Messages in this thread |  | | | Subject | Re: [PATCH 10/18] dmaengine/amba-pl08x: Get rid of pl08x_pre_boundary() | | From | "Koul, Vinod" <> | | Date | Fri, 29 Jul 2011 18:02:57 +0530 |
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On Fri, 2011-07-29 at 12:05 +0100, Russell King - ARM Linux wrote: > On Fri, Jul 29, 2011 at 04:19:20PM +0530, Viresh Kumar wrote: > > Pl080 Manual says: "Bursts do not cross the 1KB address boundary" > > > > We can program the controller to cross 1 KB boundary on a burst and controller > > can take care of this boundary condition by itself. > > > > Following is the discussion with ARM Technical Support Guys (David): > > [Viresh] Manual says: "Bursts do not cross the 1KB address boundary" > > > > What does that actually mean? As, Maximum size transferable with a single LLI is > > 4095 * 4 =16380 ~ 16KB. So, if we don't have src/dest address aligned to burst > > size, we can't use this big of an LLI. > > > > [David] There is a difference between bursts describing the total data > > transferred by the DMA controller and AHB bursts. Bursts described by the > > programmable parameters in the PL080 have no direct connection with the bursts > > that are seen on the AHB bus. > > > > The statement that "Bursts do not cross the 1KB address boundary" in the TRM is > > referring to AHB bursts, where this limitation is a requirement of the AHB spec. > > You can still issue bursts within the PL080 that are in excess o f 1KB. The > > PL080 will make sure that its bursts are broken down into legal AHB bursts which > > will be formatted to ensure that no AHB burst crosses a 1KB boundary. > > > > Based on above discussion, this patch removes all code related to 1 KB boundary > > as we are not required to handle this in driver. > > Good. I'm glad someone's finally getting to the bottom of what's > actually required and what isn't... This driver has been a mess for > quite a long time and is in desperate need of these kinds of cleanups. Typically in few dmac IPs i know of: There is max limitation of what a burst length can be for dma, and what a max transfer length can be. The dma will push/pull data into peripheral FIFO based on max_burst_length parameter (note length in dma terminology is always items not bytes, and one item is usually defined as transfer_length/src_width). For a transfer (single block or in multi-block) the dmac will break the transaction into multiple bursts and based on max_burst_length it will push/pull data to/from FIFO. Also matching FIFO thresholds with max_burst_length is also important and can be tuned based on perf/power requirements.
Sharing my knowledge, hopefully it helps :)
-- ~Vinod
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