
        test    eax, eax
        jz      .nothing
        mov     edi, eax
        mov     edx, [esi+usb_controller.HardwareFunc]
        call    [edx+usb_hardware_func.AllocTD]
        test    eax, eax
        jz      .free_and_return0
; 3. Initialize transfer queue: pointer to transfer descriptor,
; pointers in transfer descriptor, queue lock.
        mov     [edi+usb_pipe.LastTD], eax
        mov     [eax+usb_gtd.NextVirt], eax
        mov     [eax+usb_gtd.PrevVirt], eax
        mov     [eax+usb_gtd.Pipe], edi
        lea     ecx, [edi+usb_pipe.Lock]
        call    mutex_init
; 4. Initialize software part of pipe structure, except device-related fields.
        mov     al, byte [type]
        mov     [edi+usb_pipe.Type], al
        xor     eax, eax
        mov     [edi+usb_pipe.Flags], al
        mov     [edi+usb_pipe.DeviceData], eax
        mov     [edi+usb_pipe.Controller], esi
        or      [edi+usb_pipe.NextWait], -1
; 5. Initialize device-related fields:
; for zero endpoint, set .NextSibling = .PrevSibling = this;
; for other endpoins, copy device data, take the lock guarding pipe list
; for the device and verify that disconnect processing has not yet started
; for the device. (Since disconnect processing also takes that lock,
; either it has completed or it will not start until we release the lock.)
; Note: usb_device_disconnected should not see the new pipe until
; initialization is complete, so that lock will be held during next steps
; (disconnect processing should either not see it at all, or see fully
; initialized pipe).
        cmp     [endpoint], eax
        jz      .zero_endpoint
        mov     ecx, [ebx+usb_pipe.DeviceData]
        mov     [edi+usb_pipe.DeviceData], ecx
        call    mutex_lock
        test    [ebx+usb_pipe.Flags], USB_FLAG_CLOSED
        jz      .common
.fail:
; If disconnect processing has completed, unlock the mutex, free memory
; allocated in step 2 and return zero.
        call    mutex_unlock
        mov     edx, [esi+usb_controller.HardwareFunc]
        stdcall [edx+usb_hardware_func.FreeTD], [edi+usb_pipe.LastTD]
.free_and_return0:
        mov     edx, [esi+usb_controller.HardwareFunc]
        stdcall [edx+usb_hardware_func.FreePipe], edi
.return0:
        xor     eax, eax
        jmp     .nothing
.zero_endpoint:
        mov     [edi+usb_pipe.NextSibling], edi
        mov     [edi+usb_pipe.PrevSibling], edi
.common:
; 6. Initialize hardware part of pipe structure.
; 6a. Acquire the corresponding mutex.
        lea     ecx, [esi+usb_controller.ControlLock]
        cmp     [type], BULK_PIPE
        jb      @f      ; control pipe
        lea     ecx, [esi+usb_controller.BulkLock]
        jz      @f      ; bulk pipe
        lea     ecx, [esi+usb_controller.PeriodicLock]
@@:
        call    mutex_lock
; 6b. Let the controller-specific code do its job.
        push    ecx
        mov     edx, [esi+usb_controller.HardwareFunc]
        mov     eax, [edi+usb_pipe.LastTD]
        mov     ecx, [config_pipe]
        call    [edx+usb_hardware_func.InitPipe]
        pop     ecx
; 6c. Release the mutex.
        push    eax
        call    mutex_unlock
        pop     eax
; 6d. If controller-specific code indicates failure,
; release the lock taken in step 5, free memory allocated in step 2
; and return zero.
        test    eax, eax
        jz      .fail
; 7. The pipe is initialized. If this is not the first pipe for the device,
; insert it to the tail of pipe list for the device,
; increment number of pipes,
; release the lock taken at step 5.
        mov     ecx, [edi+usb_pipe.DeviceData]
        test    ecx, ecx
        jz      @f
        mov     eax, [ebx+usb_pipe.PrevSibling]
        mov     [edi+usb_pipe.NextSibling], ebx
        mov     [edi+usb_pipe.PrevSibling], eax
        mov     [ebx+usb_pipe.PrevSibling], edi
        mov     [eax+usb_pipe.NextSibling], edi
        inc     [ecx+usb_device_data.NumPipes]
        call    mutex_unlock
@@:
; 8. Return pointer to usb_pipe.
        mov     eax, edi
.nothing:
        ret
endp

