linux-kernel - Re: [PATCH 2/2] dw_dmac: return proper residue value

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Date:	Wed, 23 Jan 2013 14:52:17 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	Vinod Koul <vinod.koul@...el.com>, linux-kernel@...r.kernel.org,
	spear-devel <spear-devel@...t.st.com>
Subject: Re: [PATCH 2/2] dw_dmac: return proper residue value

On 23 January 2013 14:42, Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
> Ah, you mean the dma_cookie_status returns DMA_SUCCESS in case
> active_list is empty, don't you?

Bingo!!

> Yesterday I spent in testing of different approaches. First of all, the
> calculation of the sent amount is nicer when ctlhi/ctllo is used and it
> works fine. But the approach, when we substract length of sent
> descriptors, is losing data. Namely, we have no information how big is
> the master descriptor (it has total length of the entire chain). So, I'm
> thinking how to solve this one. Otherwise it seems simpler than mine
> first idea.

I was planning to give you another review comment which i thought i will
give later :)

It looks to be a bit wrong to have individual lengths in all descriptors leaving
the first one :)

This is the descriptor struct:

struct dw_desc {
	/* FIRST values the hardware uses */
	struct dw_lli			lli;

	/* THEN values for driver housekeeping */
	struct list_head		desc_node;
	struct list_head		tx_list;
	struct dma_async_tx_descriptor	txd;
	size_t				len;
};

Firstly i believe we can have a union of both list_heads as only one is used
at any time. Can you patch it down?

Second to solve your problem, you can add another field here: total_len.
I know it will consume 4 bytes per desc but i think that's the drawback we need
to accept.
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