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[91.79.162.197]) by smtp.googlemail.com with ESMTPSA id d2sm2075090lfh.1.2019.06.18.02.45.21 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 18 Jun 2019 02:45:21 -0700 (PDT) Subject: Re: [PATCH v1] dmaengine: tegra-apb: Support per-burst residue granularity To: Jon Hunter , Laxman Dewangan , Vinod Koul , Thierry Reding , Ben Dooks 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> <255f92e8-df61-5e9c-ba4f-e52a0bd11451@nvidia.com> From: Dmitry Osipenko Message-ID: <69cb064c-2a8e-9421-35b0-4a2273f62365@gmail.com> Date: Tue, 18 Jun 2019 12:45:20 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.1 MIME-Version: 1.0 In-Reply-To: <255f92e8-df61-5e9c-ba4f-e52a0bd11451@nvidia.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: dmaengine-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: dmaengine@vger.kernel.org 18.06.2019 11:47, Jon Hunter пишет: > > On 17/06/2019 13:41, Dmitry Osipenko wrote: >> 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 >>>>>>> Signed-off-by: Dmitry Osipenko >>>>>>> --- >>>>>>> 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. > > Yes indeed and for the ADMA we allocate and free on-demand as you are > suggesting. I don't know why it was done like this, but to make the > change it would be good to get some data about how much memory it is > consuming to see if it is actually worth it. Yeah, that's something to check in the future.