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.do...@codethink.co.uk/ >>>>>>> >>>>>>> Inspired-by: Ben Dooks <ben.do...@codethink.co.uk> >>>>>>> Signed-off-by: Dmitry Osipenko <dig...@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. > > 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.
