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

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Date:	Wed, 23 Jan 2013 11:36:18 +0200
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	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 Wed, Jan 23, 2013 at 11:22 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 23 January 2013 14:42, Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:

>> 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?

Okay, separately and later to avoid additional testing. Will it work for you?

>
> 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.

I found already better solution I guess.
I introduced two functions to get amount of sent bytes like

u32 calc_sent(u32 ctlhi, u32 ctllo)
{
 return f1(ctlhi) * f2(ctllo);
}

u32 get_sent(dwc)
{
 return calc_sent(read(CTL_HI), read(CTL_LO));
}

And usage like

/* initial residue */
desc->len - calc_sent(desc->lli.ctlhi, desc->lli.ctllo).

What do you think?

-- 
With Best Regards,
Andy Shevchenko
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