linux-kernel - Re: [PATCH v3] dmaengine: rcar-dmac: use TCRB instead of TCR for residue

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Date:   Fri, 20 Oct 2017 11:45:16 +0530
From:   Vinod Koul <vinod.koul@...el.com>
To:     Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Niklas Söderlund 
        <niklas.soderlund+renesas@...natech.se>, dmaengine@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@...esas.com>
Subject: Re: [PATCH v3] dmaengine: rcar-dmac: use TCRB instead of TCR for
 residue

On Thu, Oct 19, 2017 at 01:15:13AM +0000, Kuninori Morimoto wrote:
> From: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@...esas.com>
> 
> SYS/RT/Audio DMAC includes independent data buffers for reading
> and writing. Therefore, the read transfer counter and write transfer
> counter have different values.
> TCR indicates read counter, and TCRB indicates write counter.
> The relationship is like below.
> 
>         TCR       TCRB
> [SOURCE] -> [DMAC] -> [SINK]
> 
> In the MEM_TO_DEV direction, what really matters is how much data has
> been written to the device. If the DMA is interrupted between read and
> write, then, the data doesn't end up in the destination, so shouldn't
> be counted. TCRB is thus the register we should use in this cases.
> 
> In the DEV_TO_MEM direction, the situation is more complex. Both the
> read and write side are important. What matters from a data consumer
> point of view is how much data has been written to memory.
> On the other hand, if the transfer is interrupted between read and
> write, we'll end up losing data. It can also be important to report.
> 
> In the MEM_TO_MEM direction, what matters is of course how much data
> has been written to memory from data consumer point of view.
> Here, because read and write have independent data buffers, it will
> take a while for TCR and TCRB to become equal. Thus we should check
> TCRB in this case, too.
> 
> Thus, all cases we should check TCRB instead of TCR.
> 
> Without this patch, Sound Capture has noise after PluseAudio support
> (= 07b7acb51d2 ("ASoC: rsnd: update pointer more accurate")), because
> the recorder will use wrong residue counter which indicates transferred
> from sound device, but in reality the data was not yet put to memory
> and recorder will record it.

Applied, thanks.

-- 
~Vinod