; This procedure is called several times during initial device configuration,
; when usb_device_data structure is reallocated.
; It (re)initializes all pointers in usb_device_data.
; ebx -> usb_pipe
proc usb_reinit_pipe_list
        push    eax
; 1. (Re)initialize the lock guarding pipe list.
        mov     ecx, [ebx+usb_pipe.DeviceData]
        call    mutex_init
; 2. Initialize list of opened pipes: two entries, the head and ebx.
        add     ecx, usb_device_data.OpenedPipeList - usb_pipe.NextSibling
        mov     [ecx+usb_pipe.NextSibling], ebx
        mov     [ecx+usb_pipe.PrevSibling], ebx
        mov     [ebx+usb_pipe.NextSibling], ecx
        mov     [ebx+usb_pipe.PrevSibling], ecx
; 3. Initialize list of closed pipes: empty list, only the head is present.
        add     ecx, usb_device_data.ClosedPipeList - usb_device_data.OpenedPipeList
        mov     [ecx+usb_pipe.NextSibling], ecx
        mov     [ecx+usb_pipe.PrevSibling], ecx
        pop     eax
        ret
endp

; Part of API for drivers, see documentation for USBClosePipe.
proc usb_close_pipe
        push    ebx esi ; save used registers to be stdcall
virtual at esp
        rd      2       ; saved registers
        dd      ?       ; return address
.pipe   dd      ?
end virtual
; 1. Lock the pipe list for the device.
        mov     ebx, [.pipe]
        mov     esi, [ebx+usb_pipe.Controller]
        mov     ecx, [ebx+usb_pipe.DeviceData]
        call    mutex_lock
; 2. Set the flag "the driver has abandoned this pipe, free it at any time".
        lea     ecx, [ebx+usb_pipe.Lock]
        call    mutex_lock
        or      [ebx+usb_pipe.Flags], USB_FLAG_CAN_FREE
        call    mutex_unlock
; 3. Call the worker function.
        call    usb_close_pipe_nolock
; 4. Unlock the pipe list for the device.
        mov     ecx, [ebx+usb_pipe.DeviceData]
        call    mutex_unlock
; 5. Wakeup the USB thread so that it can proceed with releasing that pipe.
        push    edi
        call    usb_wakeup
        pop     edi
; 6. Return.
        pop     esi ebx ; restore used registers to be stdcall
        retn    4
endp

; Worker function for pipe closing. Called by usb_close_pipe API and
; from disconnect processing.
; The lock guarding pipe list for the device should be held by the caller.
; ebx -> usb_pipe, esi -> usb_controller
proc usb_close_pipe_nolock
; 1. Set the flag "pipe is closed, ignore new transfers".
; If it was already set, do nothing.
        lea     ecx, [ebx+usb_pipe.Lock]
        call    mutex_lock
        bts     dword [ebx+usb_pipe.Flags], USB_FLAG_CLOSED_BIT
        jc      .closed
        call    mutex_unlock
; 2. Remove the pipe from the list of opened pipes.
        mov     eax, [ebx+usb_pipe.NextSibling]
        mov     edx, [ebx+usb_pipe.PrevSibling]
        mov     [eax+usb_pipe.PrevSibling], edx
        mov     [edx+usb_pipe.NextSibling], eax
; 3. Unlink the pipe from hardware structures.
; 3a. Acquire the corresponding lock.
        lea     edx, [esi+usb_controller.WaitPipeListAsync]
        lea     ecx, [esi+usb_controller.ControlLock]
        cmp     [ebx+usb_pipe.Type], BULK_PIPE
        jb      @f      ; control pipe
        lea     ecx, [esi+usb_controller.BulkLock]
        jz      @f      ; bulk pipe
        add     edx, usb_controller.WaitPipeListPeriodic - usb_controller.WaitPipeListAsync
        lea     ecx, [esi+usb_controller.PeriodicLock]
@@:
        push    edx
        call    mutex_lock
        push    ecx
; 3b. Let the controller-specific code do its job.
        mov     eax, [esi+usb_controller.HardwareFunc]
        call    [eax+usb_hardware_func.UnlinkPipe]
; 3c. Release the corresponding lock.
        pop     ecx
        call    mutex_unlock
; 4. Put the pipe into wait queue.
        pop     edx
        cmp     [ebx+usb_pipe.NextWait], -1
        jz      .insert_new
        or      [ebx+usb_pipe.Flags], USB_FLAG_EXTRA_WAIT
        jmp     .inserted
.insert_new:
        mov     eax, [edx]
        mov     [ebx+usb_pipe.NextWait], eax
        mov     [edx], ebx
.inserted:
; 5. Return.
        ret
.closed:
        call    mutex_unlock
        xor     eax, eax
        ret
endp

