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Subject: Re: [PATCH v1] dmaengine: tegra-apb: Support per-burst residue
 granularity
To:     Jon Hunter <jonathanh@nvidia.com>,
        Laxman Dewangan <ldewangan@nvidia.com>,
        Vinod Koul <vkoul@kernel.org>,
        Thierry Reding <thierry.reding@gmail.com>,
        Ben Dooks <ben.dooks@codethink.co.uk>
Cc:     dmaengine@vger.kernel.org, linux-tegra@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20190613210849.10382-1-digetx@gmail.com>
 <5fbe4374-cc9a-8212-017e-05f4dee64443@nvidia.com>
 <7ab96aa5-0be2-dc01-d187-eb718093eb99@nvidia.com>
 <840fcf60-8e24-ff44-a816-ef63a5f18652@gmail.com>
 <d34c100d-e82a-bb00-22c6-c5f2f6cdb03a@nvidia.com>
From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <b47b7b89-e830-0b3e-026d-c6c7d67d3324@gmail.com>
Date:   Mon, 17 Jun 2019 15:41:18 +0300
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17.06.2019 13:57, Jon Hunter пишет:
> 
> 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@codethink.co.uk>
>>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.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.

You're right! It's a bug there! The sg_req=NULL if dma_desc is on the free list, hence
it will result in a NULL dereference. I'll fix it in v2 and will avoid the offending
call, like you're suggesting.

>> 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.

It looks like the whole point of the free list is to have a cache of preallocated
dma_desc's, but dma_desc allocation and initialization doesn't cost anything in
comparison to the free list because memory is allocated from a SLAB cache and then the
initialization will happen on CPU's cache.

So the free list is quite pointless in terms of optimization. Moreover what if driver
allocates a lot of dma_desc's and uses them just once? Looks like it will be quite a
lot of wasted memory on the free list.