linux-kernel - Re: [PATCH v2 0/3] dmaengine: add per channel capabilities api

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Date:	Mon, 28 Jan 2013 02:06:03 -0800
From:	Vinod Koul <vinod.koul@...el.com>
To:	Matt Porter <mporter@...com>
Cc:	Dan Williams <djbw@...com>, Chris Ball <cjb@...top.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Linux DaVinci Kernel List 
	<davinci-linux-open-source@...ux.davincidsp.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux MMC List <linux-mmc@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] dmaengine: add per channel capabilities api

On Mon, Jan 21, 2013 at 01:19:23PM -0500, Matt Porter wrote:
> > b) Sg segment length and numbers: Well these are capabilities, so it tells
> > you what is the maximum I can do. IMO it doesn't make sense to tie it down to
> > burst, width etc. For that configuration you are checking maximum. What this
> > needs to return is what is the maximum length it supports and maximum number of
> > sg list the h/w can use. Also if you return your burst and width capablity, then
> > any client can easily find out what is the length byte value it can hold.
>  
> Ok, so I could report the max burst size to the client, but on EDMA it's
> going to be SZ_64K-1, as that's the hw limit. Max addr width would also
> be the same or capped at the maximum enum the current API support of 8
> bytes.
Yes IMO you should report the h/w limit and let client deduce what can be done
with that h/w limit given the other parameters (burst, length etc...)
> 
> This information is not useful to the client in my case. Only the client
> knows its FIFO size, and thus the actual burst size it needs to request
> as that is a value specific to the IP the client driver controls. So it
> knows actual burst size and the width of that fifo transfer, because we
> already support telling the dmaengine driver this info in the current
> API.
Your current API way is not very generic. We need to make sure it useful on
other systems too. That is why it would make sense to query the DMA H/W
capablities and then use it with above properties to get various parameters used
for initiating a transfer, that way you dont need to set slave parameters

--
~Vinod
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