; This procedure is called when a pipe with USB_FLAG_CLOSED is removed from the
; corresponding wait list. It means that the hardware has fully forgot about it.
; ebx -> usb_pipe, esi -> usb_controller
proc usb_pipe_closed
        push    edi
        mov     edi, [esi+usb_controller.HardwareFunc]
; 1. Loop over all transfers, calling the driver with USB_STATUS_CLOSED
; and freeing all descriptors.
        mov     edx, [ebx+usb_pipe.LastTD]
        test    edx, edx
        jz      .no_transfer
        mov     edx, [edx+usb_gtd.NextVirt]
.transfer_loop:
        cmp     edx, [ebx+usb_pipe.LastTD]
        jz      .transfer_done
        mov     ecx, [edx+usb_gtd.Callback]
        test    ecx, ecx
        jz      .no_callback
        push    edx
        stdcall_verify ecx, ebx, USB_STATUS_CLOSED, \
                [edx+usb_gtd.Buffer], 0, [edx+usb_gtd.UserData]
        pop     edx
.no_callback:
        push    [edx+usb_gtd.NextVirt]
        stdcall [edi+usb_hardware_func.FreeTD], edx
        pop     edx
        jmp     .transfer_loop
.transfer_done:
        stdcall [edi+usb_hardware_func.FreeTD], edx
.no_transfer:
; 2. Decrement number of pipes for the device.
; If this pipe is the last pipe, go to 5.
        mov     ecx, [ebx+usb_pipe.DeviceData]
        call    mutex_lock
        dec     [ecx+usb_device_data.NumPipes]
        jz      .last_pipe
        call    mutex_unlock
; 3. If the flag "the driver has abandoned this pipe" is set,
; free memory and return.
        test    [ebx+usb_pipe.Flags], USB_FLAG_CAN_FREE
        jz      .nofree
        stdcall [edi+usb_hardware_func.FreePipe], ebx
        pop     edi
        ret
; 4. Otherwise, add it to the list of closed pipes and return.
.nofree:
        add     ecx, usb_device_data.ClosedPipeList - usb_pipe.NextSibling
        mov     edx, [ecx+usb_pipe.PrevSibling]
        mov     [ebx+usb_pipe.NextSibling], ecx
        mov     [ebx+usb_pipe.PrevSibling], edx
        mov     [ecx+usb_pipe.PrevSibling], ebx
        mov     [edx+usb_pipe.NextSibling], ebx
        pop     edi
        ret
.last_pipe:
; That was the last pipe for the device.
; 5. Notify device driver(s) about disconnect.
        call    mutex_unlock
        mov     eax, [ecx+usb_device_data.NumInterfaces]
        test    eax, eax
        jz      .notify_done
        add     ecx, [ecx+usb_device_data.Interfaces]
.notify_loop:
        mov     edx, [ecx+usb_interface_data.DriverFunc]
        test    edx, edx
        jz      @f
        mov     edx, [edx+USBSRV.usb_func]
        cmp     [edx+USBFUNC.strucsize], USBFUNC.device_disconnect + 4
        jb      @f
        mov     edx, [edx+USBFUNC.device_disconnect]
        test    edx, edx
        jz      @f
        push    eax ecx
        stdcall_verify edx, [ecx+usb_interface_data.DriverData]
        pop     ecx eax
@@:
        add     ecx, sizeof.usb_interface_data
        dec     eax
        jnz     .notify_loop
.notify_done:
; 6. Bus address, if assigned, can now be reused.
        call    [edi+usb_hardware_func.GetDeviceAddress]
        test    eax, eax
        jz      @f
        bts     [esi+usb_controller.ExistingAddresses], eax
@@:
        dbgstr 'USB device disconnected'
; 7. All drivers have returned from disconnect callback,
; so all drivers should not use any device-related pipes.
; Free the remaining pipes.
        mov     eax, [ebx+usb_pipe.DeviceData]
        add     eax, usb_device_data.ClosedPipeList - usb_pipe.NextSibling
        push    eax
        mov     eax, [eax+usb_pipe.NextSibling]
.free_loop:
        cmp     eax, [esp]
        jz      .free_done
        push    [eax+usb_pipe.NextSibling]
        stdcall [edi+usb_hardware_func.FreePipe], eax
        pop     eax
        jmp     .free_loop
.free_done:
        stdcall [edi+usb_hardware_func.FreePipe], ebx
        pop     eax
; 8. Free the usb_device_data structure.
        sub     eax, usb_device_data.ClosedPipeList - usb_pipe.NextSibling
        call    free
; 9. Return.
.nothing:
        pop     edi
        ret
endp

