[PATCH v2][RFC] OMAP4: sDMA driver: descriptor autoloading feature

----- Original Message ----- 
From: "S, Venkatraman" <redacted>
To: <redacted>
Cc: <redacted>; "Shilimkar, Santosh" 
[off-list ref]
Sent: Thursday, August 27, 2009 4:41 PM
Subject: [PATCH v2][RFC] OMAP4: sDMA driver: descriptor autoloading feature

(Updated version of previous patch: 
http://marc.info/?l=linux-omap&m=125012097403050&w=2)
Add sDMA driver support for descriptor autoloading feature.
Descriptor autoloading is OMAP4 sDMA hardware capability that can be 
exploited for scatter gather scenarios.

The feature works as described below
1) A sDMA channel is programmed to be in 'linked list' mode
2) The client (sDMA user) provides a list of descriptors in a linked list 
format
3) Each of the 'descriptor' (element in the linked list) contains an 
updated set of DMA configuration register values
4) Client starts DMA transfer
5) sDMA controller loads the first element to its register configuration 
memory and executes the transfer
6) After completion, loads the next element (in linked list) to 
configuration memory and executes the transfer, without MCU intervention.
7) Interrupt is generated after all transfers are completed; this can be 
configured to be done differently.

Configurations and additional features
1) Fast mode & non-fast mode
      Fast mode/non-fast decides on how the first transfer begins. In 
non-fast mode, the first element in the linked list is loaded only after 
completing the transfer according to the configurations already in the 
sDMA channel registers. In fast mode, the loading of the first element 
precedes the transfer.

2) Pause / resume of transfers
      A transfer can be paused after a descriptor set has been loaded, 
provided the 'pause bit' is set in the linked list element.
An ongoing transfer cannot be paused. If the 'pause bit' is set, transfer 
is not started after loading the register set from memory.
Such a transfer can be resumed later.

3) Descriptor types
      3 possible configurations of descriptors (initialized as linked list 
elements) are possible. Type 1 provides the maximum flexibility, which 
contains most register definitions of a DMA logical channel. Fewer options 
are present in type 2. Type 3 can just modify source/destinations address 
of transfers. In all transfers, unmodified registers settings are 
maintained for the next transfer.

Patch provides options / API for
1) Setting up a descriptor loading for DMA channel for sg type transfers
2) configuration with linked list elements
3) Starting / pause and resume of the said transfers, query state
4) Closing/Releasing the DMA channel

The patches are generated against kernel 2.6.31-rc1, tested on OMAP4 
simulator platform.

It might be easier to understand the APIs if you could provide an example. 
Could you take example of a contiguous/non-contiguous buffer and explain how 
the desciptor based DMA could be used on this.