linux-kernel - Re: [PATCH v1] dmaengine: tegra-apb: Support per-burst residue granularity

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Message-ID: <d34c100d-e82a-bb00-22c6-c5f2f6cdb03a@nvidia.com>
Date:   Mon, 17 Jun 2019 11:57:21 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Vinod Koul <vkoul@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Ben Dooks <ben.dooks@...ethink.co.uk>
CC:     <dmaengine@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] dmaengine: tegra-apb: Support per-burst residue
 granularity


On 14/06/2019 17:44, Dmitry Osipenko wrote:
> 14.06.2019 18:24, Jon Hunter пишет:
>>
>> On 14/06/2019 16:21, Jon Hunter wrote:
>>>
>>> On 13/06/2019 22:08, Dmitry Osipenko wrote:
>>>> Tegra's APB DMA engine updates words counter after each transferred burst
>>>> of data, hence it can report transfer's residual with more fidelity which
>>>> may be required in cases like audio playback. In particular this fixes
>>>> audio stuttering during playback in a chromiuim web browser. The patch is
>>>> based on the original work that was made by Ben Dooks [1]. It was tested
>>>> on Tegra20 and Tegra30 devices.
>>>>
>>>> [1] https://lore.kernel.org/lkml/20190424162348.23692-1-ben.dooks@codethink.co.uk/
>>>>
>>>> Inspired-by: Ben Dooks <ben.dooks@...ethink.co.uk>
>>>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>>>> ---
>>>>  drivers/dma/tegra20-apb-dma.c | 35 ++++++++++++++++++++++++++++-------
>>>>  1 file changed, 28 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
>>>> index 79e9593815f1..c5af8f703548 100644
>>>> --- a/drivers/dma/tegra20-apb-dma.c
>>>> +++ b/drivers/dma/tegra20-apb-dma.c
>>>> @@ -797,12 +797,36 @@ static int tegra_dma_terminate_all(struct dma_chan *dc)
>>>>  	return 0;
>>>>  }
>>>>  
>>>> +static unsigned int tegra_dma_update_residual(struct tegra_dma_channel *tdc,
>>>> +					      struct tegra_dma_sg_req *sg_req,
>>>> +					      struct tegra_dma_desc *dma_desc,
>>>> +					      unsigned int residual)
>>>> +{
>>>> +	unsigned long status, wcount = 0;
>>>> +
>>>> +	if (!list_is_first(&sg_req->node, &tdc->pending_sg_req))
>>>> +		return residual;
>>>> +
>>>> +	if (tdc->tdma->chip_data->support_separate_wcount_reg)
>>>> +		wcount = tdc_read(tdc, TEGRA_APBDMA_CHAN_WORD_TRANSFER);
>>>> +
>>>> +	status = tdc_read(tdc, TEGRA_APBDMA_CHAN_STATUS);
>>>> +
>>>> +	if (!tdc->tdma->chip_data->support_separate_wcount_reg)
>>>> +		wcount = status;
>>>> +
>>>> +	if (status & TEGRA_APBDMA_STATUS_ISE_EOC)
>>>> +		return residual - sg_req->req_len;
>>>> +
>>>> +	return residual - get_current_xferred_count(tdc, sg_req, wcount);
>>>> +}
>>>> +
>>>>  static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
>>>>  	dma_cookie_t cookie, struct dma_tx_state *txstate)
>>>>  {
>>>>  	struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
>>>> +	struct tegra_dma_sg_req *sg_req = NULL;
>>>>  	struct tegra_dma_desc *dma_desc;
>>>> -	struct tegra_dma_sg_req *sg_req;
>>>>  	enum dma_status ret;
>>>>  	unsigned long flags;
>>>>  	unsigned int residual;
>>>> @@ -838,6 +862,8 @@ static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
>>>>  		residual = dma_desc->bytes_requested -
>>>>  			   (dma_desc->bytes_transferred %
>>>>  			    dma_desc->bytes_requested);
>>>> +		residual = tegra_dma_update_residual(tdc, sg_req, dma_desc,
>>>> +						     residual);
>>>
>>> I had a quick look at this, I am not sure that we want to call
>>> tegra_dma_update_residual() here for cases where the dma_desc is on the
>>> free_dma_desc list. In fact, couldn't this be simplified a bit for case
>>> where the dma_desc is on the free list? In that case I believe that the
>>> residual should always be 0.
>>
>> Actually, no, it could be non-zero in the case the transfer is aborted.
> 
> Looks like everything should be fine as-is.

I am still not sure we want to call this for the case where dma_desc is
on the free list.

> BTW, it's a bit hard to believe that there is any real benefit from the
> free_dma_desc list at all, maybe worth to just remove it?

I think you need to elaborate a bit more here. I am not a massive fan of
this driver, but I am also not in the mood for changing unless there is
a good reason.

Cheers
Jon

-- 
nvpublic