; Part of API for drivers, see documentation for USBNormalTransferAsync.
proc usb_normal_transfer_async stdcall uses ebx edi,\
 pipe:dword, buffer:dword, size:dword, callback:dword, calldata:dword, flags:dword
; 1. Sanity check: callback must be nonzero.
; (It is important for other parts of code.)
        xor     eax, eax
        cmp     [callback], eax
        jz      .nothing
; 2. Lock the transfer queue.
        mov     ebx, [pipe]
        lea     ecx, [ebx+usb_pipe.Lock]
        call    mutex_lock
; 3. If the pipe has already been closed (presumably due to device disconnect),
; release the lock taken in step 2 and return zero.
        xor     eax, eax
        test    [ebx+usb_pipe.Flags], USB_FLAG_CLOSED
        jnz     .unlock
; 4. Allocate and initialize TDs for the transfer.
        mov     edx, [ebx+usb_pipe.Controller]
        mov     edi, [edx+usb_controller.HardwareFunc]
        stdcall [edi+usb_hardware_func.AllocTransfer], [buffer], [size], [flags], [ebx+usb_pipe.LastTD], 0
; If failed, release the lock taken in step 2 and return zero.
        test    eax, eax
        jz      .unlock
; 5. Store callback and its parameters in the last descriptor for this transfer.
        mov     ecx, [eax+usb_gtd.PrevVirt]
        mov     edx, [callback]
        mov     [ecx+usb_gtd.Callback], edx
        mov     edx, [calldata]
        mov     [ecx+usb_gtd.UserData], edx
        mov     edx, [buffer]
        mov     [ecx+usb_gtd.Buffer], edx
; 6. Advance LastTD pointer and activate transfer.
        push    [ebx+usb_pipe.LastTD]
        mov     [ebx+usb_pipe.LastTD], eax
        call    [edi+usb_hardware_func.InsertTransfer]
        pop     eax
; 7. Release the lock taken in step 2 and
; return pointer to the first descriptor for the new transfer.
.unlock:
        push    eax
        lea     ecx, [ebx+usb_pipe.Lock]
        call    mutex_unlock
        pop     eax
.nothing:
        ret
endp

; Part of API for drivers, see documentation for USBControlTransferAsync.
proc usb_control_async stdcall uses ebx edi,\
 pipe:dword, config:dword, buffer:dword, size:dword, callback:dword, calldata:dword, flags:dword
locals
last_td         dd      ?
endl
; 1. Sanity check: callback must be nonzero.
; (It is important for other parts of code.)
        cmp     [callback], 0
        jz      .return0
; 2. Lock the transfer queue.
        mov     ebx, [pipe]
        lea     ecx, [ebx+usb_pipe.Lock]
        call    mutex_lock
; 3. If the pipe has already been closed (presumably due to device disconnect),
; release the lock taken in step 2 and return zero.
        test    [ebx+usb_pipe.Flags], USB_FLAG_CLOSED
        jnz     .unlock_return0
; A control transfer contains two or three stages:
; Setup stage, optional Data stage, Status stage.
; 4. Allocate and initialize TDs for the Setup stage.
; Payload is 8 bytes from [config].
        mov     edx, [ebx+usb_pipe.Controller]
        mov     edi, [edx+usb_controller.HardwareFunc]
        stdcall [edi+usb_hardware_func.AllocTransfer], [config], 8, 0, [ebx+usb_pipe.LastTD], (2 shl 2) + 0
                ; short transfer is an error, direction is DATA0, token is SETUP
        mov     [last_td], eax
        test    eax, eax
        jz      .fail
; 5. Allocate and initialize TDs for the Data stage, if [size] is nonzero.
; Payload is [size] bytes from [buffer].
        mov     edx, [config]
        mov     ecx, (3 shl 2) + 1      ; DATA1, token is OUT
        cmp     byte [edx], 0
        jns     @f
        cmp     [size], 0
        jz      @f
        inc     ecx     ; token is IN
@@:
        cmp     [size], 0
        jz      .nodata
        push    ecx
        stdcall [edi+usb_hardware_func.AllocTransfer], [buffer], [size], [flags], eax, ecx
        pop     ecx
        test    eax, eax
        jz      .fail
        mov     [last_td], eax
.nodata:
; 6. Allocate and initialize TDs for the Status stage.
; No payload.
        xor     ecx, 3  ; IN becomes OUT, OUT becomes IN
        stdcall [edi+usb_hardware_func.AllocTransfer], 0, 0, 0, eax, ecx
        test    eax, eax
        jz      .fail
; 7. Store callback and its parameters in the last descriptor for this transfer.
        mov     ecx, [eax+usb_gtd.PrevVirt]
        mov     edx, [callback]
        mov     [ecx+usb_gtd.Callback], edx
        mov     edx, [calldata]
        mov     [ecx+usb_gtd.UserData], edx
        mov     edx, [buffer]
        mov     [ecx+usb_gtd.Buffer], edx
; 8. Advance LastTD pointer and activate transfer.
        push    [ebx+usb_pipe.LastTD]
        mov     [ebx+usb_pipe.LastTD], eax
        call    [edi+usb_hardware_func.InsertTransfer]
; 9. Release the lock taken in step 2 and
; return pointer to the first descriptor for the new transfer.
        lea     ecx, [ebx+usb_pipe.Lock]
        call    mutex_unlock
        pop     eax
        ret
.fail:
        mov     eax, [last_td]
        test    eax, eax
        jz      .unlock_return0
        stdcall usb_undo_tds, [ebx+usb_pipe.LastTD]
.unlock_return0:
        lea     ecx, [ebx+usb_pipe.Lock]
        call    mutex_unlock
.return0:
        xor     eax, eax
        ret
endp

; Part of API for drivers, see documentation for USBGetParam.
proc usb_get_param
virtual at esp
                dd      ?       ; return address
.pipe           dd      ?
.param          dd      ?
end virtual
        mov     edx, [.param]
        mov     ecx, [.pipe]
        mov     eax, [ecx+usb_pipe.DeviceData]
        test    edx, edx
        jz      .get_device_descriptor
        dec     edx
        jz      .get_config_descriptor
        dec     edx
        jz      .get_speed
        or      eax, -1
        ret     8
.get_device_descriptor:
        add     eax, usb_device_data.DeviceDescriptor
        ret     8
.get_config_descriptor:
        movzx   ecx, [eax+usb_device_data.DeviceDescrSize]
        lea     eax, [eax+ecx+usb_device_data.DeviceDescriptor]
        ret     8
.get_speed:
        movzx   eax, [eax+usb_device_data.Speed]
        ret     8
endp

; Initialize software part of usb_gtd. Called from controller-specific code
; somewhere in AllocTransfer with eax -> next (inactive) usb_gtd,
; ebx -> usb_pipe, ebp frame from call to AllocTransfer with [.td] ->
; current (initializing) usb_gtd.
; Returns ecx = [.td].
proc usb_init_transfer
virtual at ebp-4
.Size   dd      ?
        rd      2
.Buffer dd      ?
        dd      ?
.Flags  dd      ?
.td     dd      ?
end virtual
        mov     [eax+usb_gtd.Pipe], ebx
        mov     ecx, [.td]
        mov     [eax+usb_gtd.PrevVirt], ecx
        mov     edx, [ecx+usb_gtd.NextVirt]
        mov     [ecx+usb_gtd.NextVirt], eax
        mov     [eax+usb_gtd.NextVirt], edx
        mov     [edx+usb_gtd.PrevVirt], eax
        mov     edx, [.Size]
        mov     [ecx+usb_gtd.Length], edx
        xor     edx, edx
        mov     [ecx+usb_gtd.Callback], edx
        mov     [ecx+usb_gtd.UserData], edx
        ret
endp

; Free all TDs for the current transfer if something has failed
; during initialization (e.g. no memory for the next TD).
; Stdcall with one stack argument = first TD for the transfer
; and eax = last initialized TD for the transfer.
proc usb_undo_tds
        push    [eax+usb_gtd.NextVirt]
@@:
        cmp     eax, [esp+8]
        jz      @f
        push    [eax+usb_gtd.PrevVirt]
        stdcall [edi+usb_hardware_func.FreeTD], eax
        pop     eax
        jmp     @b
@@:
        pop     ecx
        mov     [eax+usb_gtd.NextVirt], ecx
        mov     [ecx+usb_gtd.PrevVirt], eax
        ret     4
endp

; Helper procedure for handling short packets in controller-specific code.
; Returns with CF cleared if this is the final packet in some stage:
; for control transfers that means one of Data and Status stages,
; for other transfers - the final packet in the only stage.
proc usb_is_final_packet
        cmp     [ebx+usb_gtd.Callback], 0
        jnz     .nothing
        mov     eax, [ebx+usb_gtd.NextVirt]
        cmp     [eax+usb_gtd.Callback], 0
        jz      .stc
        mov     eax, [ebx+usb_gtd.Pipe]
        cmp     [eax+usb_pipe.Type], CONTROL_PIPE
        jz      .nothing
.stc:
        stc
.nothing:
        ret
endp

; Helper procedure for controller-specific code:
; removes one TD from the transfer queue, ebx -> usb_gtd to remove.
proc usb_unlink_td
        mov     ecx, [ebx+usb_gtd.Pipe]
        add     ecx, usb_pipe.Lock
        call    mutex_lock
        mov     eax, [ebx+usb_gtd.PrevVirt]
        mov     edx, [ebx+usb_gtd.NextVirt]
        mov     [edx+usb_gtd.PrevVirt], eax
        mov     [eax+usb_gtd.NextVirt], edx
        call    mutex_unlock
        ret
endp

; One part of transfer is completed, run the associated callback
; or update total length in the next part of transfer.
; in: ebx -> usb_gtd, ecx = status, edx = length
proc usb_process_gtd
; 1. Test whether it is the last descriptor in the transfer
; <=> it has an associated callback.
        mov     eax, [ebx+usb_gtd.Callback]
        test    eax, eax
        jz      .nocallback
; 2. It has an associated callback; call it with corresponding parameters.
        stdcall_verify eax, [ebx+usb_gtd.Pipe], ecx, \
                [ebx+usb_gtd.Buffer], edx, [ebx+usb_gtd.UserData]
        ret
.nocallback:
; 3. It is an intermediate descriptor. Add its length to the length
; in the following descriptor.
        mov     eax, [ebx+usb_gtd.NextVirt]
        add     [eax+usb_gtd.Length], edx
        ret
endp

if USB_STDCALL_VERIFY
proc verify_regs
virtual at esp
        dd      ?       ; return address
.edi    dd      ?
.esi    dd      ?
.ebp    dd      ?
.esp    dd      ?
.ebx    dd      ?
.edx    dd      ?
.ecx    dd      ?
.eax    dd      ?
end virtual
        cmp     ebx, [.ebx]
        jz      @f
        dbgstr 'ERROR!!! ebx changed'
@@:
        cmp     esi, [.esi]
        jz      @f
        dbgstr 'ERROR!!! esi changed'
@@:
        cmp     edi, [.edi]
        jz      @f
        dbgstr 'ERROR!!! edi changed'
@@:
        cmp     ebp, [.ebp]
        jz      @f
        dbgstr 'ERROR!!! ebp changed'
@@:
        ret
endp
end if